Synthesis and structural study of stoichiometric Bi2Ti2O7 pyrochlore

Bi₂Ti₂O₇ has been synthesized using a co-precipitation route from H₂O₂/NH_3(aq) solutions of titanium with aqueous bismuth nitrate. The stoichiometric material crystallizes into a pale yellow cubic pyrochlore phase. A powder X-ray diffraction study showed this crystallization to be very temperature sensitive, the pure phase can only be obtained within a few degrees of 470°C. Time-of-flight powder neutron diffraction studies of Bi₂Ti₂O₇ (Space group , a=10.37949(4) Å at ambient temperature, Z=8, R_p=3.95%, R_wp=4.75%) revealed positional disorder in the bismuth site and in the O′ oxide site both at ambient temperature and at 2 K.     

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)).     

The crystal structure of the new ternary antimonide Dy3Cu20+xSb11−x (x≈2)

New ternary antimonide Dy₃Cu_20+xSb_11−x (x≈2) was synthesized and its crystal structure was determined by direct methods from X-ray powder diffraction data (diffractometer DRON-3M, CuKα-radiation, R_I=6.99%,R_p=12.27%,R_wp=11.55%). The compound crystallizes with the own cubic structure type: space group , Pearson code cF272, . The structure of the Dy₃Cu₂₀Sb_11−x (x≈2) can be obtained from the structure type BaHg₁₁ by doubling of the lattice parameter and subtraction of 16 atoms. The studied structure was compared with the structures of known compounds, which crystallize in the same space group with similar cell parameters.     

The crystal chemistry of Bi6TP2O15+x, T=Fe, Ni, Zn: Isomorphism and polymorphism, structural relationship to Bi6TiP2O16

The crystal structures of compounds with nominal compositions Bi₆FeP₂O_15+x (I), Bi₆NiP₂O_15+x (II) and Bi₆ZnP₂O_15+x (III) were determined from single-crystal X-ray diffraction data. They are monoclinic, space group I2, Z=2. The lattice parameters for (I) are a=11.2644(7), b=5.4380(3), c=11.1440(5) Å, β=96.154(4)°; for (II) a=11.259(7), b=5.461(4), c=11.109(7) Å, β=96.65(1)°; for (III) a=19.7271(5), b=5.4376(2), c=16.9730(6) Å, β=131.932(1)°. Least squares refinements on F² converged for (I) to R1=0.0554, wR₂=0.1408; for (II) R₁=0.0647, wR₂=0.1697; for (III) R₁=0.0385, wR₂=0.1023. The crystals are complexly twinned by 2-fold rotation about , by inversion and by mirror reflection. The structures consist of edge-sharing articulations of OBi₄ tetrahedra forming layers in the a-c plane that then continue by edge-sharing parallel to the b-axis. The three-dimensional networks are bridged by Fe and Ni octahedra in (I) and (II) and by Zn trigonal bipyramids in (III) as well as by oxygen atoms of the PO₄ moieties. Bi also randomly occupies the octahedral sites. Oxygen vacancies exist in the structures of the three compounds due to required charge balances and they occur in the octahedral coordination polyhedron of the transition metal. In compound (III), no positional disorder in atomic sites is present. The Bi-O coordination polyhedra are trigonal prisms with one, two or three faces capped. Magnetic susceptibility data for compound (I) were obtained between 4.2 and 350 K. Between 4.2 and 250 K it is paramagnetic, μ_eff=6.1 μ_B; a magnetic transition occurs above 250 K.     

Characterization of the new Bi∼6.2Cu∼6.2O8(PO4)5 oxyphosphate; a disordered compound containing 2- and 3-O(Bi, Cu)4 tetrahedra—wide polycationic ribbons

