Two-dimensional monocrystalline platelets of the perovskite structured lead–free system (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) were synthesized from platelets of layer-structured Na0.5Bi4.5Ti4O15 (NBIT), using a topochemical route. Both NBIT and NBBT platelets showed high aspect ratios with an average size of 10 μm and thickness of 0.6 μm, for the latter. A structural transformation from layer-structured NBIT to perovskite NBBT was identified in a transitional platelet, which contained phases of both NBIT and NBBT. The composition of NBBT lies at a morphotropic phase boundary (MPB), with the coexistence of monoclinic and tetragonal polymorphs confirmed by X-ray powder diffraction analysis. Transmission electron microscopy and piezoresponse force microscopy were used to investigate the domain structure, with stripe-like domains in the unpoled system transformed into lamellar domains after poling. The piezoresponse amplitude for a NBBT platelet indicated a large piezoresponse strain, corresponding to a field-induced strain Smax/Emax of around 800 pm V−1 at 5 kV mm−1, which is significantly higher than that in ceramic samples of NBBT at the same applied potential.

•Models for the defect structure in tungsten doped δ-Bi3YO6 are presented.•Despite lower vacancy concentration than in δ-Bi3YO6 high conductivity maintained.•Compensating effect of enhanced oxide ion mobility caused by lattice expansionSolid solution formation in the system Bi3Y1 − xWxO6 + 3x/2 has been studied using a combination of X-ray and neutron powder diffraction and a.c. impedance spectroscopy. Compositions in the solid solution adopt the δ-Bi2O3 type structure, with single phases evident from x = 0.00 to x = 0.20. Evidence for dopant clustering is presented and discussed. Models for the defect structure derived from diffraction studies are presented. Tungsten is proposed to adopt a tetrahedral coordination geometry, with a distorted octahedral geometry adopted by yttrium. Calculated coordination numbers for bismuth of around five are consistent with stereochemical activity of the Bi 6s2 lone pairs of electrons. Despite a significant lowering of the nominal vacancy concentration with respect to δ-Bi3YO6, as well as enhanced vacancy trapping by W6+, tungsten doping is found to have very little influence on the total conductivity of δ-Bi3YO6. This is attributed to the compensating effect of enhanced oxide ion mobility caused by lattice expansion.Download high-res image (56KB)Download full-size image

The structure of phosphate glasses of general composition 10Na2O:(20 + x/2)ZnO:(20 + x/2)CaO:(50 − x)P2O5 (0 ≤ x ≤ 20) has been investigated using IR spectroscopy, 1D 31P and 43Ca MAS Bloch decay, 31P–31P double quantum MAS-NMR and 43Ca and 67Zn static NMR techniques, as well as neutron diffraction analysis. Zinc is shown to aid glass formation in this system. Glass transition temperature and density increase with increasing cation:phosphate ratio. However, free volume calculations show structures becoming significantly more compact from x = 5 to x = 10. The structural data confirm depolymerisation of the glasses with increasing cation:phosphate ratio. Zinc oxide is found to act in a network forming role in the system, with 67Zn NMR and neutron diffraction analysis confirming zinc exhibits predominantly four-coordinate geometry. Solubility in deionised water and tris/HCl buffer solution is seen to decrease significantly with increasing x-value. This is discussed in terms of water ingress and the degree of structural openness, associated with increased cross-linking and a decrease in concentration of P–O–P linkages. pH measurements confirm invert phosphate compositions maintain physiological pH levels on immersion in water and buffer solutions for up to four weeks.

Structure and electrical conductivity in the oxide ion conducting compound Bi14YO22.5 have been investigated by powder X-ray and neutron diffraction, a.c. impedance spectroscopy, measurements of transference number and ab initio molecular dynamics (MD) simulations. Phase behaviour was studied using variable temperature X-ray diffraction and differential thermal analysis. The structure at room temperature is of the βIII tetragonal-type, details of which are discussed, including its relationship to both β and δ phases of Bi2O3. Bi14YO22.5 undergoes a reversible phase transition to a cubic δ-Bi2O3 type phase at high temperatures. This phase exhibits very high electrical conductivity, with transference numbers indicating that this conductivity is almost purely ionic in nature. MD simulations confirm 3-dimensional oxide ion transport in the βIII-phase.

