Phase formation, crystal chemistry, and properties in the system Bi2O3-Fe2O3-Nb2O5

Subsolidus phase relations have been determined for the Bi₂O₃-Fe₂O₃-Nb₂O₅ system in air (900-1075 °C). Three new ternary phases were observed—Bi₃Fe_0.5Nb_1.5O₉ with an Aurivillius-type structure, and two phases with approximate stoichiometries Bi₁₇Fe₂Nb₃₁O₁₀₆ and Bi₁₇Fe₃Nb₃₀O₁₀₅ that appear to be structurally related to Bi₈Nb₁₈O₅₇. The fourth ternary phase found in this system is pyrochlore (A₂B₂O₆O′), which forms an extensive solid solution region at Bi-deficient stoichiometries (relative to Bi₂FeNbO₇) suggesting that ≈4-15% of the A-sites are occupied by Fe³⁺. X-ray powder diffraction data confirmed that all Bi-Fe-Nb-O pyrochlores form with positional displacements, as found for analogous pyrochlores with Zn, Mn, or Co instead of Fe. A structural refinement of the pyrochlore 0.4400:0.2700:0.2900 Bi₂O₃:Fe₂O₃:Nb₂O₅ using neutron powder diffraction data is reported with the A cations displaced (0.43 Å) to 96g sites and O′ displaced (0.29 Å) to 32e sites (Bi_1.721Fe_0.190(Fe_0.866Nb_1.134)O₇, Fd3¯m (#227), ). This displacive model is somewhat different from that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which exhibits twice the concentration of small B-type cations on the A-sites as the Fe system. Bi-Fe-Nb-O pyrochlores exhibited overall paramagnetic behavior with large negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. The single-phase pyrochlore with composition Bi_1.657Fe_1.092Nb_1.150O₇ exhibited low-temperature dielectric relaxation similar to that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92; at 1 MHz and 200 K the relative permittivity was 125, and above 350 K conductive effects were observed.     

Disordered pyrochlore CsMgInF6 at high pressures

High pressure behaviour of disordered pyrochlore CsMgInF₆ (Pnma, Z=4) has been studied with powder and single-crystal X-ray diffraction to 8.0 and 6.94 GPa, respectively, in diamond anvil cells at room temperature. The material is structurally stable to at least 8.0 GPa with no ordering of the In³⁺ and Mg²⁺ cations. The P-V data are fitted by a Birch-Murnaghan equation of state with the zero-pressure bulk modulus B₀=33.4(3) GPa and the unit-cell volume at ambient pressure V₀=603.2(4) Å³ for the first pressure derivative of the bulk modulus B′=4.00. The major contribution to the bulk compressibility arises from the changes in the coordination sphere around the Cs atoms. The effect of hydrostatic pressure on the crystal structure of CsMgInF₆ is comparable to the effect of chemical pressure induced by the incorporation of ions of different sizes into the A and B sites in defect AB²⁺B′³⁺F₆ pyrochlores.     

Sr18Ru1.9Bi4.1O33: crystallization of a Ru(V)/Bi(V) oxide from molten hydroxide

Single crystals of a new complex oxide, Sr₁₈Ru_1.9Bi_4.1O₃₃, were precipitated from a mixture of molten alkali and alkaline earth metal hydroxides at 750°C. The structure was determined from a ruby-red crystal using single-crystal X-ray diffraction. Sr₁₈Ru_1.88Bi_4.12O₃₃ crystallizes in the space group C2/c (monoclinic) and has unit-cell dimensions: a=10.2102(11) Å, b=17.882(2) Å, c=19.579(2) Å, and β=102.043(2)°. The structure (refined to R₁=4.5%, wR₂=9.2%) is an unusual ABO₃ defect perovskite, with th of the oxygen positions vacant. All the A sites and half of the B sites are occupied by Sr²⁺, while the remaining B sites are occupied by Bi⁵⁺ or Ru⁵⁺. The oxygen atom vacancies are located within the Bi coordination sphere exclusively. The bonding in the BO₃ sublattice is less covalent than that in the perovskite archetype from which it is derived due to the presence of 50% Sr²⁺ on the B sites. Thus, the structure of Sr₁₈Ru_1.9Bi_4.1O₃₃ can also be viewed as being made up of isolated bismuthate anions (BiO₅⁵⁻, BiO_5.5⁶⁻ and BiO₆⁷⁻) and ruthenate anions (RuO₆⁷⁻) separated by strontium cations.     

Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi1−xYx)2(MNb)O7 (M=Fe, In) Bi-pyrochlores

Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi₂(M^IIINb^V)O₇ Bi-pyrochlores, Bi₂InNbO₇ and Bi₂FeNbO₇, as well as of their A site substituted Bi_1.5Y_0.5InNbO₇ and Bi_1.5Y_0.5FeNbO₇ variants. Highly structured diffuse distributions in the form of {110}* sheets of diffuse intensity perpendicular to the six 〈110〉 directions of real space along with 〈111〉* rods of diffuse intensity perpendicular to the four {111} real space planes are observed. The existence of this structured diffuse scattering is interpreted in terms of large amplitude, β-cristobalite-type tetrahedral rotations of the O′A₂ tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to understand the local crystal chemistry responsible for such displacive disorder. The frequency-dependent dielectric properties of Bi₂InNbO₇ and Bi₂FeNbO₇ are also investigated along with the effect upon them of A site doping with Y.     

Synthesis and structural analysis of Bi2−ySryIr2O7, a new pyrochlore solid solution

This paper presents a study of the synthesis and structural properties of the new pyrochlore-type Bi_2−ySr_yIr₂O₇ series. Ten compositions with 0.0≤y≤0.9 were prepared by solid-state reaction with thermal treatments at 873, 1073 and 1323 K under atmospheric pressure conditions. Structural refinements from X-ray powder diffraction data by the Rietveld method show that all compounds of the Bi_2−ySr_yIr₂O₇ solid solution crystallize in a α-pyrochlore structure. The main structural difference when bismuth is substituted by strontium concerns the x position of the O1 (x, ?, ?). This substitution significantly increases the Bi/Sr-O1 distance and diminishes the Ir-O1 distance; this implies that the Ir-O1-Ir bond angle increases. With the Sr substitution, the IrO₆ local configuration goes from a flattened trigonal antiprism, y<0.5, to an elongated one, y>0.5, passing through an octahedral array, y∼0.5. The electrical consequences of these structural changes observed in this system are qualitatively explained with electronic structure calculations, this behavior agrees very well with those observed in other pyrochlore systems A₂M₂O₇ (A=rare earth cations or Tl⁺, Pb²⁺, or Bi³⁺, and M=Ru or Ir).     

Crystal chemistry and crystallography of the Aurivillius phase Bi5AgNb4O18

Bi₅AgNb₄O₁₈ is a new phase, which was discovered during the phase equilibrium study of the Bi₂O₃-Ag₂O-Nb₂O₅ system. Bi₅AgNb₄O₁₈ was prepared at 750°C and is stable in air up to its melting temperature of 1160.1±5.0°C (standard error of estimate). Results of a Rietveld refinement using neutron powder diffraction confirmed that Bi₅AgNb₄O₁₈ is isostructural with Bi₃TiNbO₉, Bi₅NaNb₄O₁₈, and Bi₅KNb₄O₁₈. The structure was refined in the orthorhombic space group A2₁am, Z=2, and the lattice parameters are a=5.4915(2) Å, b=5.4752(2) Å, c=24.9282(8) Å, and V=749.52(4) Å³. The structure can be described as the m=2 member of the Aurivillius family, (Bi₂O₂)²⁺ (A_m−1B_mO_3m+1)²⁻ (where A=Bi and B=Ag, Nb), which is characterized by perovskite-like (A_m−1B_mO_3m+1)²⁻ slabs regularly interleaved with (Bi₂O₂)²⁺ layers. The octahedral [NbO₆] units are distorted with Nb-O distances ranging from 1.856(4) to 2.161(2) Å and the O-Nb-O angles ranging from 82.6(3)° to 98.5(3)°. These octahedra are tilted about the a- and c-axis by about 10.3° and 12.4°, respectively. Ag was found to substitute exclusively into the Bi-site that is located in the layer between the two distorted [NbO₆] units. Although the Ag substitutes into the Bi-site with the Bi:Ag ratio of 1:1, the existence of a superlattice was not detected using electron diffraction. A comparison of (Bi₂O₂)²⁺(A_m−1Nb_mO_3m+1)²⁻ structures (where A=Ag, Na, and K) revealed a relation between the pervoskite tolerance factor, t, and structural distortion. The reference pattern for Bi₅AgNb₄O₁₈ has been submitted to the International Centre for Diffraction Data (ICDD) for inclusion in the Powder Diffraction File.     

