Michel Drache

New phases in the binary system PbO–BiVO4: Pb2BiVO6 and Pb4BiVO8

The binary system PbO–BiVO4 exhibits two new phases, Pb4BiVO8 and Pb2BiVO6. They both melt congruently at 780 °C. They have been identified by single-crystal and powder X-ray diffraction. Pb4BiVO8 crystallizes with a triclinic cell, a= 6.221(2)A, b= 7.603(5)A, c= 10.457(4)A, α= 100.40(3)°, β= 102.18(2)°, γ= 90.03(3)° and Z= 2. Pb2BiVO6 exhibits four phase transitions at 300, 415, 475 and 600 °C. The α phase, room-temperature form, is monoclinic with a=7.642(6)A, b= 5.81 2(5)A, c= 28.85(2)A, β=100.3(1)° and Z= 8. The β and γ phases are orthorhombic with a= 8.381(6)A, b= 16.65(1)A, c= 4.859(6)A, Z= 4 and a=14.304(7)A, b=10.14(1)A, c= 9.923(4)A, Z= 8, respectively. The δ phase is tetragonal with a= 12.110(2)A, c= 9.472(8)A and Z= 8.

Solid-state reaction pathways of Sillenite-phase formation studied by high-temperature X-ray diffractometry and differential thermal analysis

Abstract This paper reports on the use of high-temperature X-ray diffractometry, in conjunction with differential thermal analysis, to study the solid-state reactions and phase transformations involved in and leading to the synthesis of various (bismuth oxide-based) Sillenite-type compounds. These, which have γ-body centered cubic (bcc) structures related to γ-Bi 2 O 3 , are generally represented by the formula Bi 12 MO 20 ; here, the second cation M=Ge, Si, Ti, Zn, Al, as well as the two-cation combination {Si 0.5 Ge 0.5 }. In each case, using the two techniques, measurements were performed on starting powder-mixtures of bismuth and cation M oxides, formulated with desired proportions to favor the Sillenite (γ-)phase formation. Henceforth, the proceeding of solid-phase reactions and occurring of phase transitions, with rising temperature, were monitored and elucidated by the evolution of the X-ray diffractometry patterns and the appearance of thermal events in the differential thermal analysis curve, as both sets of results were also correlated.

A new family of compounds with tetrahedral anions: PbnBiOn(XO4), X = V, P, As, n = 1, 2, 4. A comparison with lead oxysulfates PbnPbOn(SO4)

Abstract The thermal behavior and crystallographic characteristics of nine compounds, PbnMOn(XO4) with M = Bi, Pb, X = V, P, As, S and n = 1, 2, 4, are reported. A previously reported γ phase for Pb2BiO2(VO4) was shown to consist of a mixture of PbBiO(VO4) and Pb4BiO4(VO4) that resulted from a kinetically controlled decomposition of Pb2BiO2(VO4) at elevated temperatures. The relationship between the α, β and δ transitions for Pb2BiO2(VO4) was clarified. All of these phases contain the tetrahedral anion XO4 that imparts thermal and structural similarities as well as specifications that can be ascribed to anion size differences.

The Bi2O3–Sm2O3 system: phase diagram and electrical properties

The (Bi2O3)1 –x–(Sm2O3)x system (0 ≤x≤ 0.65) was investigated by means of X-ray diffractometry, differential thermal analysis, impedance spectroscopy and e.m.f. measurements. The corresponding equilibrium solid-phase part of the diagram is proposed. The most striking feature is the existence of a very large domain of a δ Bi2O3 f.c.c.-type solid solution. By air-quenching from this domain two different phases were obtained: for 0.03 ≤x≤ 0.07, the room-temperature quenched phase exhibits tetragonal symmetry, changing to cubic for greater values of x. Both these phases are metastable and decompose into equilibrium phases on annealing. This transformation is always accompanied by a significant decrease in the electrical conductivity.

The dependence of polymorphism on the ionic radius of Ln3+ (LnLaEr, Y) in the oxide ion conductors Bi0.775Ln0.225O1.5

Abstract This paper describes the polymorphic transitions of Bi0.775Ln0.225O1.5 (LnLaEr and Y), which is a representative composition of a rhombohedral solid solution over a limited range inside the 20–30 mol% Ln2O3. Three kinds of polymorphs are found: a hexagonal low-temperature modification (β2) isomorphous with Bi0.765Sr0.235O1.383 (BiSrO) type, a hexagonal intermediate (β1), and the well known high-temperature modification (δ) isomorphous with the fluorite type. The way the β2 phase behaves on heating can be divided into two groups depending on the Ln3+ ion: (1) one or two reversible transitions from β2 to β1, or from β2 to β1, then from β1 to δ for La, Pr, Nd, Sm, Eu and Gd; and (2) an irreversible transition from β2 to δ for the others. The thermal behaviour in group (1) closely resembles the BiSrO-type solid solution in the systems Bi2O3MO (MCa, Sr and Ba).