Carbon-coated and titanium-substituted lithium vanadium phosphate composites have been successfully prepared through a sol-gel method followed by solid-state reaction under argon. Li3V1.9Ti0.1(PO4)3-C (LVT10PC) and Li3V1.85Ti0.15(PO4)3-C (LVT15PC) were investigated using X-ray powder diffraction, thermal analysis, transmission electron microscopy, cyclic voltammetry, and galvanostatic tests. Different models for the solid solution mechanism in this system are discussed. Electrochemical tests, at a charge-discharge rate of 0.2 C, in the range 2.8–4.4 V show that LVT10PC delivers the highest discharge capacity of 121 mA h g−1 and declines to 115.7 mA h g−1 up to the 60th cycle, corresponding to a 4.4 % loss. At low levels, titanium substitution is found to increase initial discharge capacity compared to the carbon-coated unsubstituted system (LVPC). Further substitution is found to have detrimental effects on initial discharge capacity and cycling behaviour.

Oxide ion distribution, vacancy ordering and electrical conductivity has been examined in the Nb/Yb double substituted bismuth oxide based system Bi3Nb1−xYbxO7−x, using X-ray and neutron powder diffraction, reverse Monte Carlo modelling of total neutron scattering data and a.c. impedance spectroscopy. Transference number measurements confirm the system to be predominantly ionically conducting above ca. 450 °C. Niobium rich compositions show incommensurate ordering of the fluorite subcell, while increasing ytterbium content results in a commensurate fluorite, with fully disordered cation and anion sublattices. Oxide ion distribution shows both compositional and thermal dependencies. The latter is discussed with respect to its effect on the thermal variation of cubic lattice parameter. Substitution of bismuth by niobium and ytterbium in the cation sublattice of bismuth oxide leads to the creation of Frenkel interstitial oxide ions, which increase the tetrahedral vacancy concentration. The high vacancy concentration is confirmed in both Rietveld and RMC analyses of neutron data. Examination of vacancy ordering, in the x = 0.6 composition, indicates a favouring of 〈100〉 vacancy pair alignment.

The thermal behavior of the oxide ion-conducting solid electrolyte Bi4YbO7.5 was investigated using a combination of variable temperature X-ray and neutron powder diffraction, thermal analysis (DTA and TGA), and ac impedance spectroscopy. The title compound shows a fluorite-type structure throughout the measured temperature range (20–850 °C), with a phase separation at ca. 600 °C into a cubic δ-type phase and an orthorhombic phase of assumed stoichiometry Bi17Yb7O36. This type of transition is a relatively common feature in bismuth oxide-based systems and can limit their practical application. Here, the transition was carefully studied using isothermal measurements, which showed that it is accompanied by changes in oxide-ion stoichiometry, as well as significant disorder in the oxide ion sublattice in the δ-type phase. These results correlate with the observed electrical behavior. Analysis of the total neutron scattering through reverse Monte Carlo (RMC) modeling reveals details of the coordination environments for both cations. The oxide-ion vacancy distribution seems to be consistent with a favoring of ⟨100⟩ vacancy pairs, although ⟨110⟩ vacancy pairs exhibit the highest frequency as they have the maximum likelihood. A vacancy ordering model based on three vacancies per cell is presented.Keywords: ac impedance spectroscopy; bismuth oxide; bismuth ytterbium oxide; defect structure; fluorite structure; neutron diffraction; total scattering; X-ray diffraction;

•Bi polarization for < 100 > and < 110 > vacancy pairs.•MD simulation shows < 110 > preferred vacancy configuration.•Vacancy concentration higher than δ-Bi2O3 through creation of Frenkel defects.Ab-initio molecular dynamics (MD) study of oxygen ion diffusion in the oxide ion conducting solid electrolyte δ-Bi3YO6 is presented. Vacancy ordering models were tested by structure relaxation and that for < 111 > vacancy pairs was found to have the highest total energy per atom, while those for < 110 >, < 100 > and random vacancy distributions were found to have similar energies. Evidence for polarization of bismuth charge density is seen for < 100 > and < 110 > vacancy pair configurations, consistent with stereochemical activity of the Bi 6s2 lone pair. MD studies of oxygen diffusion were carried out at three temperatures. Vacancy pair alignments during these simulations were predominantly in the < 110 > direction, consistent with previous neutron total scattering results.

