•Phase transitions for stuffed tridymite AGa2O4 (A = Sr, Ca) were observed.•Rietveld refinements were performed for γ-SrGa2O4, β-SrGa2O4 and α-CaGa2O4.•γ-SrGa2O4 is the thermodynamically stable phase below 1200 °C.•The temperature for α-CaGa2O4 (Pna21) to β-CaGa2O4 transition is about 1350 °C.•DFT calculations proved the low temperature phases had the low formation energies.Temperature-induced phase transitions for stuffed tridymite AGa2O4 (A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO3 and Ga2O3 led to the formation of γ-SrGa2O4 (P21/n) below 1200 °C, and transform to β-SrGa2O4 (P21/c) when heated at 1200 °C. A similar phenomenon was found for CaGa2O4, and the temperature boundary between α-CaGa2O4 (Pna21) and the high temperature polymorph β-CaGa2O4 (P21/c) was about 1350 °C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa2O4 and α-CaGa2O4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.Stuffed tridymite γ-SrGa2O4 (P21/n) is the thermodynamically stable phase below 1200 °C, and it transforms to β-SrGa2O4 (P21/c) at 1200 °C. Similarly, the phase transition from α-CaGa2O4 (Pna21) to β-CaGa2O4 (P21/c) occurs at ~ 1350 °C. Rietveld refinements provide detailed structural information and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations.Download high-res image (328KB)Download full-size image

•The accurate solid solution range of BiMnxFe3-xO6 is determined to 0.9 ≤ x ≤ 1.3.•High quality XRD data were collected to prove the phase purity of BiMnxFe3-xO6.•Rietveld refinements suggest the cationic substitution do not change the structure.•Similar low temperature magnetism was observed for this series of oxides.•The AFM ordering temperature decreased with the increasing the Mn3+ content.BiMnxFe3-xO6 (x = 1) represents a new type of oxide structure containing Bi3+ and competing magnetic super-exchanges. In literature, multiple magnetic states were realized at low temperatures in BiMnFe2O6, and the hypothetical parent compounds (BiMn3O6, BiFe3O6) were predicted to be different in magnetism. Herein, we performed a careful study on the syntheses of BiMnxFe3-xO6 at ambient pressure, and the solid solution range was determined to be 0.9 ≤ x ≤ 1.3 by Rietveld refinements on high-quality powder X-ray diffraction data. Due to the very similar cationic size of Mn3+ and Fe3+, and possibly the structural rigidity, there was no significant structure change in the whole range of solid solution. The magnetic behavior of BiMnxFe3-xO6 (x = 1.2, 1.22, 1.26, 1.28 and 1.3) was generally similar to BiMnFe2O6, while the relative higher concentration of Mn3+ led to the decreasing of the antiferromagnetic ordering temperature.Download high-res image (418KB)Download full-size image

AlPO4-5 is the most extensively studied material in exploring the crystallization mechanism of aluminophosphate molecular sieves, and it can be synthesized using 4-dimethylaminopyridine (DMAP) as an organic structure-directing agent (OSDA). Achieving the AlPO4-5 intermediate in the crystalline form would be of great importance and significance for this purpose. In this study, we proposed a concept that by carefully regulating the synthesis conditions, crystalline alumimophosphate intermediates could be rationally prepared. A crystalline AlPO4-5 intermediate, (C7H11N2)2(H3O)(Al3P4O16)·(H2O)2.5 (1), was, therefore, synthesized by significantly increasing the amounts of orthophosphoric acid and DMAP. The intermediate 1 consists of alternating organic DMAP and inorganic [Al3P4O16]3− layers with abundant hydrogen bonds and can transform to the AlPO4-5 framework upon heating in air or through a steam-assisted conversion (SAC) method. To unravel the role of DMAP in the transformation, pure AlPO4-5 (2) was also synthesized by altering the synthesis conditions. The supramolecular assembly templating (SAT) effects of DMAP in 1 and 2 were verified via fluorescence spectra. Thus, the π–π stacking interactions between the DMAP molecules and the abundant hydrogen bonds between the adjacent layers are considered to be important for the phase transformation of 1. This crystalline intermediate would promote the understanding of the crystallization mechanism in the system of aluminophosphate molecular sieves.

•Octahedron-based framework aluminoborates provide a powerful alternative strategy to prepare redox molecular sieves with different transition metals.•Cr-PKU-1 is a dual-centered solid acid catalyst and shows higher activity than mono-centered solid catalyst in the heterogeneously-catalyzed Strecker reaction.•Site-isolated Lewis or Brönsted acid centers independently activate the Strecker reaction and undergo two-type parallel reaction pathways.Cr-PKU-1, an octahedron-based redox molecular sieve, was found to be an efficient, environmentally benign and easily recoverable catalyst for the heterogeneously-catalyzed Strecker reaction under the mild conditions. This material was evidenced to be a dual-centered solid acid with a large density of Brönsted acid (borate hydroxyl groups, B-OH) and Lewis acid sites (Cr3+), and therefore showed much higher catalytic activity than mono-central solid acid in the formation of α-aminonitriles. Although commonly considered as Lewis acid centers, electron-deficient boron atoms (tri-coordinated BO3) in Cr-PKU-1 failed to exhibit the desired catalytic activity in the present investigations probably due to steric hindrance against the large organic substrates. A plausible mechanism about two-centered catalysis in Strecker reaction were proposed based on the comprehensive and sufficient analysis. This work illustrates for the first time the possible application in the Strecker reaction using the dual-centered redox molecular sieve, and highlights their potential ability to address the various functions involved in the reaction mechanism.Download high-res image (177KB)Download full-size image

