Preparation and structure of NaSr0.5Al2B2O7 and NaCa0.5Al2B2O7

Two compounds NaSr_0.5Al₂B₂O₇ and NaCa_0.5Al₂B₂O₇, have been found to crystallize into a new structure type by Rietveld refinement from X-ray powder diffraction data. Their structure belongs to hexagonal space group P6₃/m, with lattice parameters of , for NaSr_0.5Al₂B₂O₇ and , for NaCa_0.5Al₂B₂O₇, respectively. The structure is built up by [Al₂B₂O₇]²⁻ double layer and Na⁺/Ca²⁺ or Na⁺/Sr²⁺ ions alternatively stacking along the c-axis. The sites in the inter-double layer are fully occupied jointly by Na and Ca or Sr, but the intra-double layer sites are only half occupied solely by Na. A mechanism of the transition of the structure from CaAl₂B₂O₇ to present structure type by replacing only 1% Ca by Na (2%) as observed by Chang and Keszler (Mater. Res. Bull. 33 (1998) 299) is also proposed.     

Synthesis and characterization of a new four-layer Aurivillius phase Bi2SrNa2Nb4O15 and its protonated form

A new four-layer Aurivillius phase Bi₂SrNa₂Nb₄O₁₅ has been synthesized by solid-state reaction of Bi₂SrNb₂O₉ and NaNbO₃ at 1100 °C. The detailed structure determination of Bi₂SrNa₂Nb₄O₁₅ performed by powder X-ray diffraction (XRD) shows that it crystallizes in the space group I4/mmm [a∼3.9021(1) Å, c∼40.7554(10) Å]. Protonated form of Bi₂SrNa₂Nb₄O₁₅ was obtained by the substitution of bismuth oxide sheets with protons via acid treatment. The conversion into the protonated forms was achieved easily using 6 M HCl at room temperature. Preservation of the structure of the perovskite-like slabs and contraction in the c-axis were confirmed by X-ray analysis. The compositions of the resulting products were determined to be H_1.8[Sr_0.8Bi_0.2Na₂Nb₄O₁₃] by X-ray fluorescence spectroscopy (XFS) and thermogravimetry.     

Defect and dopant properties of the Aurivillius phase Bi4Ti3O12

Computer modelling techniques have been used to investigate the defect and oxygen transport properties of the Aurivillius phase Bi₄Ti₃O₁₂. A range of cation dopant substitutions has been considered including the incorporation of trivalent ions (M³⁺=Al, Ga and In). The substitution of In³⁺ onto the Bi site in the [Bi₂O₂] layer is predicted to be the most favourable. The calculations suggest that lanthanide (Ln³⁺) doping at the dilute limit preferentially occurs in the [Bi₂O₂] layer, with probable distribution over both the [Bi₂O₂] and the perovskite A-site at higher dopant levels. It is predicted that the reduction process involving Ti³⁺ and oxygen vacancy formation is energetically favourable. The energetics of oxide vacancy migration between various oxygen sites in the structure have been investigated.     

Synthesis crystal structure and ionic conductivity of Ca0.5Bi3V2O10 and Sr0.5Bi3V2O10

Two new compounds Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀ have been synthesized in the ternary system: MO-Bi₂O₃-V₂O₅ system (M=M²⁺). The crystal structure of Sr_0.5Bi₃V₂O₁₀ has been determined from single crystal X-ray diffraction data, space group and Z=2, with cell parameters a=7.1453(3) Å, b=7.8921(3) Å, c=9.3297(3) Å, α=106.444(2)°, β=94.088(2)°, γ=112.445(2)°, V=456.72(4) Å³. Ca_0.5Bi₃V₂O₁₀ is isostructural with Sr_0.5Bi₃V₂O₁₀, with, a=7.0810(2) Å, b=7.8447(2) Å, c=9.3607(2) Å, α=106.202(1)°, β=94.572(1)°, γ=112.659(1)°, V=450.38(2) Å³ and its structure has been refined by Rietveld method using powder X-ray data. The crystal structure consists of infinite chains of (Bi₂O₂) along c-axis formed by linkage of BiO₈ and BiO₆ polyhedra interconnected by MO₈ polyhedra forming 2D layers in ac plane. The vanadate tetrahedra are sandwiched between these layers. Conductivity measurements give a maximum conductivity value of 4.54×10⁻⁵ and 3.63×10⁻⁵ S cm⁻¹ for Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀, respectively at 725 °C.     