A study of electrical and structural characteristics of compositions in the Bi3Ta1 − xNbxO7 system, using X-ray and neutron powder diffraction and AC impedance spectroscopy, is presented. The electrical conductivity increases with increasing niobium content. A full solid solution is observed which adopts an incommensurately ordered pseudo-cubic fluorite structure (type II). Analysis of the defect structure of the x = 0.50 composition shows chains of niobate/tantalate octahedra as a likely structural motif. A small degree of non-linearity in the thermal expansion of the cubic subcell lattice parameter is observed.Highlights► Bi3Ta1 − xNbxO7 system exhibits type II incommensurate structure. ► Defect structure is consistent with chains of tantalate/niobate octahedra. ► Lattice parameter thermal expansion correlated with oxide ion distribution changes.

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi14W1 − xLaxO24 − 3x/2 (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi2O3 related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi2O3 type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ800 = 1.34 S cm− 1), but also shows significant phase separation at lower temperatures.Highlights► Polymorphism and phase separation in Bi14W1 − xLaxO24 − 3x/2. ► Very high conductivity in δ-phase polymorph, especially for x = 0.75. ► Only x = 0.75 composition is cubic at RT, but shows a degree of phase separation. ► Conductivity behaviour is correlated with the observed phase behaviour.

Structure and electrical behaviour are reported for the system Bi1-xTaxO1.5 + x (0.167 ≤ x ≤ 0.250). In the compositional range 0.200 < x ≤ 0.250 an incommensurately modulated pseudo-cubic phase (type II) is observed, with the appearance of a larger pseudo-cubic phase in the region 0.167 ≤ x ≤ 0.200. Structural analysis of the type II phases by neutron diffraction reveals subtle changes in the oxide ion distribution with temperature, associated with changes in the incommensurate modulation parameter. Analysis of the defect structure of the type II phase reveals chains of tantalate octahedra as a likely structural motif. It is proposed that these chains facilitate an electronic contribution to total conductivity at low temperatures through electron hopping along the chains. Changes in oxide ion vacancy ordering may explain the observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plots of total conductivity.Highlights► Oxide ion conductors Bi1-xTaxO1.5 + x (0.167 ≤ x ≤ 0.250). ► Two pseudo-cubic phases observed. ► Chains of tantalate octahedra in type II incommensurate phase. ► Oxide ion distribution correlated to electrical behaviour and thermal expansion.

The defect structure of the title compound has been analyzed by reverse Monte Carlo (RMC) modeling of neutron total scattering data. The composition exhibits both diffuse scattering and weak superlattice ordering of the cubic fluorite subcell in neutron diffraction patterns. Combined Rietveld analysis of X-ray and neutron data at room temperature reveals oxide ion scattering on three crystallographic sites. Analysis of the RMC model reveals Bi coordination numbers consistent with stereochemical activity of the Bi 6 s2 lone pair electrons. Integration of the O–M–O angular distribution function gives an angular ratio consistent with predominantly < 110> vacancy ordering in this system.

Structure and electrical conductivity of Bi14WO24 as a function of temperature have been examined by X-ray and neutron powder diffraction, a.c. impedance spectroscopy and differential thermal analysis. The room temperature structure was successfully refined using a monoclinic subcell model in space group I2/m. However, additional reflections in the neutron data are consistent with a large supercell of dimensions a = 17.3780(1) Å, b = 17.3891(1) Å, c = 26.1785(2) Å and β = 90.270(1)°, as previously proposed. Transitions to tetragonal and cubic phases are observed at ca. 35 °C and 780 °C, respectively. The structure of the high temperature polymorph is confirmed as a fully disordered δ-Bi2O3 type phase. Analysis of the defect structure is consistent with a predominantly tetrahedral environment for tungsten, as seen at low temperatures. The conductivity behaviour is correlated with the appearance of the δ-phase at high temperatures and exhibits a value of 0.97 S cm− 1 at 800 °C.Highlights► High oxide ion conduction in Bi14WO24 at elevated temperatures. ► Analysis of the ambient temperature structure confirms a large monoclinic cell. ► The defect structure of HT phase consistent with predominantly tetrahedral W. ► Conductivity behaviour is correlated with the observed phase transitions.