The present work is dedicated to the XRD, ED and HREM characterization of a new bismuth copper oxyphosphate Bi_∼6.2Cu_∼6.2O₈(PO₄)₅ (a=11.599(2)Å, , c=37.541(5)Å, R₁=0.0755, Rw₂₌0.174, G.S Pn2₁a). The relatively long size of its c parameter is due to the arrangement along this direction of two kinds of ribbon-like polycations formed by edge sharing O(Bi, Cu)₄ tetrahedra. The existence of such cations is characterized by the b∼5.2 Å value intrinsic to the ribbons structure and commonly found in bismuth oxyphosphate materials. In the title compound, 2-tetrahedra wide [Bi_∼2.4Cu_∼3.6O₄]^6.4+ and 3-tetrahedra wide [Bi_∼5Cu_∼3O₆]⁹⁺ ribbons are isolated by phosphate groups and alternate along c. The interstitial site created between two different sizes ribbons is occupied by Cu²⁺ cations disordered over several close crystallographic sites. The mixed Bi³⁺/Cu²⁺ nature of certain edge-of-ribbons positions induces a disorder over several configurations of the phosphate groups. The concerned oxygen atoms form the environment of the disordered interstitial Cu²⁺ cations which occupy tunnels formed by the phosphate anions. The high-resolution electron microscope study enables a precise correlation between the observed images and the refined crystal structure, evidencing the polycations visualization. Furthermore, this material being the second example of partially disordered compound similar chemical system, some topological rules can be deduced. The b-axis doubling was observed by ED and HREM and is assigned to the ordering of interstitial Cu²⁺ within tunnels cations. A partial intra-tunnel ordering was also observed.     

Preparation and crystal structure of a new bismuth chromate: Bi8(CrO4)O11

Single crystals of a new bismuth chromate, Bi₈(CrO₄)O₁₁, were prepared by hydrothermal reaction of NaBiO₃·nH₂O in K₂CrO₄ solution. The bismuth chromate crystallizes in the monoclinic space group P2₁/m with a=9.657(3), b=11.934(3), c=13.868(2)Å and β=104.14(1)°, Z=4 and the final R factors are R=0.038 and R_w=0.041 for 3541 unique reflections. The crystal structure has a slab built up by (CrO₄)²⁻ tetrahedra and distorted bismuth polyhedra which are five-fold pyramids, six-fold trigonal prisms and octahedra. The distance of lone pair from nucleus for bismuth atoms ranges from 0.29 to 1.12 Å, depending on the coordination environment. Bi₈(CrO₄)O₁₁ decomposes to Bi₁₄CrO₂₄ and a small amount of an unknown phase at 796 °C.     

Ab initio structure determination and Rietveld refinement of Bi10Mo3O24 the member n=3 of the Bi2n+4MonO6(n+1) series

Bi₂O₃-MoO₃ system shows a large panoply of phases depending on Bi/Mo ratio, among them, the low temperature phases of the homologous series Bi_2(n+2)Mo_nO_6(n+1) with n=3, 4, 5 and 6. They exhibit, alike most of the phases of this system, strong fluorite sub-network. Nevertheless, a multitechnique approach has been followed in order to solve the crystal structure of the n=3 member, i.e. Bi₁₀Mo₃O₂₄. From ab initio indexing X-ray powder pattern cell parameters were derived. It belongs to the monoclinic system, space group C2, with cell parameters: a=23.7282(2) Å, b=5.64906(6) Å, c=8.68173(9) Å, β=95.8668(7)° with Z=2. The matrix relating this cell with the fluorite one is 4 0 1/0 1 0/ 0  and a cationic localization was derived. HRTEM allowed the cationic Bi and Mo order to be modified and specified, as well as to build up a full structural ab initio model on the basis of crystal chemistry considerations. Simultaneous Rietveld refinement of multipattern X-ray and neutron powder diffraction data taking advantage of the neutron scattering length for O location have been performed. The goodness of the model was ascertained by low reliability factors, weighted R_b=4.97% and R_f=3.21%. This complex Bi₁₀Mo₃O₂₄ structure, with 5Bi, 2Mo and 13O in different crystallographic positions of the asymmetric unit, shows good agreement between observed and calculated patterns within the data resolution. Moreover, the determination of this structure sets the basis for the crystallographic characterization of the complete family Bi_2(n+2)Mo_nO_6(n+1), whose guidelines are also evidenced in this paper.     