A structural study of the perovskite series Na0.75Ln0.25Ti0.5Nb0.5O3

The ternary stoichiometric perovskite compounds, Na_0.75Ln_0.25Ti_0.5Nb_0.5O₃ (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm) are intermediate members of the NaNbO₃-Na_0.5Ln_0.5TiO₃ solid solution series. The compounds were synthesized by standard ceramic methods at 1300 °C followed by annealing at 800 °C and quenching to ambient conditions. Rietveld analysis of the powder X-ray diffraction patterns shows that the compounds with Ln ranging from Pr to Tm adopt the orthorhombic space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) and the GdFeO₃ structure. In contrast, Na_0.75La_0.25Ti_0.5Nb_0.5O₃ adopts the orthorhombic space group Cmcm (a≈b≈c≈2a_p; Z=4). All cations located at the A- and B-sites are disordered in these compounds. The unit cell parameters and cell volumes of the compounds decrease regularly with increasing atomic number of the Ln cation. The Pbnm compounds with Ln from Sm to Tm have A-site cations in eight-fold coordination. A-site cations in the Pr and Nd compounds are considered to be in ten-fold coordination. Analysis of the crystal chemistry of the Pbnm compounds shows that B-site cations enter the second coordination sphere of the A-site cations for compounds with Ln from Tb to Tm as the A-B intercation distances are less than the maximum A-^IIO(2) bond lengths. The [111] tilt angles of the (Ti,Nb)O₆ polyhedra in the Pbnm compounds increase with increasing atomic number from 11.1° to 15.8° and are less than those observed in lanthanide orthoferrite and orthoscandate perovskites. These data are considered as relevant to the sequestration of lanthanide fission products in perovskite and the structure of lanthanide-bearing perovskite-structured minerals.     

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.     

Crystal structure, magnetic, and dielectric properties of Aurivillius-type Bi3Fe0.5Nb1.5O9

Bi₃Fe_0.5Nb_1.5O₉ was synthesized using conventional solid state techniques and its crystal structure was refined by the Rietveld method using neutron powder diffraction data. The oxide adopts an Aurivillius-type structure with non-centrosymmetric space group symmetry A2₁am (a=5.47016(9) Å, b=5.43492(9) Å, c=25.4232(4) Å), analogous to other Aurivillius compounds that exhibit ferroelectricity. The Fe and Nb cations are disordered on the same crystallographic site. The [(Fe,Nb)O₆] octahedra exhibit tilting and distortion to accommodate the bonding requirements of the Bi cations located in the perovskite double layers. Magnetic measurements indicate non-Curie-Weiss-type paramagnetic behavior from 300 to 6 K. Measurements of dielectric properties and electrical resistivity exhibited changes near 250-260 °C and are suggestive of a ferroelectric transition.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.     

Crystal structure refinements of the three-layer Aurivillius ceramics Bi2Sr2−xAxNb2TiO12 (A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Three-layer Aurivillius ceramics Bi₂SrCaNb₂TiO₁₂, Bi₂Sr_1.5Ca_0.5Nb₂TiO₁₂, Bi₂Sr₂Nb₂TiO₁₂, Bi₂Sr_1.5Ba_0.5Nb₂TiO₁₂, and Bi₂SrBaNb₂TiO₁₂ were formed via solid-state synthesis and their structures characterized by combined Rietveld analysis of powder X-ray and neutron diffraction data. Static disorder was observed in the form of mixed cation occupancies between the Bi and the Sr, Ca, or Ba on the A sites in the perovskite block, as well as between the Nb and Ti sites. The degree of site mixing between the Bi site in the (Bi₂O₂)²⁺ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi₂SrBaNb₂TiO₁₂ displayed the greatest degree of Bi-A site static disorder. Bond valence sum (BVS) calculations showed an increase in A site BVS with average A site cation radius. All compositions except Bi₂SrCaNb₂TiO₁₂ had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca²⁺ to remain on the A site while Ba²⁺ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi₂O₂)²⁺ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO₆ octahedra were observed in structural models that included full oxygen occupancy. However, excellent structure models and more reasonable BVS values were obtained by assuming oxygen vacancies in the TiO₆ octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of at 900°C.     