The defect structure and electrical properties of the fast oxide ion-conducting solid electrolyte δ-Bi3YO6 have been studied using a combination of total neutron scattering analysis, energy minimization methods, and AC impedance spectroscopy. Conventional structural analysis using the Rietveld method reveals the oxide ions to be distributed over three crystallographic sites at room temperature, with a small change in this distribution at 800 °C. Analysis of short-range correlations using a total neutron scattering approach yields information on Bi and Y coordination environments. Careful analysis of the angular distribution functions derived from reverse Monte Carlo modeling of the total scattering data reveals physical evidence for a predominance of ⟨110⟩ vacancy ordering in this system. This ordering is confirmed as the lowest energy configuration in parallel energy minimization simulations.

An investigation of electrical conductivity and defect structure in the fluorite related system Bi3 + xNb0.8W0.2O7.1 + 3x/2 (0.0 ≤ x ≤ 2.0) using a.c. impedance spectroscopy and powder neutron diffraction is presented. All compositions exhibit incommensurate ordering of the cubic fluorite sublattice, with only the x = 2.0 composition showing evidence for an order–disorder transition above 650 °C. Non-linear behaviour in the thermal expansion of the cubic lattice parameter and the Arrhenius plot of total conductivity are discussed in terms of the defect structure.

Double substitution of bismuth in bismuth oxide is investigated in the system Bi3Nb1 − xErxO7 − x using X-ray and neutron diffraction and a.c. impedance spectroscopy. A full solid solution is observed with the basic fluorite structure maintained throughout. Long-range superlattice ordering observed at low x-value compositions is replaced by short-range ordering as the erbium content increases. Nb5+ and Er3+ dopant cations are believed to adopt a distorted octahedral coordination geometry, with evidence for clustering of these ions. Significant compositional changes are observed in the oxide ion distribution, but there is little significant thermal change in the average oxide ion distribution to account for observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plot of total conductivity. These phenomena may well be associated with more subtle changes in local ordering of vacancies.

Aluminoborosilicate glasses of general composition (0.14 − x/2.2)SiO2:(0.07 − x/4.2)B2O3:(0.09 − x/3.3)Na2O:0.40CaO:xAl2O3 (0.00 ≤ x ≤ 0.30) were synthesised using standard melt quenching methods and analysed by 11B, 23Na, 27Al and 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Calculations based on electroneutrality considerations were used to check the consistency of the NMR assignments in both glasses and cements. Glass–ionomer cement (GIC) formation was tested with all the prepared glass compositions, but only the samples with higher concentrations of Al2O3, x ≥ 0.20, produced sustainable cements. The compressive strengths (CS) of the cements varied from 66.7 to 74.2 MPa, although the differences were not statistically significant. The leaching of aluminium from the glass into the cement matrix varied between 6 and 12 mol%. It is proposed that aluminium plays a key role in glass–ionomer cements, not only through leaching of Al3+ ions from the glass into the cement matrix during formation, but also through reinforcement of the resulting cement by the residual glass particles which have high alumina content and confer greater mechanical strength.

A detailed investigation using X-ray and neutron powder diffraction as well as ac impedance spectroscopy in the substituted bismuth oxide system Bi7Nb2 − 2xY2xO15.5 − 2x is presented. Combined refinement of variable temperature X-ray and neutron powder diffraction data reveal both compositional and temperature dependencies of the oxide ion distribution in this system. Oxide ions are found to be distributed over three crystallographically distinct sites, with one site exclusively associated with the dopant cations Nb5+ and Y3+ leading to a distorted octahedral geometry for these ions. Changes in the oxide ion distribution are correlated with a non-linear thermal expansion as well as non-Arrhenius behaviour of total conductivity at high temperatures.

The structure of Bi3TaO7 has been examined by X-ray and neutron powder diffraction. The structure is that of an ordered defect fluorite and has been refined using a cubic δ-Bi2O3 type subcell model. A defect structure model is proposed with Ta5+ in predominantly a distorted octahedral geometry. These tantalate octahedra are thought to form corner sharing chains in a similar way to that proposed in the isostructural phase Bi3NbO7. The thermal variation of conductivity and lattice parameter appear to be correlated with subtle changes in the oxide ion distribution.

Tungsten doping into Bi7Nb2O15.5 was investigated using a combination of X-ray and neutron diffraction and ac impedance spectroscopy. Only limited solid solution formation is observed in the system Bi7Nb2 − 2xW2xO15.5 + x, with a defect fluorite type phase observed up to around x = 0.2. The defect structure of this phase shows, that Nb5+ and W6+ ions are likely to adopt a distorted octahedral coordination geometry, with Bi3+ ions in predominantly four-pyramidal geometry. Non-linearity in the Arrhenius plot of conductivity as well as the thermal variation of the cubic lattice parameter are interpreted in terms of a subtle redistribution of oxide ions in the structure at elevated temperatures.