•Octahedral aluminoborate: alternative candidate to prepare redox molecular sieves.•High substitution upper-limit and strong accommodation capacity for some transition metals.•Sec-alcohols oxidized to ketones or other over-oxidative products.•Cr3+ functioned as catalytic center to modulate the formation of free radicals.•A mechanism proposed to demonstrate the probable reaction pathway.Octahedral-based redox molecular sieves M-PKU-1 (MCr, Fe) were synthesized by isomorphous metal-substitution and used as catalysts for catalytic dehydrogenation of sec-alcohols using H2O2 as oxidant. Various characterizations, such as X-ray diffraction Rietveld refinement and XPS spectrum, confirmed that transition metals were embedded successfully inside the PKU-1 framework with a high level (about ∼50 atom %) and presented in the valence state of +3. Molecular probe analyzes suggested that Cr sites catalyzed the quick formation of active OH radicals, however strongly suppressed the generation of superoxide O2- ions. Under the performed reaction conditions, 10%Cr-PKU-1 exhibited excellent catalytic performances (>99 % selectivity) and kept favorable recyclable stability. A hypothetical mechanism was proposed, which involved a Cr3+-Cr2+-Cr3+ circle when the oxidative dehydrogenation reaction happened. Furthermore, qualitative and quantitative analyzes were performed to illustrate the stepwise by-products generated in the probable pathway due to the over-oxidization, but the selectivity to the two proposed pathways seemed to be in equal portions and didn’t have any obvious preference. Obviously, our preliminary results still merit further exploration; we believe however, they would provide helpful information to better understand the structure-activity relationship and the key function of Cr-PKU-1 in the catalytic activation of H2O2.Download high-res image (97KB)Download full-size image

An octahedron-based gallium borate (Ga-PKU-1) with an open framework was prepared by the boric acid flux method and studied by XRD, SEM, FT-IR, TGA-DSC, BET and NH3-TPD. The borate hydroxyl groups in the 18-ring membered channels functioned as Brönsted acid centers and were proved to play an important role in the dehydration of isopropanol to propylene with high selectivity. However, the other two gallium borates, Ga4B2O9 and GaBO3, with similar elemental compositions showed completely different acidities and catalytic performance for isopropanol decomposition. Our results indicated that such a great difference in selectivity most probably originated from the changes in the coordination microenvironment from GaO6 to GaO5. The relationship between the structure and activity was discussed in detail, and an intrinsic mechanism for the selectivity preference was proposed according to the obtained results.

“114” oxides have shown intriguing physical properties while their performance in photocatalysis has not yet been reported probably due to the instability in aqueous solution. YBaZn3GaO7 is an exception, which is stable and indeed shows observable photocatalytic H2 evolution (∼2 μmol/h/g) in methanol aqueous solution under UV light. This activity was enhanced to 23.6 μmol/h/g by a full replacement of Y3+ by Sc3+. Optical absorption spectra and theoretical calculations show no significant difference upon Sc3+-doping. Instead, a systematic analysis of the structure evolution by Rietveld refinements for Y1–xScxBaZn3GaO7 (0 ≤ x ≤ 1) suggests that the increase of the catalytic activity is likely due to the decrease of the structural defects and thus the lower level of recombination rate of e– and h+. In detail, Sc3+ substitution leads to a shrinkage of YO6 octahedra, and successively the adjustment of the Zn2+/Ga3+ occupancy behaviors in tetrahedra sites. The photocatalytic H2 evolution rate was further optimized to 118.2 μmol/h/g in methanol solution and 42.9 μmol/h/g in pure water for 1 wt % Pt-loaded ScBaZn3GaO7. Here, the relatively less investigated nonmagnetic “114” oxides were, for the first time, proved to be good candidates for photocatalytic water reduction.

There emerge great interests in the syntheses of metastable polyborates; however, most are involved with the high-pressure technique. A facile method to synthesize metastable rare earth borates at ambient pressure is eagerly required for the large-scale production and property investigation. Here we demonstrate the critical role of Bi3+ substitutions in the stabilization of metastable β-REB3O6 (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, and Y) at ambient pressure, where the Bi3+-to-RE3+ substitutions would efficiently reduce the synthetic temperatures to 735–820 °C, well below the upper limit of thermodynamically stable window (840–980 °C). Partial solid solutions of β-RE1–xBixB3O6 were prepared, and the ranges of the solution were also studied experimentally. The thermal behaviors of β-RE0.8Bi0.2B3O6 were investigated by differential thermal analyses and powder X-ray diffraction, and they were divided into two categories; that is, β-RE0.8Bi0.2B3O6 (RE = Sm, Eu, Gd) transfers to α-RE0.8Bi0.2B3O6 with further increasing the temperature to 950 °C, while β-RE0.8Bi0.2B3O6 (RE = Tb, Dy, Ho, Er, and Y) decomposes into hexagonal REBO3 and B2O3. In particular, the allowed concentration of Bi3+ in β-Gd1–xBixB3O6 was 0.10 ≤ x ≤ 0.25, and these samples show bright blue emissions under UV excitation, which suggests the high efficiency of light absorption and high potential as phosphors with further doping of other activators.

The study of perovskites has been active for a long time. Here, we rationally designed and prepared a double perovskite, LaBaZnSbO6, by selecting Zn2+ and Sb5+ with large size and charge differences, and, indeed, complete B-site ordering can be achieved. Careful study using powder X-ray diffraction data pinpointed its space group to be I2/m, which has rarely been seen in double perovskites. Thereafter, an interesting observation of Sr2+-doping-induced symmetry evolution from I2/m to P21/n was confirmed in the complete solid solutions LaBa1−xSrxZnSbO6, where the tilting system also transferred from a−a−c0 to a−a−c+. The transition boundary is around x = 0.4. It can also be visualized by the variation of θ (defined as c/[(a + b)/2]), which is associated with the anisotropic shrinkage of the unit cell lattice and indeed shows a minimum at x = 0.4. Such a successive modulation of both the structural symmetry and the average La/Ba/Sr–O bond distances (revealed by Rietveld refinements) motivated us to study the Eu3+ luminescence in La0.95Eu0.05Ba1−xSrxZnSbO6. Interestingly, the maximum of charge transfer absorption of Eu3+ shows a precise changing tendency with the A–O bond distances along with the Sr2+ doping, clearly revealing the structure–luminescence correlations.

