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).     

Tl1−xNaxNb2PO8 and Related Compounds, Isostructural with Ca0.5+xCs2Nb6P3O24

Compounds A_2/3A′_1/3M₂XO₈ (A=Tl, Rb, Cs; A′=Na, Ag; M=Nb, Ta; X=P, As) have been synthesized using the ceramic method. The sodium and potassium compounds (A= Na and K) have been prepared by an ion exchange reaction starting from their thallium analogues. These materials are isotypic with Tl_1−xNa_xNb₂PO₈ (x=0.21) the structure of which has been determined by using X-ray single-crystal data. The space group is R32, the cell constants are a_H=13.369(2), c_H=10.324(3) Å and z=9. This compound is isostructural with Ca_0.5+xCs₂ Nb₆P₃O₂₄. Its three-dimensional framework [Nb₂PO₈]_n, built up from NbO₆ octahedra and corner-sharing PO₄ tetrahedra, delimits tunnels running along c_H and cavities accommodating Tl⁺ and Na⁺ cations, respectively. The K_2/3Na_1/3Nb₂PO₈ structure, refined using X-ray powder data, showed that K⁺ cations are spread like the Tl+ ones over many sites, but more excentred from the tunnel axis. The isotypy of these compounds is also revealed by the similarity of the infrared and Raman spectra. The nonlinear optical study showed a behavior similar to that of the KDP for all the compounds. The ionic conductivity measurements gave high activation energies and low conductivity values for these materials.     

Structural studies of five layer Aurivillius oxides: A2Bi4Ti5O18 (A=Ca, Sr, Ba and Pb)

The room temperature structures of the five layer Aurivillius phases A₂Bi₄Ti₅O₁₈ (A=Ca, Sr, Ba and Pb) have been refined from powder neutron diffraction data using the Rietveld method. The structures consist of [Bi₂O₂]²⁺ layers interleaved with perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks. The structures were refined in the orthorhombic space group B2eb (SG. No. 41), Z=4, and the unit cell parameters of the oxides are a=5.4251(2), b=5.4034(1), c=48.486(1); a=5.4650(2), b=5.4625(3), c=48.852(1); a=5.4988(3), b=5.4980(4), c=50.352(1); a=5.4701(2), b=5.4577(2), c=49.643(1) for A=Ca, Sr, Ba and Pb, respectively. The structural features of the compounds were found similar to n=2-4 layers bismuth oxides. The strain caused by mismatch of cell parameter requirements for the [Bi₂O₂]²⁺ layers and perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks were relieved by tilting of the TiO₆ octahedra. Variable temperature synchrotron X-ray studies for Ca and Pb compounds showed that the orthorhombic structure persisted up to 675 and 475 K, respectively. Raman spectra of the compounds are also presented.     

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO₂NO₃, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2) pm for M=Pb; a=396.337(5), c=1412.83(3) pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1) pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3) pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO₂]⁺ and [NO₃]⁻ layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M²⁺ cations in BiMO₂NO₃ are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)₂Sr₂Ca_n−1Cu_nO_x.     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

Structure Study of Bi2.5Na0.5Ta2O9 and Bi2.5Nam−1.5NbmO3m+3 (m=2-4) by Neutron Powder Diffraction and Electron Microscopy

The crystal structures of Bi_2.5Na_0.5Ta₂O₉ and Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=3,4) have been investigated by the Rietveld analysis of their neutron powder diffraction patterns (λ=1.470 Å). These compounds belong to the Aurivillius phase family and are built up by (Bi₂O₂)²⁺ fluorite layers and (A_m-1B_mO_3m+1)^2- (m=2-4) pseudo-perovskite slabs. Bi_2.5Na_0.5Ta₂O₉ (m=2) and Bi_2.5Na_2.5Nb₄O₁₅ (m=4) crystallize in the orthorhombic space group A2₁am, Z=4, with lattice constants of a=5.4763(4), b=5.4478(4), c=24.9710 (15) and a=5.5095(5), b=5.4783(5), c=40.553(3) Å, respectively. Bi_2.5Na_1.5Nb₃O₁₂ (m=3) has been refined in the orthorhombic space group B2cb, Z=4, with the unit-cell parameters a=5.5024(7), b=5.4622(7), and c=32.735(4) Å. In comparison with its isostructural Nb analogue, the structure of Bi_2.5Na_0.5Ta₂O₉ is less distorted and bond valence sum calculations indicate that the Ta-O bonds are somewhat stronger than the Nb-O bonds. The cell parameters a and b increase with increasing m for the compounds Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=2-4), causing a greater strain in the structure. Electron microscopy studies verify that the intergrowth of mixed perovskite layers, caused by stacking faults, also increases with increasing m.     