A detailed structural study of selected compositions in the Bi3Nb1 − xZrxO7 − x/2 system using combined X-ray and neutron powder diffraction techniques is presented. Two distinct composition regions are evident. A limited pseudo-cubic solid solution range is observed up to ca. x = 0.4 and details of the defect structure are presented and compared with conductivity data. At higher Zr content, a polyphasic system is observed, with diffraction patterns dominated by a cubic δ-type phase. Evidence suggests that Zr is excluded from this phase and that the phase separation results in a bismuth rich δ-type phase characterised by a significantly larger unit cell parameter as well as high conductivity.

Ageing effects in the defect fluorite structure of Bi3Nb1−xYxO7−x have been studied using powder neutron diffraction. Two compositions at x = 0.4 and x = 0.6 were studied after annealing for prolonged times (400–500 h) at 410 and 550 °C. The results show that significant changes in oxide ion distribution occur at 550 °C, but that at 410 °C only the lower x-value composition exhibited such a change. A comparison of lattice parameter variation with temperature for annealed and unannealed samples suggests that at higher temperatures the oxide ion distribution is independent of thermal history.

Calcium and aluminium incorporation into a sodium borosilicate glass with base composition 0.30Na2O ∶ 0.46SiO2 ∶ 0.24B2O3 was investigated. Glasses of general compositions (0.30 − x/3.3)Na2O ∶ xCaO ∶ (0.46 − x/2.2)SiO2 ∶ (0.24 − x/4.2)B2O3 (Cx) and (0.3 − x/3.3)Na2O ∶ xAl2O3 ∶ (0.46 − x/2.2)SiO2 ∶ (0.24 − x/4.2)B2O3 (Ax) were prepared by melt quenching. All the samples were tested for glass-ionomer cement formation. Glasses in the Cx system did not yield sustainable cements on mixing at glass to liquid ratios of 2.4 ∶ 1. The glasses of system Ax were unmixable at glass to liquid ratios between 2.4 ∶ 1 and 1.5 ∶ 1, with the exception of the sample with composition x = 0.3, which was mixable at a glass to liquid ratio of 2.4 ∶ 1, producing a cement with an average compressive strength of 69.3 MPa (σ = 6.1 MPa). 11B, 23Na, 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy was used to characterize the structure of the glass and cement samples. The results showed that the calcium boroaluminosilicate glass compositions have the potential to be used as solid components in the preparation of glass-ionomer cement formulations.

Defect structure and conductivity behaviour are discussed in the solid solution Bi3Nb1−xYxO7−x(0.0 ≤ x ≤ 1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800 °C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions.

The use of Arrhenius plots of total conductivity derived from ac impedance spectroscopy measurements in characterisation of phase transitions is discussed with reference to Bi4V2O11−δ and the BIMEVOX family of oxide ion-conducting solid electrolytes. The technique is shown to be superior in some cases to standard laboratory methods such as thermal analysis and variable temperature X-ray diffraction. Examples include the identification of the metastable intermediate ε-Bi4V2O11−δ, β-phase stability in BIMGVOX and a kinetic study of the γ → γ’ transition in BIZNVOX.

A combination of variable temperature X-ray powder diffraction, ac impedance spectroscopy and differential thermal analysis (DTA) has been used to characterise phase transitions in the Bi2ZrxV1−xO5.5−(x/2)−δ, BIMEVOX system. At low substitution levels α-, β-, and γ-phase BIMEVOX structures are observed as a function of temperature with some evidence for an intermediate phase, ɛ, seen between β and γ phases on heating. At x = 0.10 the β-phase is observed at room temperature with a transition to the γ-phase at around 500 °C. There is evidence for a subtle phase transition at around 200 to 250 °C in the variation of refined cell parameters and the Arrhenius plot of conductivity for this composition that may be associated with a vacancy ordering phenomenon. For the x = 0.19 composition, despite near equivalence of a and b unit cell parameters, variation in the unit cell c-parameter, DTA and Arrhenius plots of conductivity show evidence of a β↔γ transition between 450 and 500 °C and that the true symmetry of the low temperature phase is likely to be orthorhombic.