Investigation of structural and hydrogen absorption properties in the LaNi5−xPtx-H2 system

The substitution of nickel by platinum in the binary LaNi₅ compound (CaCu₅ structure type, a=5.019(1) Å, c=3.981(1) Å, space group P6/mmm) and its effect on the hydrogenation properties was studied. The phase LaNi_5−xPt_x has a homogeneity domain ranging from x=0 to 5. For x<3, platinum substitutes almost exclusively on site 3g and also replaces nickel on site 2c for x>3. Contrary to what is observed in other systems, the hydrogen absorption plateau pressure was found to increase as a function of the cell volume. Powder neutron diffraction experiments were conducted for two deuterated compounds with x=0.25 and 0.75. Deuterium partial ordering occurs in the case of x=0.25 leading to a symmetry decrease to the space group P6mm (LaNi_4.75Pt_0.25D_5.23, a=4.225(1) Å, c=5.357(1) Å, Z=1, R_Bragg=3.3%). For x=0.75, an orthorhombic superstructure based on the CaCu₅-type lattice was found (LaNi_4.25Pt_0.75D_2.61, a_orth=√3a_hex=9.089(1) Å, b_orth=b_hex=5.272(1) Å, c_orth=2c_hex=8.145(1) Å, Z=4, SG Ibam, R_Bragg=6.1%).     

Thermoelectric and structural properties of a new Chevrel phase: Ti0.3Mo5RuSe8

The new Chevrel phase Ti_0.3Mo₅RuSe₈ has been synthesized and characterized by quantitative microprobe analysis, powder X-ray diffraction, and high-temperature thermoelectric properties measurements. The thermoelectric properties of this compound are compared to the previously reported data for other related Chevrel phases. We report also the results of Rietveld analysis of powder X-ray diffraction data for Ti_0.3Mo₅RuSe₈. This compound adopts the rhombohedral Chevrel phase structure (space group , Z=3) with hexagonal lattice constants a=9.75430(25) Å and c=10.79064(40) Å. The low level of incorporation and low scattering power of Ti precluded the identification of the Ti positions, and Rietveld refinement was carried out only for the Mo₅RuSe₈ framework of Ti_0.3Mo₅RuSe₈ (R_p=10.5%, R_wp=14.6%). Rietveld analysis was also used to refine the structure of the unfilled phase Mo₅RuSe₈ (, Z=3, a=9.63994(8) Å, c=10.97191(11) Å, R_p=8.0%, R_wp=10.5%). Comparisons between the two structures are made.     

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.     

Ab initio structure determination of new compound Li4CaB2O6

A new compound, Li₄CaB₂O₆, has been synthesized by solid-state reaction and its structure has been determined from powder X-ray diffraction data by direct methods. The refinement was carried out using the Rietveld methods and the final refinement converged with R_p=10.4%, R_wp=14.2%, R_exp=4.97%. This compound belongs to the orthorhombic space group Pnnm, with lattice parameters a=9.24036(9) Å, b=8.09482(7) Å, and c=3.48162(4) Å. Fundamental building units are isolated [BO₃]³⁻ anionic groups, which are all parallel to the a-b plane stacked along the c-axis. The Ca atoms are six-coordinated by the O atoms to form octahedral coordination polyhedra, which are joined together through edges along the c-axis, forming infinitely long three-dimensional chains. The Li atoms have a four-fold and a five-fold coordination with O atoms that lead to complex Li-O-Li chains that also extend along the c-axis. The infrared spectrum of Li₄CaB₂O₆ was also studied, which is consistent with the crystallographic study.     

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.     

One-dimensional inorganic arrangement in the bismuth oxalate hydroxide Bi(C2O4)OH

Single crystals of Bi(C₂O₄)OH were obtained by the slow diffusion of Bi³⁺ cations through silica gel impregnated with oxalic acid. The structure was solved in the Pnma space group with a=6.0853(2) Å, b=11.4479(3) Å, c=5.9722(2) Å, leading to R=0.0188 and wR=0.0190 from 513 unique reflections. The bismuth coordination polyhedron is a BiO₆E pentagonal bipyramid with the lone pair E sitting at an axial vertex. The opposite axial vertex is occupied by a hydroxyl oxygen atom, which is also an equatorial corner of a neighboring BiO₆E bipyramid. The sharing of the hydroxyl oxygen atoms build zig-zag chains running down the [100] direction. These chains are aligned in a sheet parallel to the (010) plane and are further connected through oxalate ions to form a three-dimensional arrangement. On heating, Bi(C₂O₄)OH decomposes to the meta-stable quadratic β-Bi₂O₃ phase.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.     