Displacive disorder and dielectric relaxation in the stoichiometric bismuth-containing pyrochlores, Bi2MNbO7 (M=In and Sc)

The structural disorder and temperature-dependent dielectric properties of two Bi-based niobate pyrochlore systems which have both previously been reported to occur at the ideal Bi₂(M^IIINb^V)O₇ stoichiometry without any compositional disorder on the pyrochlore A site, namely the Bi₂InNbO₇ (BIN) and Bi₂ScNbO₇ (BSN) pyrochlore systems, have been carefully re-investigated. It is established that A site stoichiometric, Bi-based niobate pyrochlores can indeed exist. Electron diffraction is used to investigate the nature of the displacive disorder therein both at room temperature as well as at close to liquid nitrogen temperature. The characteristic structured diffuse scattering observed arises from β-cristobalite-like, 1-d correlated rotations and associated translations of chains of corner-connected O′Bi₄ tetrahedra. The temperature-dependent dielectric properties including the low temperature dielectric relaxation properties of these A site stoichiometric, Bi-based niobate pyrochlores are also reported as are the micro-Raman spectra thereof. The experimental results suggest that the dipoles as well as the glassy relaxation behaviour in these Bi-based pyrochlores are directly related to these β-cristobalite-like, correlated rotations of 〈110〉 chains of corner-connected O′Bi₄ tetrahedra.     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.     

Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 °C). Phases containing Mn²⁺, Mn³⁺, and Mn⁴⁺ were all observed. Ternary compound formation was limited to pyrochlore (A₂B₂O₆O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi₂(Mn,Nb)₂O₇) suggesting that ≈14-30% of the A-sites are occupied by Mn (likely Mn²⁺). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi₂O₃:Mn₂O_3±x:Nb₂O₅ using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn²⁺ on A-sites and Mn³⁺ on B-sites (Bi_1.6Mn²⁺_0.4(Mn³⁺_0.8Nb_1.2)O₇, (?227), a=10.478(1) Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi_1.600Mn_1.200Nb_1.200O₇ was ≈128 with tan δ=0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.     

Structural Study of an Unusual Cubic Pyrochlore Bi1.5Zn0.92Nb1.5O6.92

Single-phase pyrochlore-type specimens of Bi_1.5Zn_0.92Nb_1.5O_6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A₂B₂O₆O′ (Fd&3macr;m, a=10.5616(1) Å) with both Bi and Zn mixed on the A-sites. However, refinements revealed significant local deviations from the ideal pyrochlore arrangement which were caused by apparent displacive disorder on both the A and O′ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ∼0.39 Å from the ideal eight-fold coordinated positions. The displacements occur along the six 〈112〉 directions perpendicular to the O′-A-O′ links. In addition, the O′ ions were randomly displaced by ∼0.46 Å along all 12 〈110〉 directions. Crystal-chemical considerations suggest the existence of short-range correlations between the O′ displacements and both the occupancy of the A-sites (i.e., Bi or Zn) and the directions of the A-cation displacements. The combined A-cation and O′ displacements change the coordination sphere of the A-cations from 8 to (5+3); the resulting coordination environment of the A-cations bears similarities to that of the (5+1)coordinated Zn in zirconolite-like Bi₂Zn_2/3Nb_4/3O₇. The observed displacive disorder in the A₂O′ network of the Bi_1.5Zn_0.92Nb_1.5O_6.92 structure involves atoms associated with the lowest-frequency vibrational bending mode, and is likely responsible for both the high dielectric constant and the dielectric relaxation reported for this compound.     