The crystal structure of CaNa4(P3O9)2 has been determined using single crystal X-ray diffraction. The 23Na and 31P magic angle spinning solid-state NMR data are consistent with the centrosymmetric space group assignment of C2/c and the cation distribution of two distinct sodium sites. The structure determination confirms the presence of the cyclic triphosphate unit as opposed to a cyclic hexaphosphate or metaphosphate chains. Phosphorus and sodium edge EXAFS data are discussed with respect to the structure.

The structures of 1,2,3-indantrionemonooxime have been investigated in gas, solution and solid states using a combination of quantum-chemical calculations, NMR spectroscopy and X-ray crystallography and has been shown to exist exclusively as the oxime tautomer. Two basic rotameric forms are proposed, the ‘closed’ form which shows intramolecular hydrogen bonding between the oxime group and one of the carbonyl oxygens and the ‘open’ form which shows no intramolecular interaction. Quantum-chemical calculations indicate that the ‘closed’ form is favoured in the gas phase and in non-polar solvents, while in polar solvents the ‘open’ form is stabilised through intermolecular interactions. These results are confirmed by 13C solution state NMR spectroscopy. In the solid state only the ‘open’ rotamer is observed with the crystal structure stabilised by intermolecular hydrogen bonding. Two crystallographically non-equivalent forms of this rotamer are observed in the crystal structure and also confirmed in the 13C solid state NMR spectrum.

The structure of phosphate glasses of general composition 10Na2O:(20 + x/2)ZnO:(20 + x/2)CaO:(50 − x)P2O5 (0 ≤ x ≤ 20) has been investigated using IR spectroscopy, 1D 31P and 43Ca MAS Bloch decay, 31P–31P double quantum MAS-NMR and 43Ca and 67Zn static NMR techniques, as well as neutron diffraction analysis. Zinc is shown to aid glass formation in this system. Glass transition temperature and density increase with increasing cation:phosphate ratio. However, free volume calculations show structures becoming significantly more compact from x = 5 to x = 10. The structural data confirm depolymerisation of the glasses with increasing cation:phosphate ratio. Zinc oxide is found to act in a network forming role in the system, with 67Zn NMR and neutron diffraction analysis confirming zinc exhibits predominantly four-coordinate geometry. Solubility in deionised water and tris/HCl buffer solution is seen to decrease significantly with increasing x-value. This is discussed in terms of water ingress and the degree of structural openness, associated with increased cross-linking and a decrease in concentration of P–O–P linkages. pH measurements confirm invert phosphate compositions maintain physiological pH levels on immersion in water and buffer solutions for up to four weeks.

Oxide ion distribution, vacancy ordering and electrical conductivity has been examined in the Nb/Yb double substituted bismuth oxide based system Bi3Nb1−xYbxO7−x, using X-ray and neutron powder diffraction, reverse Monte Carlo modelling of total neutron scattering data and a.c. impedance spectroscopy. Transference number measurements confirm the system to be predominantly ionically conducting above ca. 450 °C. Niobium rich compositions show incommensurate ordering of the fluorite subcell, while increasing ytterbium content results in a commensurate fluorite, with fully disordered cation and anion sublattices. Oxide ion distribution shows both compositional and thermal dependencies. The latter is discussed with respect to its effect on the thermal variation of cubic lattice parameter. Substitution of bismuth by niobium and ytterbium in the cation sublattice of bismuth oxide leads to the creation of Frenkel interstitial oxide ions, which increase the tetrahedral vacancy concentration. The high vacancy concentration is confirmed in both Rietveld and RMC analyses of neutron data. Examination of vacancy ordering, in the x = 0.6 composition, indicates a favouring of 〈100〉 vacancy pair alignment.