•Ba6(RE1−xTbx)9B79O138 (RE = La, Y, 0 ≤ x ≤ 1) were prepared by solid state reaction.•XRD show a shrinkage or expansion of unit cell in La- or Y-system, respectively.•The cyan/green ratio of Tb3+ was applied to evaluate the RE-site symmetry.•The C/G value remains unchanged at ∼0.65 in Ba6(La1−xTbx)9B79O138.•The C/G increases quickly in the range of 0.50 ≤ x ≤ 0.90 in Ba6(Y1−xTbx)9B79O138.Extensive studies on photoluminescence (PL) were devoted to the well-known “REBaB9O16” family, while the structure remained unsolved until the recent work on the structure determination of its new analogue Ba6Bi9B79O138. Accordingly, a re-visit to “REBaB9O16” with the new formula was rationally performed by the syntheses and Tb3+-PL study on Ba6La9B79O138 (BLBO) and Ba6Y9B79O138 (BYBO). Powder XRD patterns of Ba6(RE1−xTbx)9B79O138 (RE = La and Y, 0 ≤ x ≤ 1) show a linear shrinkage or expansion of the unit cell in BLBO: xTb3+ and BYBO: xTb3+, respectively. The ratio of Tb3+ cyan/green (C/G) emission was proposed to evaluate the evolution principle of the RE-site symmetry in both series of phosphors. The C/G value remains unchanged in BLBO: xTb3+, while it is sensitive to the Tb3+-to-Y3+ substitution in BYBO: xTb3+. For example, C/G keeps almost constant (∼0.30) when 0.10 ≤ x ≤ 0.5; a dramatic increase of C/G (from 0.33 to 0.67) occurs in the range of 0.50 ≤ x ≤ 0.90, accompanied with a RE-site symmetry transition from centrosymmetric to noncentrosymmetric; the value of C/G remains unchanged thereafter. The Ce3+-to-Tb3+ sensitization effect is applied in Ba6(RE0.3−yCeyTb0.7)9B79O138 (RE = La or Y, 0 ≤ y ≤ 0.3), and the samples with y = 0.2 possess the strongest green emission.

In contrast to α- and β-Ga2O3 which have already been studied as photocatalysts for pure water splitting, there has been no report on the γ-phase. A comparative study on α-, β- and γ-Ga2O3 all prepared by a precipitation method was therefore performed. The as-prepared gallium oxides were phase-identified by powder X-ray diffraction, where γ-Ga2O3 possessed the most broad reflection peaks due to poor crystallization. Scanning electron microscopy and N2 adsorption–desorption experiments confirmed the morphology, and the specific surface areas were 144.3, 30.7, and 77.3 m2 g−1 for γ-, β- and α-Ga2O3, respectively. Photocatalytic H2 evolution efficiency in pure water was determined to be in the order of γ-Ga2O3 > α-Ga2O3 > β-Ga2O3, and the efficiencies were all much higher than that of P25–1 wt% Ag. A slight hydrolysis process was observed for γ-Ga2O3. Both lowering the pH value (∼4.5) by H2SO4 and adding sacrificial agent (CH3OH) were applied to prohibit the hydrolysis completely. Eventually, 1 wt% Ag was loaded as a cocatalyst in order to not only improve the stability but also to increase the H2 generation rate to 742 μmol h−1 g−1 in pure water. In addition, for this particular photocatalyst, the optimal apparent quantum yield achieved at 254 nm was 8.34%. Our work represents the first study of γ-Ga2O3 in the application of photocatalytic water splitting, and indeed it might have a high potential in solar energy conversion.

•PbGaBO4 was prepared by conventional and solution-assisted solid state reactions.•The bandgap energy is 4.10 eV from UV–Vis reflectance spectroscopy.•Theoretical calculations were performed to analyze the electronic structure.•The intrinsic UV-light water reduction activity was observed for PbGaBO4.•The optimal H2 evolution rate achieved 41.9(9) μmol/h/g by loading cocatalysts.Metal borates are well-known optical materials while their semiconducting behavior, i.e. photocatalytic property, was less studied. Inspired by the recent publications on gallium borates with good photocatalytic performances, we investigated photocatalytic water reduction activity of another gallium borate, PbGaBO4, comprising edge-sharing GaO6-chains. Both conventional and solution assisted solid state reaction methods were employed for the preparation. Phase purity and the crystallinity of both samples were identified by powder X-ray diffraction and scanning electron microscopy. The UV–Vis reflectance spectra suggested the wide bandgap characteristic (4.10 eV) and theoretical calculations pointed out that the 6s and 5p orbitals of Pb2+ contribute to both the bottom of CB and the top of VB. However the ionic bonding of PbO in PbGaBO4 led to a slight hydrolysis during the photocatalysis process, which can be suppressed by loading cocatalysts of Pt and RuOx. In addition, such a surficial modification significantly improved the photocatalytic water reduction ability, with an optimal H2 evolution rate of 41.9(9) μmol/h/g under UV-light irradiation.

A novel microporous aluminoborate, denoted as PKU-3, was prepared by the boric acid flux method. The structure of PKU-3 was determined by combining the rotation electron diffraction and synchrotron powder X-ray diffraction data with well resolved ordered Cl– ions in the channel. Composition and crystal structure analysis showed that there are both proton and chlorine ions in the channels. Part of these protons and chlorine ions can be washed away by basic solutions to activate the open pores. The washed PKU-3 can be used as an efficient catalyst in the Strecker reaction with yields higher than 90%.

