•Alkali–metal ion extraction using polytetrafluoroethylene as an extraction reagent (AEP) is applicable to highly air- and solvent-sensitive compounds.•AEP is applicable to mixed-anion compounds.•AEP can be utilized to modify chemical compositions and physical properties of alkali–metal ion containing inorganic compounds.Na+ extraction from Na2Ti2Sb2O has been attempted using polytetrafluoroethylene (PTFE; empirical formula CF2) as the extraction reagent. Systematic increase in both lattice parameters a along the intralayer direction and c along the interlayer direction was observed with respect to the reacted PTFE amounts, suggesting the successful deintercalation of the interlayer Na+. In addition, ferromagnet-like behavior, which is rare for systems consisting of ions with low magnetic moments, such as Ti3+(d1), was observed in the Na+-deintercalated Na2Ti2Sb2O. This result suggests that the method of alkali–metal ion extraction using polytetrafluoroethylene as an extraction reagent (AEP) can be utilized in order to induce interesting and important properties in compounds including highly air- and solvent-sensitive mixed-anion compounds such as Na2Ti2Sb2O.Performance of polytetrafluoroethylene (PTFE) as a reagent to deintercalate Na+ from a highly air- and solvent-sensitive compound Na2Ti2Sb2O2 has been studied.

A new upconversion (UC) material was designed by flocculating a Ca2Nb3O10– nanosheet, which acts as thermal and structural stabilizer, with Ho3+ photoactivator, Yb3+ sensitizer, and Y3+ space filler. The flocculated product consists of the restacked nanosheets and the rare-earth ions in the internanosheet gallery. The restacked sheet faces of the Ca2Nb3O10– nanosheet building blocks are self-organized in a parallel manner, and their crystallographic coherency extends to three layers on average. On the other hand, the nanosheet building blocks are randomly staggered along the in-layer direction. Chemical composition of the flocculated product was estimated as (Ho0.096Yb0.23Y0.164)Ca1.76□0.24Nb3O10·1.4H2O. Heat treatment of the flocculated product at 500 °C was necessary in order to suppress nonradiative energy loss via OH vibration and to induce UC emission. Even after the heat treatment, perovskite-type atomic arrangement of the Ca2Nb3O10– nanosheet building block was retained. Upon laser irradiation at 980 nm, two UC emission bands around 550 and 660 nm were observed, and the emission was visible to the eye. The result from this study suggests that flocculation of nanosheets, as building blocks, with counterions is a promising way to design bulk functional materials that are rather difficult or impossible to prepare by conventional synthetic approaches.

Interlayer Rb+ of the perovskite-type layered oxyfluoride RbSrNb2O6F was ion-exchanged with H+, and the protonated phase was reacted with aqueous solution of tetrabutylammonium hydroxide to exfoliate it into nanosheets. The resulting nanosheet suspension exhibits Tyndall scattering of a laser beam, indicating its colloidal nature. Elemental composition of the nanosheet was estimated as Sr0.98Nb2O6F0.97δ−, which was quite close to that of the layer unit of the precursor. The homogeneously unilamellar nature of this nanosheet was confirmed by atomic force and transmission electron microscopy observations and X-ray scattering results. The optical absorption edge of the nanosheet suspension was observed around at 293 nm, and two well-defined peaks with their maxima at 229 and 278 nm were observed. Furthermore, the aqueous suspension of the nanosheet exhibits fluorescence emission in the UV-blue region. These properties of the oxyfluoride nanosheets are quite different from those of its oxide analogues without F–, such as LnNb2O7– nanosheets (Ln = La3+, Eu3+, Sm3+), suggesting that anion-site replacement of oxide nanosheets can be utilized to optimize or induce various properties.

Alkali-metal-ion extraction reactions using poly(tetrafluoroethylene) (PTFE; AEP reactions) were performed on two kinds of α-NaFeO2-type layered compounds: Na0.68(Li0.68/3Ti1–0.68/3)O2 and K0.70(Li0.70/3Sn1–0.70/3)O2. At 400 °C in flowing argon, these layered compounds were reacted with PTFE. By these reactions, alkali-metal ions in the layered compounds were successfully extracted, and TiO2 and SnO2 with rutile-type structure were formed. The structural similarity between the alkali-metal-ion-extracted layered compounds and the binary metal oxide products in these unique alkali-metal-ion extraction reactions was interpreted in terms of their interatomic distance distribution by atomic pair distribution function analysis. The results of this study indicate that PTFE is an effective agent to extract alkali-metal ions from layered compounds, and AEP reaction is not limited to the previously reported γ-FeOOH-type layered titania K0.8(Li0.27Ti1.73)O4, but is also applicable to other layered titania and other non-titanium-based layered metal oxides. Therefore, it was clarified that AEP reactions are widely applicable routes to prepare various compounds, including those that are difficult to synthesize by other reactions.