Treatment of [Al4(μ4-O)(OBui)10(BuiOH)] 1 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in boiling toluene produced [Al5(μ5-O)(μ-OBui)8(OBui)5] 2, whereas removal of BuiOH from 1 by azeotropic fractional distillation with toluene gave [Al8(μ4-O)2(μ-OH)2(μ-OBui)10(OBui)8] 3. X-Ray crystallography revealed that in 2 the μ5-oxo atom is bonded to one apical six-coordinated Al and four basal five-coordinated Al atoms, whilst in 3 the centrosymmetrical molecule comprises two tetranuclear fragments each containing a μ4-oxo atom bonded to five-coordinated Al atoms and bridged by two μ-hydroxo ligands. Alcohol exchange of 2 with BunOH gave [Al5(μ5-O)(μ-OBui)8(OBui)4(OBun)] 4 and of 1 with CF3CH2OH gave [Al4(μ4-O)(OBui)6(OCH2CF3)4(CF3CH2OH)] 6. Other compounds synthesised were [Al4(OCH2CF3)12] 7, [Al4(μ4-O)(OCH2CF3)10(CF3CH2OH)] 8, [Al{OCH(CF3)2}3(THF)] 9, and [Al{OCH(CF3)2}3(Et2O)] 10. Using 17O enriched (10%) water, samples of 1–4, 6 and 8 labelled at the oxo atom were prepared and their 17O chemical shifts measured in addition to 1H and 27Al NMR chemical shifts. Possible reaction pathways for the formation of 2 and 3 from 1 are discussed. Structural aspects of aluminium oxoalkoxides are discussed.

Aluminoborosilicate glasses of general composition (0.14 − x/2.2)SiO2:(0.07 − x/4.2)B2O3:(0.09 − x/3.3)Na2O:0.40CaO:xAl2O3 (0.00 ≤ x ≤ 0.30) were synthesised using standard melt quenching methods and analysed by 11B, 23Na, 27Al and 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Calculations based on electroneutrality considerations were used to check the consistency of the NMR assignments in both glasses and cements. Glass–ionomer cement (GIC) formation was tested with all the prepared glass compositions, but only the samples with higher concentrations of Al2O3, x ≥ 0.20, produced sustainable cements. The compressive strengths (CS) of the cements varied from 66.7 to 74.2 MPa, although the differences were not statistically significant. The leaching of aluminium from the glass into the cement matrix varied between 6 and 12 mol%. It is proposed that aluminium plays a key role in glass–ionomer cements, not only through leaching of Al3+ ions from the glass into the cement matrix during formation, but also through reinforcement of the resulting cement by the residual glass particles which have high alumina content and confer greater mechanical strength.

Two-dimensional (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) platelets with a size of up to ca. 5 μm and thickness of 0.2–0.5 μm were introduced as fillers into a polymer matrix to prepare energy storage composites for the first time. The NBBT platelets were treated with an aqueous solution of H2O2 and coated with polyvinylpyrrolidone (PVP) before mixing with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)). The final composite was denoted as NBBT@PVP/P(VDF–HFP). Composites were prepared with NBBT@PVP loadings from 1 to 30 vol%. The relative permittivity of the composites increased significantly with increasing NBBT@PVP loading, while the breakdown strength decreased. To improve the breakdown strength of the composites, a sandwich-structure of multilayer films was developed, which used NBBT@PVP/P(VDF–HFP) composites with 1 vol% NBBT loadings as central hard layers and the composites with 30 vol% NBBT loadings as neighboring soft layers. The five-layered film, which contained three central hard layers and neighboring soft layers, showed excellent energy storage properties. The breakdown strength and the maximum energy storage density of the film reached 258 kV mm−1 and 14.95 J cm−3, respectively. The energy efficiency remained 0.9 at an electric field of 200 kV mm−1. The findings provide a new approach to produce energy storage materials with high performance.

Two-dimensional monocrystalline platelets of the perovskite structured lead–free system (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) were synthesized from platelets of layer-structured Na0.5Bi4.5Ti4O15 (NBIT), using a topochemical route. Both NBIT and NBBT platelets showed high aspect ratios with an average size of 10 μm and thickness of 0.6 μm, for the latter. A structural transformation from layer-structured NBIT to perovskite NBBT was identified in a transitional platelet, which contained phases of both NBIT and NBBT. The composition of NBBT lies at a morphotropic phase boundary (MPB), with the coexistence of monoclinic and tetragonal polymorphs confirmed by X-ray powder diffraction analysis. Transmission electron microscopy and piezoresponse force microscopy were used to investigate the domain structure, with stripe-like domains in the unpoled system transformed into lamellar domains after poling. The piezoresponse amplitude for a NBBT platelet indicated a large piezoresponse strain, corresponding to a field-induced strain Smax/Emax of around 800 pm V−1 at 5 kV mm−1, which is significantly higher than that in ceramic samples of NBBT at the same applied potential.