Very recently, a novel barium bismuth borate Ba6Bi9B79O138 (BBBO) was shown to be a new analogue of REBaB9O16 (RE = rare earth), which is a family of excellent phosphor hosts, because it has two types of cationic positions suitable for bi- and tri-valent activators (or sensitizers). In this study, we prepared Eu3+-to-Bi3+ substituted solid solutions Ba6(Bi1−xEux)9B79O138 (0 ≤ x ≤ 1) and performed a systematic UV-photoluminescence investigation. As anticipated, efficient energy transfer occurs in the whole series of compounds and the majority of the energy absorbed by Bi3+ was transmitted to Eu3+, causing a great enhancement of Eu3+ emission. More interestingly, the Eu3+-to-Bi3+ substitution leads to a centrosymmetric-to-noncentrosymmetric transition of the local coordination symmetry of the trivalent cations due to the size difference between Eu3+ and Bi3+. This is observed in the synergetic changing of the wavelength of Bi3+ 1S0 → 3P1 absorption (excitation spectra) and the red-to-orange emission ratio (emission spectra) when changing x. BBBO is a new polyborate host with a freshly resolved structure whose interesting preliminary results suggest the further development of BBBO-based phosphors.

Understanding structure–property relationships are key to the rational design of superior materials. The series of well-known photoluminescent hosts with the chemical formula REBaB9O16 (RE = La–Lu, Y) have been extensively studied but no structural details have been reported. We prepared a new analogue Ba6Bi9B79O138 (BBBO) and proved the isomorphism by X-ray diffraction studies, in spite of the fact that they possess different cationic compositions. In addition, we successfully prepared complete solid solutions of Ba6(Bi1−xEux)9B79O138 (0 ≤ x ≤ 1). Single crystal XRD characterizations on a suitable crystal of BBBO indicated an ordered structure model with the noncentrosymmetric space group R3. It possesses a layered-type structure composed of BO4–BiO6–BO4 sandwiched layers and polyborate inter-layer groups. Ba2+ atoms are located in the inter-layer cavities. There are three and two independent Bi and Ba atoms, respectively. In other words, we have taken a step forward in understanding the structure of REBaB9O16 by analysing a new analogue. We believe this will help the further interpretation and development of REBaB9O16-based optical materials.

Borates are well-known candidates for optical materials, but their potentials in photocatalysis are rarely studied. Ga3+-containing oxides or sulfides are good candidates for photocatalysis applications because the unoccupied 4s orbitals of Ga usually contribute to the bottom of the conducting band. It is therefore anticipated that Ga4B2O9 might be a promising photocatalyst because of its high Ga/B ratio and three-dimensional network. Various synthetic methods, including hydrothermal (HY), sol–gel (SG), and high-temperature solid-state reaction (HTSSR), were employed to prepare crystalline Ga4B2O9. The so-obtained HY-Ga4B2O9 are micrometer single crystals but do not show any UV-light activity unless modified by Pt loading. The problem is the fast recombination of photoexcitons. Interestingly, the samples obtained by SG and HTSSR methods both possess a fine micromorphology composed of well-crystalline nanometer strips. Therefore, the excited e– and h+ can move to the surface easily. Both samples exhibit excellent intrinsic UV-light activities for pure water splitting without the assistance of any cocatalyst (47 and 118 μmol/h/g for H2 evolution and 22 and 58 μmol/h/g for O2 evolution, respectively), while there is no detectable activity for P25 (nanoparticles of TiO2 with a specific surface area of 69 m2/g) under the same conditions.

Band structure engineering is an efficient technique to develop desired semiconductor photocatalysts, which was usually carried out through isovalent or aliovalent ionic substitutions. Starting from a UV-activated catalyst ZnGa2S4, we successfully exploited good visible light photocatalysts for H2 evolution by In3+-to-Ga3+ and (Cu+/Ga3+)-to-Zn2+ substitutions. First, the bandgap of ZnGa2–xInxS4 (0 ≤ x ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn1–2y(CuGa)yGa1.7In0.3S4 (y = 0.1, 0.15, 0.2) provided a further and significant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn0.7Cu0.15Ga1.85In0.3S4 possessed the highest H2 evolution rate under pure visible light irradiation using S2– and SO32– as sacrificial reagents (386 μmol/h/g for the noble-metal-free sample and 629 μmol/h/g for the one loaded with 0.5 wt % Ru), while the binary hosts ZnGa2S4 and ZnIn2S4 (synthesized using the same procedure) show 0 and 27.9 μmol/h/g, respectively. The optimal apparent quantum yield reached to 7.9% at 500 nm by tuning the composition to Zn0.6Cu0.2Ga1.9In0.3S4 (loaded with 0.5 wt % Ru).

A series of new lanthanoid thioarsenates [Ln(teta)(μ–η1:η2:η1-AsIIIS3)]n {Ln = Ce (Ia), Pr (Ib), Nd (Ic), and Sm (Id); teta = triethylenetetramine} and [Ln(teta)(en)(μ–η1:η1:η1-AsVS4)]n {Ln = La (IIa), Ce (IIb), Pr (IIc), and Nd (IId); en = ethylenediamine} were prepared by the solvothermal reaction of K3AsO3, S, LnCl3 and organic amines and structurally characterized. Compounds Ia–d crystallise in the orthorhombic space group Aba2 and display 1-D neutral chains [Ln(teta)(μ–η1:η2:η1-AsIIIS3)]n, which represent the first examples of 1-D organic hybrid lanthanoid sulfides built up from trigonal-pyramidal [AsIIIS3]3− acting as tetradentate bridging ligands to interlink [Ln(teta)]3+ ions, while compounds IIa–d crystallise in the orthorhombic space group P212121 and consist of other 1-D neutral chains [Ln(teta)(en)(μ–η1:η1:η1-AsVS4)]n, which are built up from the linkages of the tetrahedral [AsVS4]3− ion and the [Ln(teta)(en)]3+ ion. To learn more about the influence of lanthanide contraction on the formation of lanthanoid thioarsenates, three organic hybrid lanthanoid thioarsenates [Ln(teta)(en)AsVS4] [Ln = Dy (IIIa), Ho (IIIb), and Tm (IIIc)] with the neutral molecular structure type in the monoclinic centrosymmetric space group P21/c are also presented. Their optical and magnetic properties have been investigated, and density functional theory calculations of Ia and IIa have also been performed.