Synthesis and structural studies of cation-substituted Aurivillius phases ASrBi2Nb2TiO12

We report here a novel method to prepare high-quality samples of the three layered oxides ASrBi₂Nb₂TiO₁₂ACa, Sr or Ba using the pre-formed intermediates ABi₂Nb₂O₉ and SrTiO₃. The room temperature structures were refined using synchrotron X-ray and Neutron powder diffraction data in the orthorhombic space group B2cb. This symmetry arises as a consequence of cooperative tilting of the BO₆ octahedra in the [ASrNb₂TiO₁₀]²⁻ perovskite-like slabs and a polar displacement of the cations. The structure is characterized by extensive cation disorder but lacks appreciable oxygen vacancies.     

Cu22Bi12S21Cl16—A mixed conductor with fast one-dimensional copper(I) ion transport

Melting reactions of Cu, CuCl, S, and Bi₂S₃ yield black, shiny needles of Cu₂₂₍₁₎Bi₁₂S₂₁₍₁₎Cl₁₆₍₁₎. The compound decomposes peritectically at 649(5) K. Oxidation state +I of the copper atoms is supported by Cu-K-XANES. The compound crystallizes in the hexagonal space group P6/m with a=2116.7(7) pm and c=395.17(5) pm. Seven anions coordinate each of the two independent bismuth cations in the shape of mono-capped trigonal prisms. These polyhedra share edges and faces to form trigonal and hexagonal tubes running along [0 0 1]. The hexagonal tubes are centered by chloride ions, which are surrounded by disordered copper cations. The majority of copper cations are distributed over numerous sites between the tubes. The Joint Probability Density Function (JPDF) reveals a continuous pathway along [0 0 1]. The high mobility of the copper cations along [0 0 1] was demonstrated by impedance spectroscopy and DC polarization measurements on single crystals. The ionic conductivity at 450 K is about σ_ion=0.06 S cm⁻¹, and the activation energy for Cu⁺ ion conduction is E_a=0.44 eV. The chemical diffusion coefficient of copper is in the order of D_cu^δ=10¹⁹ cm⁻³ at 420 K. The electronic band gap (p-type conductor) was determined as E_g=0.06 eV. At room temperature the thermal conductivity of a pressed pellet is about κ=0.3 W K⁻¹ m⁻¹ and the Seebeck coefficient is S=43 μV K⁻¹.     

Preparation, crystal structure and thermal expansion of a new bismuth barium borate, BaBi2B4O10

Single crystals of a new compound, BaBi₂B₄O₁₀ were grown by cooling a melt with the stoichiometric composition. The crystal structure of the compound has been solved by direct methods and refined to R₁=0.049 (wR=0.113) on the basis of 1813 unique observed reflections (|F_o|>4σ|F_o|). It is monoclinic, space group P2₁/c, a=10.150(2), b=6. 362(1), c=12.485(2) Å, β=102.87(1)^o, V=786.0(2) Å³, Z=4. The structure is based upon anionic thick layers that are parallel to (001). The layers can be described as built from alternating novel borate [B₄O₁₀]⁸⁻_∞ chains and bismuthate [Bi₂O₅]⁴⁻_∞ chains extended along b-axis. The borate chains are composed of [B₃O₈]⁷⁻ triborate groups of three tetrahedra and single triangles with a [BO₂]⁻ radical. The borate chains are interleaved along the c-axis with rows of the Ba²⁺ cations so that the Ba atoms are located within the layers. The layers are connected by two nonequivalent Ba-O bonds as well as by two equivalent Bi-O bonds with bond valences in the range of 0.2-0.3 v.u.Thermal expansion of BaBi₂B₄O₁₀ studied by high-temperature X-ray powder diffraction in the temperature range of 20-700 °C (temperature step 30-35 °C) is highly anisotropic. While the b and c unit-cell parameters increase almost linearly on heating, temperature dependencies of a parameter and β monoclinic angle show nonlinear behavior. As a result, on heating orientation of thermal expansion tensor changes, and bulk thermal expansion increases from 20×10⁻⁶ °C⁻¹ at the first heating stage up to 57×10⁻⁶ °C⁻¹ at 700 °C that can be attributed to the increase of thermal mobility of heavy Bi³⁺ and Ba²⁺ cations.     