We have designed a new approach to synthesize brookite, i.e., to extract alkali-metal ions from K0.8Ti1.73Li0.27O4 (KTLO) and to apply simultaneous heat treatment to the remaining lepidocrocite-type layers of TiO6 octahedra. For the alkali-metal ion extraction and the simultaneous heat treatment, KTLO was heated at 400 °C with polytetrafluoroethylene (PTFE) in flowing Ar. PTFE has been found to be an effective agent to extract strongly electropositive alkali-metal ions from KTLO because of the strong electronegativity of F as its component. The product of this reaction consists of a mixture of brookite, K2CO3, LiF, and PTFE derivatives, indicating the complete extraction of K+ and Li+ from KTLO and formation of brookite from the lepidocrocite-type layer of TiO6 octahedra as a template. This brookite has a partial replacement of O2− with F− and/or slight oxygen deficiency; thus, its color is light-bluish gray. Fully oxidized brookite formation and complete decomposition of PTFE derivatives have been achieved by further heating in flowing air, and coproduced alkali-metal salts have been removed by washing in water. Powder X-ray diffraction, Raman spectroscopy, and chemical analysis results have confirmed that the final brookite product treated at 600 °C is single phase, and it is white. The method to extract alkali-metal ions from a crystalline material using PTFE is drastically different from the common methods such as soft-chemical and electrochemical reactions. It is likely that this new synthetic approach is applicable to other layered systems to prepare a diverse family of compounds, including novel metastable ones.

The bulk layered compound KLa0.90Sm0.05Nb2O7 and its exfoliated form of La0.90Sm0.05Nb2O7 nanosheet have been prepared, and their photoluminescence properties have been characterized. The photoluminescence emission of KLa0.90Sm0.05Nb2O7 via host excitation is negligibly low in intensity, whereas the emission via direct Sm3+ excitation has apparently been observed. On the contrary, the emission via direct Sm3+ excitation is preferentially quenched, and the emission via host excitation becomes far more predominant than that via direct Sm3+ excitation for the exfoliated La0.90Sm0.05Nb2O7 nanosheet. From the comparison of photoluminescence properties between the bulk layered compound KLa0.90Sm0.05Nb2O7 and its exfoliated form of La0.90Sm0.05Nb2O7 nanosheet, the relative enhancement of the host excitation-mediated photoluminescence by exfoliation of bulk Ln-photoactivated layered phosphors into nanosheets as a general trend has become more apparent.

Magnetic properties of a series of orthorhombic NaLnTiO4 (Ln = Sm, Eu, Gd, Tb, Dy, Ho and Er) characterized by heat capacity measurements are reported. Magnetic transitions are observed for all NaLnTiO4 investigated except for Ln = Eu. The change in magnetic entropy closely agrees with the expected theoretical values except for Ln = Sm and Gd. Peak profiles in temperature dependence of heat capacity indicate that Ln = Sm, Tb, Dy, Ho and Er phases possess Ising square lattices. Furthermore, Ln = Gd also exhibits the similar property regardless of its S state; there seems some degree of anisotropy in its magnetic lattice. Magnetization and heat capacity data indicate that superexchange through oxygen is the dominant spin interaction in this system.

A series of orthorhombic NaLnTiO4 (Ln = lanthanides) has been synthesized and characterized. These compounds all crystallize in A-site ordered n = 1 Ruddlesden–Popper phase (An+1BnO3n+1) structure. The weak ferromagnetism with large magnetization has been observed for Ln = Tb and Dy, and non-linear field dependence of magnetization without hysteresis has been observed for Ln = Er and Ho. By performing the symmetry operations, two possible canted spin arrangements have been proposed.