Scheelite compounds with Eu3+ substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A1–1.5xEux□0.5xWO4 and A0.64–0.5yEu0.24Liy□0.12–0.5yWO4 (A = Ca, Sr; □ = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu3+. All compounds crystallize in I41/a with a disordered arrangement of A2+, Eu3+, □ at the A-site. The title compounds are all good red phosphors with a high R/O ratio (∼10), indicating that Eu3+ is located at a significantly distorted cavity. A1–1.5xEux□0.5xWO4 shows a saturation phenomenon at a high doping level, x = 0.20. With the incorporation of Li+, the emission intensity was generally enhanced compared to the Li+-free samples, moreover, an increase of the Li+ content reduces the content of vacancies, resulting in further increase of the luminescence intensity.

Rietveld refinements were performed on CaBaZn2Ga2−xAlxO7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga3+/Al3+ preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al3+. This represents the first example of a cationic ordering in “114” compounds. By substituting Al3+ into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1–O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P63mc to Pbn21. All the title compounds crystallized in P63mc, which is unexpected. A comparison with the selected “114” magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other “114” oxides.

Sol–gel method was applied to prepare homogenous and highly crystalline phosphors with the formulas α-GdB5O9:xTb3+ (0 ≤ x ≤ 1), α-Gd1−xCexB5O9 (0 ≤ x ≤ 0.40), α-GdB5O9:xCe3+, 0.30Tb3+ (0 ≤ x ≤ 0.15) and α-GdB5O9:0.20Ce3+, xTb3+ (0 ≤ x ≤ 0.10). The success of the syntheses was proved by the linear shrinkage or expansion of the cell volumes against the substitution contents. In α-GdB5O9:xTb3+, an efficient energy transfer from Gd3+ to Tb3+ was observed and there was no luminescence quenching. The exceptionally high efficiency of the f–f excitations of Tb3+ implies that these phosphors may be good green-emitting UV-LED phosphors. For α-Gd1−xCexB5O9, Ce3+ absorbs the majority of the energy and transfers it to Gd3+. Therefore, the co-doping of Ce3+ and Tb3+ leads to a significant enhancement in the green emission of Tb3+. Our current results together with the study on α-GdB5O9:xEu3+ in the literature indicate that α-GdB5O9 is a good phosphor host with advantages including controllable preparation, diverse cationic doping, the absence of concentration quenching, and effective energy transfer.

An open-framework gallium borate with intrinsic photocatalytic activities to water splitting has been discovered. Small inorganic molecules, H3BO3 and H3B3O6, are confined inside structural channels by multiple hydrogen bonds. It is the first example to experimentally show the structural template effect of boric acid in flux synthesis.

Single crystals of pure aluminoborate PKU-1 (Al3B6O11(OH)5·nH2O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the R3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by 27Al NMR results. This distortion of Al3O6 octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported R3̅ to the presently reported R3. We applied a pretreatment to prepare Al3+/Cr3+ aqueous solutions; as a consecquence, a very high Cr3+-to-Al3+ substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr3+ substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr3+. We also systematically investigated the thermal behaviors of PKU-1:xCr3+ (0 ≤ x ≤ 0.50) by thermogravimetric–differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr3+-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ x ≤ 0.20, PKU-1:xCr3+ transferred to the PKU-5:xCr3+ (Al4B6O15:xCr3+) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al4B2O9:xCr3+. For x > 0.20 in PKU-1:xCr3+, the heat treatment led to a composite of Cr3+-substituted PKU-5 and Cr2O3, so the doping upper limit of Cr3+ in PKU-5 structure is around 20 atom %.

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H2O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd1−xEux[B6O9(OH)3] (x = 0.10–1) and their dehydrated products, α-Gd1−xEuxB5O9. Efficient energy transfer from Gd3+ to Eu3+ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B6O9(OH)3], the re-crystallization to α-EuB5O9, and further decomposition to α-EuB3O6. During those processes, the Eu3+ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu3+. It is observed that the symmetry of the Eu3+ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescence efficiency when excited by Eu3+ CT transition. For the first time, complete solid solutions α-Gd1−xEuxB5O9 were synthesized directly by the sol–gel method, and their luminescence properties were also studied.

•An organic-free hydrothermal method was applied to obtain chalcopyrite CuInS2.•Bulk CuInS2 was prepared by annealing Cu2S and In2S3 at 600–750 °C under vacuum.•All samples show good photocatalytic activities for nitrate reduction.•All catalysts were stable after the photocatalytic reactions.We performed a facile and organic-free hydrothermal method (thiourea-oxalic acid co-molten system) to prepare chalcopyrite CuInS2 nano-crystallite. Its intrinsic photocatalytic activities to nitrate reduction in aqueous solutions were investigated in comparison to bulk CuInS2 prepared by annealing Cu2S and In2S3 at 600–750 °C under vacuum. All samples show good and similar photocatalytic activities for nitrate reduction, for example, the conversion rate is about two times of that in a blank experiment. All CuInS2 catalysts were stable after the photocatalytic reactions, indicated by the identical XRD patterns before and after the reaction.