Anionic conduction properties of BiCaPb mixed oxides

Abstract This study of the Bi 2 O 3 CaOPbO system has determined the equilibrium domains of existence of three solid solutions which belong respectively to the fcc (δ-Bi 2 O 3 ), rhombohedral (BiSrO) and bcc (anti-α-AgI) structural types. The first two solid solutions can be preserved at room temperature in a metastable or/and a stable form within a significant range of composition. They have been investigated by impedance spectroscopy and transport number measurements. In all cases, the substitution of Bi 3+ by the divalent cations leads to a decrease of the conductivity. However, for a constant M 2+ concentration, the conductivity increases slightly with the Pb 2+ /Ca 2+ ratio without significant modification of the activation energy. Transport number measurements, performed particularly on the δ-phase, have shown, at low temperature (≤400 °C), a very attractive quick response to any P O2 variation. The corresponding Nernst potentials were shown to be significantly higher than expected from the classical O 2 /O 2- electrode equilibrium.

High temperature X-ray diffraction: An essential tool for phase diagram investigation. evidence of its role in the system Bi2O3-SrO

Abstract Previous descriptions of the Bi2O3-SrO system lead to significant discrepancies due to the use of different techniques. A new investigation, using high temperature X-ray diffraction unambiguously characterizes the different equilibrium phases as a fonction of the temperature. The most striking differences are: the presence of a fully reversible s2 - s1 phase transition in the rhombohedral domain; the thermal phenomenon observed at 780°C for the SrBi2O4 composition has to be associated to the decomposition into two solid solutions; the Sr6Bi2O9 compound is not part of the Bi2O3-SrO system but, due to a necessary stabilizing BiIII - BiV oxydation phenomenon, it must be formulated Sr6BiVyBiIII2−yO9+y with y > 0.

The crystal structure, phase transition, and dielectric properties of K2Bi3(PO4)3O, a new oxyphosphate

The title compound exhibits a transition at about 745 K detectable by DSC and dielectric measurements. The structure was determined at ambient temperature and above the transition. In both cases, the cell is orthorhombic with a = 13.139(4), b = 10.413(6), c = 9.239(6)A at 295 K and a = 13.302(4), b = 10.506(7), c = 9.240(5) A at 795 K, Z = 4. The structures were refined in space group Pnma to R values of 0.048 and 0.058 using 1472 and 1154 unique single-crystal reflections, respectively. The two structures are very similar and can be described as a three-dimensional network of distorted BiO6 octahedra and PO4 tetrahedra. At high temperature, one of the PO4 tetrahedra is statistically disordered over two positions related by the mirror. Below the transition temperature, ordering of this ion leads to doubling of the c parameter and formation of satellite reflections (h, k, l + 12). Extinction rules indicate P21ca as a possible supercell space group.

Synthesis and structure of some Co(III)-Bi(III) heterometallic complexes: [Co(NH3)5NCS][Bi(EDTA)]2.4H2O, trans-[Co(NH3)4(NO2)2][Bi(EDTA)(H2O)].2H2O, and [Co(NH3)4(CO3)][Bi(EDTA)].3H2O

Abstract Three Co(III)–Bi(III) heterometallic complexes: pentaammine-thiocyanato-cobalt(III) ethylenediaminetetraacetatobismuthate(III) dihydrate, [Co(NH3)5NCS](Bi(EDTA)]2·4H2O, (I), trans-tetraamminedinitrocobalt(III) ethylenediaminetetraacetato(aqua)bismuthate(III) dihydrate (trans-[Co(NH3)4(NO2)2][Bi(EDTA)(H2O)]·2H2O, (II), and tetraammine(carbonato)-cobalt(III) ethylenediaminetetraacetatobismuthate(III) trihydrate [Co(NH3)4(CO3)][Bi(EDTA)]·3H2O, (III) were synthesized and characterized by X-ray crystallography. Thermal decomposition of the complexes proceeds in several stages, dehydration, deamination, and finally pyrolysis of ligands leaving sillenite and traces of Co3O4 in the case of (II) and (III), and an unknown phase in the case of (I). Complex (II) is thus a potential precursor for Bi/Co mixtures in the ratio of 1/1.

Anionic conduction properties of the fluorite-type phase in the Bi2O3-Y2O3-PbO system

Abstract The Bi2O3-Y2O3-PbO system study has evidenced a fluorite structure phase easily air-quenchable in a large domain of composition. The investigation of its thermal behaviour by X-ray diffraction, impedance spectroscopy and transport number measurements has shown the existence of two modifications of this fluorite phase, both of which having high oxide conductivities. The isothermal conductivity decreases when Bi3+ is substituted by Pb2+ (or Y3+) but increases as Pb Y+Pb ratio also increases.