Preparation and comparison of the photoelectronic properties of Sr2Nb2O7 and Ba0.5Sr0.5Nb2O6

The two alkaline earth niobates Sr₂Nb₂O₇ and Ba_0.5Sr_0.5Nb₂O₆ have been prepared, their electronic properties measured, and their photoresponses compared. The indirect band gap in Sr₂Nb₂O₇ is 3.86 eV compared with 3.38 eV for Ba_0.5Sr_0.5Nb₂O₆. Hence, photoanodes composed of Sr₂Nb₂O₇ respond to much less of the “white” light spectrum than those made from Ba_0.5Sr_0.5Nb₂O₆. Nevertheless, their electrical outputs at an anode potential of 0.8 eV with respect to SCE in 0.2 M sodium acetate under “white” xenon arc irradiation of 1.25 W/cm² are comparable.     

Extension of the “1201” Family to Strontium-Rich Chromite and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Two new Sr-rich “1201”-type oxides, Bi_0.4Sr_2.5Cr_1.1O_4.9 and Bi_0.4Sr_2.5Fe_1.1O₅ have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi_0.4M_0.1Sr_0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe³⁺ and 50% Fe⁴⁺ (also confirmed by Mössbauer), and chromium could be a mixture of Cr³⁺ and Cr⁶⁺ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.     

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.     

Mechanochemical-thermal preparation and structural studies of mullite-type Bi2(GaxAl1−x)4O9 solid solutions

A series of Bi₂(Ga_xAl_1−x)₄O₉ solid solutions (0≤x≤1), prepared by mechanochemical processing of Bi₂O₃/Ga₂O₃/Al₂O₃ mixtures and subsequent annealing, was investigated by XRD, EDX, and ²⁷Al MAS NMR. The structure of the Bi₂(Ga_xAl_1−x)₄O₉ solid solutions is found to be orthorhombic, space group Pbam (No. 55). The lattice parameters of the Bi₂(Ga_xAl_1−x)₄O₉ series increase linearly with increasing gallium content. Rietveld refinement of the XRD data as well as the analysis of the ²⁷Al MAS NMR spectra show a preference of gallium cations for the tetrahedral sites in Bi₂(Ga_xAl_1−x)₄O₉. As a consequence, this leads to a far from random distribution of Al and Ga cations across the whole series of solid solutions.     

Crystal structure of the ‘mixed-layer’ Aurivillius phase Bi5TiNbWO15

The crystal structure of the Aurivillius phase Bi₅TiNbWO₁₅ has been analyzed in detail using powder X-ray and neutron diffraction. The structure can be described as a regular intergrowth of alternating single and double perovskite-like layers sandwiched between fluorite-like bismuth oxide layers, such that the layer sequence is … [WO₄]-[Bi₂O₂]-[BiTiNbO₇]-[Bi₂O₂] …. There is complete ordering of tungsten within the B sites of the single perovskite layer, so that the structure can be described as a direct intergrowth of the ‘component’ Aurivillius phases Bi₂WO₆ and Bi₃TiNbO₉. At 25 °C the structure adopts the polar orthorhombic space group I2cm, , , .     

Crystal structure of Ca12Al14O32Cl2 and luminescence properties of Ca12Al14O32Cl2:Eu

The crystal structure of Ca₁₂Al₁₄O₃₂Cl₂ was determined from laboratory X-ray powder diffraction data (CuKα₁) using the Rietveld method, with the anisotropic displacement parameters being assigned for all atoms. The crystal structure is cubic (space group , Z=2) with lattice dimensions a=1.200950(5) nm and V=1.73211(1) nm³. The reliability indices calculated from the Rietveld method were R_wp=8.48% (S=1.21), R_p=6.05%, R_B=1.27% and R_F=1.01%. The validity of the structural model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The reliability indices calculated from the MPF were R_B=0.75% and R_F=0.56%. In the structural model there are one Ca site, two Al sites, two O sites and one Cl site. This compound is isomorphous with Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4. Europium-doped sample Ca₁₂Al₁₄O₃₂Cl₂:Eu²⁺ was prepared and the photoluminescence properties were presented. The excitation spectrum consisted of two wide bands, which were located at about 268 and 324 nm. The emission spectrum, when excited at 324 nm, resulted in indigo light with a peak at about 442 nm.     