β-BiB3O6 was compressed in a diamond anvil cell at room temperature and studied using a combinstion of in situ high-pressure Raman scattering and angle-dispersive synchrotron X-ray diffraction. The results reveal that β-BiB3O6 retains its structure below 9.0 GPa and undergoes a structural phase transition that starts at ∼11.5 GPa and finishes at ∼18.5 GPa. No other phase transition occurred with increasing pressure up to ∼46 GPa. It was also found that the high-pressure phase is different from the already-reported five polymorphs of BiB3O6. Therefore, the new phase is denoted as ζ-BiB3O6, which could be indexed by an orthorhombic unit cell (a ≈ 12.5 Å, b ≈ 6.7 Å, c ≈ 4.0 Å) from the powder XRD pattern collected at 22.2 GPa. Moreover, ζ-BiB3O6 can be quenched to ambient conditions. The investigation of the pressure dependence of the lattice parameters reveals that both β-BiB3O6 and ζ-BiB3O6 exhibit a large amount of crystallographic anisotropy. An unusual expansion of the c-axis of ζ-BiB3O6 was observed. Assignments for the Raman spectra of β-BiB3O6, γ-BiB3O6, and δ-BiB3O6 under ambient conditions were also performed. Currently, we cannot solve the crystal structure of ζ-BiB3O6 but give some speculations based on its relationship with β-BiB3O6.

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or –OH may lead to emission quenching. We investigated the PL properties of Sm1−xEux[B9O13(OH)4]·H2O (x = 0–1.00) and their dehydrated products α-Sm1−xEuxB5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu3+ activators very well. Sm3+ and Eu3+ are basically separated luminescent activators. Comparatively, Sm3+ shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu3+ luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu3+ are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu3+, even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu3+ in the amorphous state is the lowest. The high efficiency of the f–f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu3+ CT transition. In addition, α-Sm1−xEuxB5O9 was synthesized by a sol–gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.

ACo3(P2O7)2 (A=Ca, Sr, Ba, Pb) has been synthesized hydrothermally. Dc and ac magnetic susceptibilities are measured at various conditions. The magnetic interactions in all four compounds are dominantly ferromagnetic. All four compounds has a ferromagnetic-like long-range ordering at 8.0 K for CaCo, 6.4 K for SrCo, 7.6 K for PbCo, 8.0 K for BaCo. The ordering of PbCo at 7.6 K may contain a glassy component. Moreover, there is an additional phase transition for PbCo at 3.6 K. Once enhancing the external field to 3 kOe, the second phase transition is suppressed. Isothermal magnetizations of SrCo, PbCo and BaCo are quite similar, showing a field-induced metamagnetic transition at 2 K. For PbCo, the low-field ground state is weak ferromagnetic, and the critical filed is 10 kOe. Differently, CaCo is a typical soft weak ferromagnet.Graphical abstractDc and ac magnetic susceptibilities were measured on ACo3P4O14 (A=Ca, Sr, Ba, Pb). At 2 K, PbCo3P4O14 is a field-induced metamagnet, while CaCo3P4O14 is a soft ferromagnet.Highlights► Pure ACo3(P2O7)2 (A=Ca, Sr, Ba, Pb) samples are hydrothermally prepared. ► Dc and ac magnetic data shows they all have a FM-like ordering at 6–8 K. ► CaCo3(P2O7)2 is a typical soft ferromagnet. ► PbCo3(P2O7)2 has a metamagnetic transition at 2 K with a critical field of 10 kOe.

We successfully obtained pure α-CoB4O7 powder sample by a two-step synthesis method. First, a new cobalt polyborate with unknown crystal structure was synthesized by boric acid flux method in a sealed system. Subsequently, α-CoB4O7 powder was obtained by the thermal decomposition of this new phase at 800 °C. The purity was carefully checked by Le Bail fitting of its powder XRD with good convergence. The magnetic susceptibilities in paramagnetic region agree well with Curie–Weiss law with θ = −17.7 K, C = 3.13 cm3 K mol−1. Ac magnetic measurements confirm its long-range AFM ordering at ∼5 K. The field-dependent magnetization curve at 1.85 K indicates a spin-flop-like phase transition, with Hc = 35 kOe. Considering the relatively weak magnetic super-exchange, we believe this transition is due to the large magnetic anisotropy of Co2+.Graphical abstractHighlights► Pure polycrystalline α-CoB4O7 is prepared by two-step synthesis method. ► Dc and ac magnetic susceptibilities show an AFM ordering at ∼5 K. ► Isothermal magnetization curve at 1.85 K shows a spin-flop-like transition. ► High critical field (35 kOe) may relate to the large magnetic anisotropy of Co2+.

•Bi2Ga4O9 shows an intrinsic dual edge of light absorption at ∼433 and ∼382 nm.•The estimated valence and conduction band potentials allow water splitting.•Bi2Ga4O9–0.5 wt.% RuOx photocatalyzes overall water splitting under visible light.•The optimal gas generation rates are 41.5 (H2) and 19.6 (O2) μmol/h/g.•The apparent quantum yield at 420 nm is 0.09%.Visible-light-driven overall water splitting via semiconductors is one of the most challenging topics in photocatalysis, because it raises harsh requirements for photocatalysts both thermodynamically and kinetically. With the rationale of combining Bi3+ and Ga3+, we developed an oxide photocatalyst, Bi2Ga4O9 (loaded with RuOx), capable of overall water splitting under visible light due to its specific band structure, i.e., suitable potentials for valence and conduction bands, and most importantly its characteristic of anisotropic charge migration. Band structure engineering of Fe3+-to-Ga3+ doping and a sol–gel synthetic method were both applied to further enhance the light-harvesting ability and eventually lead to optimal gas generation rates of 41.5 and 19.6 μmol/h/g for H2 and O2, respectively. The apparent quantum yield at 420 nm is 0.09%; nevertheless, it represents a successful effort to develop a single-phase visible light catalyst for overall water splitting.Download high-res image (107KB)Download full-size image