Ln2BaPdO5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) all crystallizes in Nd2BaZnO5 structure which is proposed to have a Shastry–Sutherland (S–S) lattice of Ln3+ ions. Ln2BaPdO5 (Ln = Nd, Sm, Gd, Dy, Ho) exhibits antiferromagnetic transitions at temperatures between 2 and 5 K. In addition, metamagnetic transitions have been observed for Ln2BaPdO5 (Ln = Nd, Dy, Ho) below 2 K. Specific heat of Ln2BaPdO5 (Ln = Ho, Dy) exhibits symmetric peaks at TN, and residual entropy below TN is negligibly small. On the contrary, specific heat of Ln2BaPdO5 (Ln = Nd, Sm, Gd) exhibits Schottky type anomalies followed by sharp peaks at TN, and residual entropy below TN is large. From these results, the applicability of the S–S model to Ln2BaPdO5 with various Ln3+ ions is proposed.

Synthesis and characterization of Sr1−xAxPd3O4 (A = Na, Bi) are reported. All the doped phases crystallize in the cubic symmetry with the space group Pm3m. Both Hall and thermopower measurement results indicate that substitutions of Na+ and Bi3+ into the Sr2+ site introduce hole and electron carriers, respectively. As the dopant concentration increases, effective magnetic moment, electrical conductivity and carrier concentration of these phases increase and thermopower decreases. The highest power factor obtained among these doped samples is 0.5 μW/K2 cm at room temperature for Sr0.8Na0.2Pd3O4.

Eu3+-activated double perovskite-type nanosheet phosphor (K1.5Eu0.5)Ta3O10 has been prepared by the soft chemical exfoliation reaction of K(K1.5Eu0.5)Ta3O10 bulk precursor. The lateral size of the nanosheet product ranges from 0.1 to a few micrometers, and the thickness is uniformly 2.4(2) nm. The lattice parameter along the sheet direction obtained by the in-plane X-ray diffraction is 0.3933(3) nm, corresponding to the typical O−Ta−O distances. The photoluminescence emission can be obtained either by host or by direct photoactivator excitation, but the host excitation yields much higher emission intensity than does the direct photoactivator excitation. The emission spectra show relatively sharp peaks from the 5D0 → 7FJ manifold transitions of Eu3+. The highest emission intensity is observed around 704 nm (far-red) from the 5D0 → 7F4 transition of Eu3+.

Eu3+-doped perovskite nanosheets, La0.90Eu0.05Nb2O7, have been prepared by the soft chemical exfoliation reaction of K1−xHxLa0.90Eu0.05Nb2O7 with a tetrabuthylammonium hydroxide aqueous solution. The resulting colloidal La0.90Eu0.05Nb2O7 nanosheet suspension exhibits photoluminescence emission from the 5D0 to 7FJ manifold transitions of Eu3+ by either direct excitation of Eu3+ or host excitation, whereas no host emission was observed at room temperature. In the case of the bulk precursors K1−xHxLa0.90Eu0.05Nb2O7, the direct excitation yields more intense emission than the host excitation. On the contrary, the most intense emission from the La0.90Eu0.05Nb2O7 nanosheets was observed by exciting at the broad excitation band maximum (353 nm). The difference in the photoluminescence properties between the La0.90Eu0.05Nb2O7 nanosheets and their bulk precursors seems to be related to the dimensionality of these host structures and the confinement of the energy-transfer process between the host layer units and Eu3+ activators.

The alkali-metal ion extraction ability of an inert material, polytetrafluoroethylene (PTFE; empirical formula CF2), was clarified by characterizing a partially alkali-metal ion-extracted layered compound, K0.8(Li0.27Ti1.73)O4. Washing K0.8(Li0.27Ti1.73)O4 in water extracts only 44% of the interlayer K+ and no intralayer Li+; on the other hand, 53% of the interlayer K+ and approximately 10% of the intralayer Li+ ions were extracted from K0.8(Li0.27Ti1.73)O4 by the reaction with PTFE at 350 °C under flowing Ar. A systematic decrease in the lattice parameters a and c along the intralayer directions and an increase in b along the interlayer direction were observed, consistent with the alkali-metal ion deintercalation amount as a function of the reaction temperatures and the reacted PTFE amounts. After the reaction with K0.8(Li0.27Ti1.73)O4:CF2 = 1:0.6 in mol, the lattice parameter b increased to 1.5607(3) nm from 1.5522(2) of the pristine K0.8(Li0.27Ti1.73)O4, and this change in the lattice parameter was approximately one order of magnitude larger than those in a and c.