Superstructure of a phosphor material Ba3MgSi2O8 determined by neutron diffraction data

Ba₃MgSi₂O₈, a phosphor host examined for use in white-light devices and plant-growth lamps, was synthesized at 1225 °C in air. Its crystal structure has been determined and refined by a combined powder X-ray and neutron Rietveld method (, Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) Å³; R_p/R_wp=3.79%/5.03%, χ²=4.20). Superstructure reflections, observed only in the neutron diffraction data, provided the means to establish the true unit cell and a chemically reasonable structure. The structure contains three crystallographically distinct Ba atoms—Ba1 resides in a distorted octahedral site with S₆ () symmetry, Ba2 in a nine-coordinate site with C₃ (3) symmetry, and Ba3 in a ten-coordinate site with C₁ (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.     

Ab initio structure determination of new compound Ba3(BO3)(PO4)

The crystal structure of new compound Ba₃BPO₇ was determined by ab initio method from high-resolution conventional X-ray powder diffraction data. The Rietveld refinement converged to R_p=5.92%, R_wp=8.87%, R_exp=5.00% with the following details: Hexagonal, space group P6₃mc, a=5.4898 (1) Å, c=14.7551(1) Å, Z=2. The basic unit of the structure is the [BaO₁₀]-[BO₃]-[PO₄] polar polyhedra-chain composed of Ba1-B-P-O cluster. These chains, running along c-axis, stack in a HCP mode to build the whole structure with triangular prism channels. The channels are parallel to c-axis too, in which Ba2 and Ba3 are located.     

Layered ruthenium hexagonal perovskites: The new series [Ba2Br2−2x(CO3)x][Ban+1RunO3n+3] with n=2, 3, 4, 5

Single crystals of the title compounds were prepared by solid state reactions from barium carbonate and ruthenium metal using a BaBr₂ flux and investigated by X-ray diffraction method using Mo(Kα) radiation and a Charge Coupled Device (CCD) detector. A structural model for the term n=2, Ba₅Ru₂Br₂O₉ (1) was established in the hexagonal symmetry, space group P6₃/mmc, a=5.8344(2) Å, c=25.637(2) Å, Z=2. Combined refinement and maximum-entropy method (MEM) unambiguously show the presence of CO₃²⁻ ions in the three other compounds (2, 3, 4). Their crystal structures were solved and refined in the trigonal symmetry, space group , a=5.8381(1) Å, c=15.3083(6) Å for the term n=3, Ba₆Ru₃Br_1.54(CO₃)_0.23O₁₂ (2), and space group , a=5.7992(1) Å, c=52.866(2) Å and a=5.7900(1) Å, c=59.819(2) Å for the terms n=4, Ba₇Ru₄Br_1.46(CO₃)_0.27O₁₅ (3), and n=5, Ba₈Ru₅Br_1.64(CO₃)_0.18O₁₈ (4), respectively. The structures are formed by the periodic stacking along [0 0 1] of (n+1) hexagonal close-packed [BaO₃] layers separated by a double layer of composition [Ba₂Br_2−2x(CO₃)_x]. The ruthenium atoms occupy the n octahedral interstices created in the hexagonal perovskite slabs and constitute isolated dimers Ru₂O₉ of face-shared octahedra (FSO) in 1 and isolated trimers Ru₃O₁₂ of FSO in 2. In 3 and 4, the Ru₂O₉ units are connected by corners either directly (3) or through a slab of isolated RuO₆ octahedra (4) to form a bidimensional arrangement of RuO₆ octahedra. These four oxybromocarbonates belong to the family of compounds formulated [Ba₂Br_2−2x(CO₃)_x][Ba_n+1Ru_nO_3n+3] where n represents the thickness of the octahedral string in hexagonal perovskite slabs. These compounds are compared to the oxychloride series.     

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.