New ferromagnets of Sr8ARe3Cu4O24 (A=Sr, Ca) with an ordered perovskite structure

New phases Sr₈ARe₃Cu₄O₂₄ (A=Sr,Ca) were discovered under high-pressure/high-temperature condition. X-ray powder diffraction and electron diffraction studies for these phases indicated that they have an ordered perovskite-type structure with cubic lattices of ∼8 Å. They showed ferromagnetism at room temperature when they were synthesized under high-oxygen-pressure condition. The Ca-containing phase has a very high T_c of 440 K with a spontaneous magnetization of ∼1 μ_B/f.u.     

The study of Bi3B5O12: synthesis, crystal structure and thermal expansion of oxoborate Bi3B5O12

The paper presents a new data on the crystal structure, thermal expansion and IR spectra of Bi₃B₅O₁₂. The Bi₃B₅O₁₂ single crystals were grown from the melt of the same stoichiometry by Czochralski technique. The crystal structure of Bi₃B₅O₁₂ was refined in anisotropic approximation using single-crystal X-ray diffraction data. It is orthorhombic, Pnma, a=6.530(4), b=7.726(5), c=18.578(5) Å, V=937.2(5) Å³, Z=4, R=3.45%. Bi³⁺ atoms have irregular coordination polyhedra, Bi(1)O₆ (d(B-O)=2.09-2.75 Å) and Bi(2)O₇ (d(B-O)=2.108-2.804 Å). Taking into account the shortest bonds only, these polyhedra are considered here as trigonal Bi(1)O₃ (2.09-2.20 Å) and tetragonal Bi(2)O₄ (2.108-2.331 Å) irregular pyramids with Bi atoms in the tops of both pyramids. The BiO₄ polyhedra form zigzag chains along b-axis. These chains alternate with isolated anions [B₂^IVB₃^IIIO₁₁]⁷⁻ through the common oxygen atoms to form thick layers extended in ab plane. A perfect cleavage of the compound corresponds to these layers and an imperfect one is parallel to the Bi-O chains. The Bi₃B₅O₁₂ thermal expansion is sharply anisotropic (α₁₁≈α₂₂=12, α₃₃=3×10⁻⁶ °C⁻¹) likely due to a straightening of the flexible zigzag chains along b-axis and decreasing of their zigzag along c-axis. Thus the properties like cleavage and thermal expansion correlate to these chains.     

The non-centrosymmetric borate oxides, MBi2B2O7 (M=Ca, Sr)

Two novel noncentrosymmetric borates oxides, MBi₂B₂O₇ or MBi₂O(BO₃)₂ (MCa, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600-700 °C range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi₂B₂O₇ crystallizes in the orthorhombic Pna2₁ space group with a=8.9371(5) Å, b=5.4771(3) Å, c=12.5912(7) Å, Z=4, R_wp=0.118, χ²=2.30. SrBi₂B₂O₇ crystallizes in the hexagonal P6₃ space group with a=9.1404(4) Å, c=13.0808(6) Å, Z=6, R_wp=0.115, χ²=4.15. Large displacement parameters suggest the presence of disorder in SrBi₂B₂O₇ as also revealed by diffuse 2×a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO₃ triangles and MO₆ trigonal prisms forming six-membered rings in which Bi₂O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi-O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi₂B₂O₇ (d_eff=2d_eff(KDP)).     