Synthesis, characterization and crystal structure of K2Bi(PO4)(MoO4)

A new potassium bismuth phosphate-molybdate K₂Bi(PO₄)(MoO₄) has been synthesized by the flux method and characterized by single-crystal and powder X-ray diffraction, IR spectroscopic studies. The compound crystallizes in the orthorhombic system with the space group Ibca and the cell parameters: a=19.7037(10), b=12.4752(10), c=7.0261(10). This phase exhibits an original layered structure, in which the [Bi(PO₄)(MoO₄)]_∞ layers consist of [Bi₂Mo₂O₁₈]_∞ chains linked through single PO₄ tetrahedra. The K⁺ cations interleaved between these layers exhibit a monocapped distorted cubic coordination.     

Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

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.     

K2Mg5−xSn3 and K3Mg18Tt11 (Tt=Sn, pb) with two types of Mg-Sn/Pb frameworks

K₂Mg_5−xSn₃ (x=0.28) and K₃Mg₁₈Tt₁₁ (Tt=Sn, Pb) have been synthesized by reacting the mixture of the corresponding pure elements at high temperature, and structurally characterized by single-crystal X-ray diffraction studies. K₂Mg_5−xSn₃ (x=0.28) is isostructural with Ni_7−xSbQ₂ (Q=Se, Te) series and features 2D corrugated [Mg_5−xSn₃] layers that are separated by K⁺ cations. The structure of K₃Mg₁₈Tt₁₁ (Tt=Sn, Pb) is closely related to the Ho₂Rh₁₂As₇ structural type and features 3D [Mg₁₈Tt₁₁] framework composed of 1D [Mg₁₈Tt₁₁] columns that are interconnected via Mg-Tt bonds, forming 1D hexagonal tunnels occupied by the K+ cations. Electronic structure calculations indicate that Mg atoms can function as either electron donor or as a participator in the network along with Tt atoms. Magnetic property measurements and band structure calculations indicate that these compounds are metallic.     

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.     

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.     

Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Ta₂O₉ (A=Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and the crystal structures were characterized by powder X-ray diffraction. The results showed that the structure of ABi₂Ta₂O₉ (A=Ca, Sr) is orthorhombic, while that of BaBi₂Ta₂O₉ is tetragonal. First-principles calculations of the electronic band structures and density of states (DOS) revealed that the conduction bands of these photocatalysts are mainly attributable to the Ta 5d+Bi 6p+O 2p orbitals, while their valence bands are composed of hybridization with O 2p+Ta 5d+Bi 6s orbitals. Photocatalytic activities for water splitting were investigated under UV light irradiation and indicated that these photocatalysts are highly active even without co-catalysts. The formation rate of H₂ evolution from an aqueous methanol solution is about 2.26 mmol h^-1 for the photocatalyst SrBi₂Ta₂O₉, which is much higher than that of CaBi₂Ta₂O₉ and BaBi₂Ta₂O₉. The photocatalytic properties are discussed in close connection with the crystal structure and the electronic structure in details.     

Crystal Structure Approach of the Disordered New Compounds Bi∼1.2M∼1.2PO5.5 (M=Mn, Co, Zn): The Role of Oxygen-Centered Tetrahedra Linkage in the Structure of Bismuth-Transition Metal Oxy-phosphates