•Photocatalytic water splitting ability of Ga-PKU-1 was systematically studied.•Various cocatalysts were loaded, including CoOx, RuOx, Pt, Au, NiOx and Ag.•Overall water splitting into stoichiometric H2 and O2 was achieved.•Ga-PKU-1 remained stable after three cycles of photocatalytic experiments.We present a systematic investigation on the photocatalytic ability of an open-framework gallium borate, Ga-PKU-1. It was reported to be intrinsically active for water reduction or oxidation, however, acquiring sacrificial reagents. Here the loading of CoOx, RuOx, Pt, Au, NiOx and Ag as cocatalysts greatly enhanced the catalytic activity, which eventually lead to overall water splitting into stoichiometric H2 (28.4 μmol/h/g) and O2 (14.5 μmol/h/g) for 1 wt.% RuOx–1 wt.% Pt loaded Ga-PKU-1. After three cycles of photocatalytic water splitting experiments, it gave the constant H2 evolution rate with no degradation of the catalyst.Download full-size image

•A highly crystallized tetragonal β-In2S3 exhibits a partial ordering of In3 + vacancy.•It contained flower-like spherical particles, further composed of very thin petals.•Tetragonal β-In2S3 has an intrinsic photocatalytic water reduction activity.•The optimal H2 evolution rate was 442 μmol/h/g under pure visible light irradiation.•Nitrate conversion rate achieved 3.26 mg N/h under the irradiation of a Xe lamp.A highly crystallized tetragonal β-In2S3 was synthesized with a partial ordering of In3 + vacancy confirmed by Rietveld refinements, which was therefore considered as an intermediate phase between conversional cubic and tetragonal β-In2S3. Benefiting from the high crystallinity and hierarchical morphology, it exhibited an intrinsic visible light photocatalytic activity in both water and nitrate reduction. The optimized H2 evolution rate was 442 μmol/h/g and the nitrate conversion rate was as high as 3.26 mg N/h. Apparent quantum yields of water splitting were also studied from 400 to 650 nm with an optimal value of 1.45% at 400 nm.Download high-res image (481KB)Download full-size image

Photocatalytic nitrate removal is potentially a green and low-cost technique for water purification; however, no effective visible light photocatalyst has been developed during the past thirty years. Here, we prove that CuInS2 (CIS) is a highly efficient visible light photocatalyst for nitrate removal. By the simultaneous loading of Pt, Ru and Au, it exhibited a high record of nitrate conversion rate of 8.32 mg N h−1 under pure visible light (λ > 400 nm). Highly concentrated NO3− (100 ppm N) can be completely converted into N2 in a few hours, without any over-reduction to ammonia nor production of H2. Under the irradiation of a monochromatic beam from 400 to 650 nm, CIS was most sensitive at 500 nm with an optimal apparent quantum yield of 23.85%. The mechanism was proposed as adsorption–reduction reactions and supported by the fact that the photocatalytic efficiency decreased when halogen ions were added as competing anions. The necessity of using a sacrificial agent during nitrate reduction was validated by a thorough discussion. Herein CIS remained effective when various or harmless sacrificial agents were used. Our work presents a new generation of photocatalysts for nitrate removal and pushes it forward to possible applications in industry.

The study of perovskites has been active for a long time. Here, we rationally designed and prepared a double perovskite, LaBaZnSbO6, by selecting Zn2+ and Sb5+ with large size and charge differences, and, indeed, complete B-site ordering can be achieved. Careful study using powder X-ray diffraction data pinpointed its space group to be I2/m, which has rarely been seen in double perovskites. Thereafter, an interesting observation of Sr2+-doping-induced symmetry evolution from I2/m to P21/n was confirmed in the complete solid solutions LaBa1−xSrxZnSbO6, where the tilting system also transferred from a−a−c0 to a−a−c+. The transition boundary is around x = 0.4. It can also be visualized by the variation of θ (defined as c/[(a + b)/2]), which is associated with the anisotropic shrinkage of the unit cell lattice and indeed shows a minimum at x = 0.4. Such a successive modulation of both the structural symmetry and the average La/Ba/Sr–O bond distances (revealed by Rietveld refinements) motivated us to study the Eu3+ luminescence in La0.95Eu0.05Ba1−xSrxZnSbO6. Interestingly, the maximum of charge transfer absorption of Eu3+ shows a precise changing tendency with the A–O bond distances along with the Sr2+ doping, clearly revealing the structure–luminescence correlations.

Sol–gel method was applied to prepare homogenous and highly crystalline phosphors with the formulas α-GdB5O9:xTb3+ (0 ≤ x ≤ 1), α-Gd1−xCexB5O9 (0 ≤ x ≤ 0.40), α-GdB5O9:xCe3+, 0.30Tb3+ (0 ≤ x ≤ 0.15) and α-GdB5O9:0.20Ce3+, xTb3+ (0 ≤ x ≤ 0.10). The success of the syntheses was proved by the linear shrinkage or expansion of the cell volumes against the substitution contents. In α-GdB5O9:xTb3+, an efficient energy transfer from Gd3+ to Tb3+ was observed and there was no luminescence quenching. The exceptionally high efficiency of the f–f excitations of Tb3+ implies that these phosphors may be good green-emitting UV-LED phosphors. For α-Gd1−xCexB5O9, Ce3+ absorbs the majority of the energy and transfers it to Gd3+. Therefore, the co-doping of Ce3+ and Tb3+ leads to a significant enhancement in the green emission of Tb3+. Our current results together with the study on α-GdB5O9:xEu3+ in the literature indicate that α-GdB5O9 is a good phosphor host with advantages including controllable preparation, diverse cationic doping, the absence of concentration quenching, and effective energy transfer.

Rietveld refinements were performed on CaBaZn2Ga2−xAlxO7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga3+/Al3+ preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al3+. This represents the first example of a cationic ordering in “114” compounds. By substituting Al3+ into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1–O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P63mc to Pbn21. All the title compounds crystallized in P63mc, which is unexpected. A comparison with the selected “114” magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other “114” oxides.