High-pressure single-crystal X-ray diffraction of Tl2SeO4

The effect of pressure on the crystal structure of thallium selenate (Tl₂SeO₄) (Pmcn, Z=4), containing the Tl⁺ cations with electron lone pairs, has been studied with single-crystal X-ray diffraction in a diamond anvil cell up to 3.64 GPa at room temperature. No phase transition has been observed. The compressibility data are fitted by a Murnaghan equation of state with the zero-pressure bulk modulus B₀=29(1) GPa and the unit-cell volume at ambient pressure V₀=529.6(8) Å³ (B′=4.00). Tl₂SeO₄ is the least compressible in the c direction, while the pressure-induced changes of the a and b lattice parameters are quite similar. These observations can be explained by different pressure effects on the nine- and 11-fold coordination polyhedra around the two non-equivalent Tl atoms. The SeO₄²⁻ tetrahedra are not rigid units and become more distorted. Their contribution to the compressibility is small. The effect of pressure on the isotypical oxide materials A₂TO₄ with the β-K₂SO₄ structure is discussed. It appears that the presence of electron lone pairs on the Tl⁺ cation does not seem to influence the compressibility of Tl₂SeO₄.     

Preparation of a new pyrochlore-type compound Na0.32Bi1.68Ti2O6.46(OH)0.44 by hydrothermal reaction

A new pyrochlore-type Na_0.32Bi_1.68Ti₂O_6.46(OH)_0.44 with the cubic cell of a=10.339(5) Å was prepared by hydrothermal reaction using TiO₂ (anatase) and Bi₂O₃ in NaOH solution. This compound was obtained when the molar ratio of NaOH/TiO₂ was above 2 and the reaction temperature was above 240 °C. The TG-curve of as-prepared sample showed a mass loss of 0.8 mass% which was caused by release of OH group. This compound decomposed to a pyrochlore-type compound and a layered-type Na_0.5Bi_4.5Ti₄O₁₅ above 800 °C. The optical band gap of Na_0.32Bi_1.68Ti₂O_6.46(OH)_0.44 was estimated to be 2.5 eV.     

Order-disorder at Fe sites in SrFe2/3B″1/3O3 (B″=Mo, W, Te, U) tetragonal double perovskites

We have prepared SrFe_2/3B″_1/3O₃ (B″=Mo, U, Te, and W) double perovskites in polycrystalline form by ceramic methods. Phases with B″=U, Te and W have been studied by X-ray powder diffraction and the results have been compared with neutron diffraction data available for B″=Mo. At room temperature, the stoichiometric samples crystallize in the tetragonal crystal system (space group I4/m, Z=4). Cell parameters when B″=U, Te and W are a=5.6936(1) Å, c=8.0637(1)Å; a=5.5776(1) Å, c=7.9144(3) Å and a=5.5707(3) Å, c=7.9081(5) Å, respectively.The Mössbauer spectra at room temperature for all compounds show hyperfine parameters belonging to two Fe³⁺ sites located at lattice positions with different degrees of distortion. This is in agreement with diffraction data that indicate that the series of compounds display different degrees of Fe-site disorder, which increases in the following sequence: Mo<U<Te<W.     

Synthesis, structure and characterisation of the n=4 Aurivillius phase Bi5Ti3CrO15

The n=4 Aurivillius phase, Bi₅Ti₃CrO₁₅, was synthesised by solid state reaction. Rietveld analysis of high resolution neutron diffraction data demonstrated this material to adopt the polar space group A2₁am at room temperature, transforming to the aristotype I4/mmm structure above 650 °C. This phase transition is coincident with an anomaly in DSC signal and relative permittivity, which are characteristic of a ferroelectric-paraelectric phase transition. Bi₅Ti₃CrO₁₅ exhibits paramagnetic behaviour at low temperature, with short range antiferromagnetic interactions, but no evidence for long range magnetic ordering. This is considered a consequence of significant disorder of Ti and Cr over the available octahedral sites, as demonstrated by analysis of neutron diffraction data.