The crystal structures of some recently published bismuth-transition metal oxy-phosphates are described as the association of complex infinite one-dimensional polycations and phosphate anions. The complex cations are built from oxygen-centered tetrahedra sharing edges to form infinite ribbons of n tetrahedra width. This structural concept allows one to describe the essential structural features of new highly disordered bismuth-transition metal oxy-phosphates, Bi_∼1.2M_∼1.2PO_5.5 (M=Mn, Co, Zn). The new compounds have been synthesized and structurally characterized by single-crystal X-ray diffraction. The three compounds crystallize in the orthorhombic space group Ibam (No. 72), Z=8. The lattice parameters are a=15.079(2), b=11.247(2), c=5.437(1) Å for M=Mn, a=14.752(3), b=11.205(3), c=5.434(2) Å for M=Co and a=14.809(2), b=11.214(1), c=5.440(1) Å for M=Zn. Because of a high disorder over several cationic sites, only an approach of the crystal structure determination has been achieved. Actually, the structure is characterized by perfectly defined ribbons parallel to the (010) plane and built from a central chain of edge-shared OBi₄ tetrahedra running along the c axis and linked by edges to two other edge-shared O(Bi,M)₄ tetrahedra chains. The positions at the border of ribbons are randomly occupied by bismuth and M atoms. The formula of the three tetrahedra width ribbons is (O₃Bi_2.4M_1.6)^+4.4. The phosphate ions and M²⁺ cations are disordered in the interspace between the ribbons.     

Synthesis and Structure of New BiMn2MO6 Compounds Where M=P, As, or V

The new compounds BiMn₂PO₆, BiMn₂AsO₆, and BiMn₂ VO₆ have been prepared and shown to be structurally related to several other BiA₂MO₆ compounds. The structure of BiMn₂PO₆ was refined from neutron powder diffraction data in space group Pnma with a=12.04 Å, b=5.37 Å, c=8.13 Å, and Z=4. It contains (BiO₂)¹⁻ chains and (PO₄)³⁻ tetrahedra. The observed fivefold coordination for the Mn²⁺ cations is unusual for Mn in this oxidation state.     

The crystal structure of a new bismuth tellurium oxychloride Bi0.87Te2O4.9Cl0.87 from neutron powder diffraction data

A new bismuth tellurium oxychloride was obtained by reaction of BiOCl and TeO₂ in air. According to energy dispersive X-ray spectroscopy and neutron powder diffraction refinement the composition of the substance was determined as Bi_0.87Te₂O_4.9Cl_0.87. The new compound crystallizes in the trigonal system space group R 3¯ (#148), Z=6, a=4.10793(4), c=31.1273(4) Å, χ²=3.20, wR_p=0.0369. Bi_0.87Te₂O_4.9Cl_0.87 has a new type of layered structure constructed by Bi-Te-O layers separated by chloride ions. The Te atoms in Bi_0.87Te₂O_4.9Cl_0.87 show an unusual umbrella-like environment. A comparison with known related structures has been made.     

Crystal structure of lanthanum bismuth silicate Bi2−xLaxSiO5 (x∼0.1)

A melting and glass recrystallization route was carried out to stabilize a new tetragonal form of Bi₂SiO₅ with bismuth partially substituted by lanthanum. The crystal structure of Bi_2−xLa_xSiO₅ (x∼0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, , c=15.227(1) Å, V=224.18 Å³, Z=2; reliability factors: R_Bragg=5.65%, R_p=14.6%, R_wp=16.8%, R_exp=8.3%, χ²=8.3 (X-ray) and R_Bragg=2.40%, R_p=8.1%, R_wp=7.5%, R_exp=4.2%, χ²=3.3 (neutrons); 11 structural parameters refined).The main effect of lanthanum substitution is to introduce, by removing randomly some bismuth 6s² lone pairs, a structural disorder in the surroundings of (Bi₂O₂)²⁺ layers, that is in the (SiO₃)²⁻ pyroxene files arrangement. It results in a symmetry increase relatively to the parent compound Bi₂SiO₅, which is orthorhombic. The two structures are compared.     

New thallium iodates—Synthesis, characterization, and calculations of Tl(IO3)3 and Tl4(IO3)6, [Tl3Tl(IO3)6]

Two new thallium iodates have been synthesized, Tl(IO₃)₃ and Tl₄(IO₃)₆ [Tl⁺₃Tl³⁺(IO₃)₆], and characterized by single-crystal X-ray diffraction. Both materials were synthesized as phase-pure compounds through hydrothermal techniques using Tl₂CO₃ and HIO₃ as reagents. The materials crystallize in space groups R-3 (Tl(IO₃)₃) and P-1 (Tl₄(IO₃)₆). Although lone-pairs are observed for both I⁵⁺ and Tl⁺, electronic structure calculations indicate the lone-pair on I⁵⁺ is stereo-active, whereas the lone-pair on Tl⁺ is inert.