Structural disorder in Ba0.6Sr0.4Al2O4

The structural disorder in Ba_0.6Sr_0.4Al₂O₄ (space group P6₃22) was investigated by X-ray powder diffraction and selected-area electron diffraction (SAED). The initial structural model was determined using direct methods, and it was further modified by the combined use of Rietveld method and maximum-entropy method (MEM). MEM-based pattern fitting method was subsequently applied, resulting in the final reliability indices of R_wp=9.61%, R_p=6.96%, R_B=1.40% and S=1.25. The electron density distribution was satisfactorily expressed by the split-atom model in which the strontium/barium and oxygen atoms were split to occupy the lower symmetry sites. The diffuse scattering in SAED was mainly attributable to the positional disorder of oxygen atoms.     

Crystal structure, thermal and magnetic properties of Cr2V4O13

Cr₂V₄O₁₃, a tetravanadate of Cr³⁺ has been prepared by repeated heating of stoichiometric amounts of Cr₂O₃ and V₂O₅ and its crystal structure is refined by Rietveld refinement of the powder XRD data. This compound crystallizes in a monoclinic lattice with unit cell parameters: a=8.2651(3), b=9.2997(3), c=14.5215(5) Å, β=102.618(3)°, V=1089.21(6) Å³ and Z=4 (Space group: P2₁/c). The U shaped (V₄O₁₃)⁶⁻ formed by corner connected VO₄ tetrahedra links the Cr₂O₁₀ (dimers of two edge shared CrO₆ octahedra) forming a three dimensional network structure of Cr₂V₄O₁₃. This compound is stable up to 635 °C and peritectically decomposes to orthorhombic CrVO₄ and V₂O₅ above this temperature. A possible long range antiferromagnetic ordering below 10 K is suggested from the squid magnetometry and electron paramagnetic resonance (epr) spectroscopic studies of Cr₂V₄O₁₃.     

On the mechanism of propene oxidation on Bi3Mo2FeO12 and Bi2Mo2Fe2O12 catalysts

Oxidation of propene on Bi₃Mo₂FeO₁₂ and Bi₂Mo₂Fe₂O₁₂ and Bi₂(MoO₄)₃ has been compared with the interaction of allyl iodide and diallyl oxalate with these compounds. Bi₂Mo₂Fe₂O₁₂ was found to be the most active and the least selective in propene oxidation. The three compounds show at the same time the same activity in acrolein formation from allyl iodide and oxalate. The results are interpreted in terms of the two-center mechanism of propene oxidation [9, 10]. The role of iron in elementary steps of propene oxidation on Bi–Mo–Fe–O catalysts is also discussed.

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Bi₃Mo₂FeO₁₂, Bi₂Mo₂Fe₂O₁₂ Bi₂(MoO₄)₃ . Bi₂Mo₂Fe₂O₁₂ . . , . Bi–Mo–Fe–O.

     

Effect of Niobium doping on structural, thermal, sintering and electrical properties of Bi4V1.8Cu0.2O10.7

Doping Bi₄V_1.8Cu_0.2O_10.7 with niobium has led to the formation of the Bi₄V_1.8Cu_0.2−xNb_xO_10.7+3x/2 solid solution. X-ray diffraction and thermal analysis have shown that only the compound with x=0.05 presents a tetragonal symmetry with a γ^′ polymorph while the other compositions are of β polymorph. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at temperatures higher than 820 °C present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. Among all compositions studied the sample with x=0.05 presents the highest value of the conductivity.     

X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15

The crystal structures of NaK₂B₉O₁₅ (, , , β=94.080(1)°, R_p=0.047, R_wp=0.059, R_B=0.026), Na(Na_.17K_.83)₂B₉O₁₅ (, , , β=94.228(2)°, R_p=0.053, R_wp=0.068, R_B=0.026), and (Na_.80K_.20)K₂B₉O₁₅ (, , , β=94.071(1)°, Z=4, R_p=0.041, R_wp=0.052, R_B=0.023) were refined in the monoclinic space groups P2₁/c(Z=4) using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K₃B₉O₁₅. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK₂B₉O₁₅ decomposes at about 700 °C in accordance with the peritectic reaction NaK₂B₉O₁₅↔K₅B₁₉O₃₁+liquid. The thermal expansion of NaK₂B₉O₁₅ and Na(Na_.17K_.83)₂B₉O₁₅ is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK₂B₉O₁₅ is revealed: heating of NaK₂B₉O₁₅ by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na_1−xK_x)₃B₉O₁₅ by 0.04 at% K.