Scheelite compounds with Eu3+ substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A1–1.5xEux□0.5xWO4 and A0.64–0.5yEu0.24Liy□0.12–0.5yWO4 (A = Ca, Sr; □ = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu3+. All compounds crystallize in I41/a with a disordered arrangement of A2+, Eu3+, □ at the A-site. The title compounds are all good red phosphors with a high R/O ratio (∼10), indicating that Eu3+ is located at a significantly distorted cavity. A1–1.5xEux□0.5xWO4 shows a saturation phenomenon at a high doping level, x = 0.20. With the incorporation of Li+, the emission intensity was generally enhanced compared to the Li+-free samples, moreover, an increase of the Li+ content reduces the content of vacancies, resulting in further increase of the luminescence intensity.

Understanding structure–property relationships are key to the rational design of superior materials. The series of well-known photoluminescent hosts with the chemical formula REBaB9O16 (RE = La–Lu, Y) have been extensively studied but no structural details have been reported. We prepared a new analogue Ba6Bi9B79O138 (BBBO) and proved the isomorphism by X-ray diffraction studies, in spite of the fact that they possess different cationic compositions. In addition, we successfully prepared complete solid solutions of Ba6(Bi1−xEux)9B79O138 (0 ≤ x ≤ 1). Single crystal XRD characterizations on a suitable crystal of BBBO indicated an ordered structure model with the noncentrosymmetric space group R3. It possesses a layered-type structure composed of BO4–BiO6–BO4 sandwiched layers and polyborate inter-layer groups. Ba2+ atoms are located in the inter-layer cavities. There are three and two independent Bi and Ba atoms, respectively. In other words, we have taken a step forward in understanding the structure of REBaB9O16 by analysing a new analogue. We believe this will help the further interpretation and development of REBaB9O16-based optical materials.

A series of new lanthanoid thioarsenates [Ln(teta)(μ–η1:η2:η1-AsIIIS3)]n {Ln = Ce (Ia), Pr (Ib), Nd (Ic), and Sm (Id); teta = triethylenetetramine} and [Ln(teta)(en)(μ–η1:η1:η1-AsVS4)]n {Ln = La (IIa), Ce (IIb), Pr (IIc), and Nd (IId); en = ethylenediamine} were prepared by the solvothermal reaction of K3AsO3, S, LnCl3 and organic amines and structurally characterized. Compounds Ia–d crystallise in the orthorhombic space group Aba2 and display 1-D neutral chains [Ln(teta)(μ–η1:η2:η1-AsIIIS3)]n, which represent the first examples of 1-D organic hybrid lanthanoid sulfides built up from trigonal-pyramidal [AsIIIS3]3− acting as tetradentate bridging ligands to interlink [Ln(teta)]3+ ions, while compounds IIa–d crystallise in the orthorhombic space group P212121 and consist of other 1-D neutral chains [Ln(teta)(en)(μ–η1:η1:η1-AsVS4)]n, which are built up from the linkages of the tetrahedral [AsVS4]3− ion and the [Ln(teta)(en)]3+ ion. To learn more about the influence of lanthanide contraction on the formation of lanthanoid thioarsenates, three organic hybrid lanthanoid thioarsenates [Ln(teta)(en)AsVS4] [Ln = Dy (IIIa), Ho (IIIb), and Tm (IIIc)] with the neutral molecular structure type in the monoclinic centrosymmetric space group P21/c are also presented. Their optical and magnetic properties have been investigated, and density functional theory calculations of Ia and IIa have also been performed.

Very recently, a novel barium bismuth borate Ba6Bi9B79O138 (BBBO) was shown to be a new analogue of REBaB9O16 (RE = rare earth), which is a family of excellent phosphor hosts, because it has two types of cationic positions suitable for bi- and tri-valent activators (or sensitizers). In this study, we prepared Eu3+-to-Bi3+ substituted solid solutions Ba6(Bi1−xEux)9B79O138 (0 ≤ x ≤ 1) and performed a systematic UV-photoluminescence investigation. As anticipated, efficient energy transfer occurs in the whole series of compounds and the majority of the energy absorbed by Bi3+ was transmitted to Eu3+, causing a great enhancement of Eu3+ emission. More interestingly, the Eu3+-to-Bi3+ substitution leads to a centrosymmetric-to-noncentrosymmetric transition of the local coordination symmetry of the trivalent cations due to the size difference between Eu3+ and Bi3+. This is observed in the synergetic changing of the wavelength of Bi3+ 1S0 → 3P1 absorption (excitation spectra) and the red-to-orange emission ratio (emission spectra) when changing x. BBBO is a new polyborate host with a freshly resolved structure whose interesting preliminary results suggest the further development of BBBO-based phosphors.

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or –OH may lead to emission quenching. We investigated the PL properties of Sm1−xEux[B9O13(OH)4]·H2O (x = 0–1.00) and their dehydrated products α-Sm1−xEuxB5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu3+ activators very well. Sm3+ and Eu3+ are basically separated luminescent activators. Comparatively, Sm3+ shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu3+ luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu3+ are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu3+, even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu3+ in the amorphous state is the lowest. The high efficiency of the f–f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu3+ CT transition. In addition, α-Sm1−xEuxB5O9 was synthesized by a sol–gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.

An octahedron-based gallium borate (Ga-PKU-1) with an open framework was prepared by the boric acid flux method and studied by XRD, SEM, FT-IR, TGA-DSC, BET and NH3-TPD. The borate hydroxyl groups in the 18-ring membered channels functioned as Brönsted acid centers and were proved to play an important role in the dehydration of isopropanol to propylene with high selectivity. However, the other two gallium borates, Ga4B2O9 and GaBO3, with similar elemental compositions showed completely different acidities and catalytic performance for isopropanol decomposition. Our results indicated that such a great difference in selectivity most probably originated from the changes in the coordination microenvironment from GaO6 to GaO5. The relationship between the structure and activity was discussed in detail, and an intrinsic mechanism for the selectivity preference was proposed according to the obtained results.