B. Frit

The crystal chemistry and dielectric properties of the Aurivillius family of complex bismuth oxides with perovskite-like layered structures

Abstract Many Aurivillius phases of ferroelectric complex bismuth oxides are currently known. With the formula Bi 2 A m−1 B m O 3m+3 , they all consist of Bi 2 O 2 layers interleaved with perovskite-like A m−1 B m O 3m+1 layers. Their crystal chemistry and their chemical stability in relation to the number, m of octahedral sheets within the perovskite-like layers and to various cationic substitutions, have been analysed. The possibility of their ordered intergrowth, i.e. the formation of the homologous series Bi 4 A 2n−1 B 2n+1 O 6n+9 of mixed-layer structures has been examined and a comprehensive review of their dielectric properties with respect to various chemical substitutions and to different formation processes has been achieved, permitting an outline of their future potential applications.

Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new γ- and δ-phases

Abstract The joint lattice-dynamical model study of the vibrational and elastic properties of paratellurite (α-TeO2) and tellurite (β-TeO2) has been conducted. Emphasis was laid on the crystal chemistry aspects of Raman spectra of these lattices. Results were used to interpret the Raman spectra of two new polymorphs (γ and δ) of tellurium dioxide, and to clarify their relationships with the spectrum of pure TeO2 glass.

Crystal structure, Raman spectrum and lattice dynamics of a new metastable form of tellurium dioxide: γ-TeO2

Abstract The crystal structure of a new metastable form of tellurium dioxide, γ-TeO2 (orthorhombic, P212121 (no. 18); a=4.898 A , b=8.576 A , c=4.351 A ; Z=4) was solved ab initio and refined to RB=0.0387 and Rp=0.115, on the basis of a Rietveld analysis of its powder X-ray diffraction pattern. Each Te atom is coordinated to four oxygen atoms, and its coordination polyhedron has a view of distorted trigonal bipyramid (disphenoid) TeO4E with one equatorial corner occupied by lone pair E. These units frame a three-dimensional network of the same type as the α-TeO2 one. There exist two different kinds of Te–O–Te bridges in γ-TeO2; one of them is nearly symmetric, and the other is highly asymmetric. The former bridges constitute polymeric chains along the Oz-axis. Such a characterization of the γ-TeO2 structure is supported by the analysis of the Raman spectra using the lattice dynamical model treatment in which the lattice vibrations are considered jointly with the elastic properties. All the longwave frequencies and the elastic constants were thus estimated. Possible relations between the structure of the TeO2 glass and the γ-phase are discussed.

Crystal structure of Bi2W2O9, the n=2 member of the homologous series (Bi2O2)BVInO3n+1 of cation-deficient Aurivillius phases

The crystal structure of Bi n 2 nW n 2 nO n 9 n has been solved by single crystal X-ray diffraction data analysis and refined to R=0.046 for 991 independent reflections. Bi n 2 nW n 2 nO n 9 n crystallizes with orthorhombic symmetry, Pna2 n 1 n space group, Z=4, a=5.440(1), b=5.413(1), c=23.740(5) A. The model previously proposed by Watanabe et al., i.e. Bi n 2 nO n 2 n layers interleaved with ReO n 3 n-like slabs of W n 2 nO n 7 n, has been confirmed. The distortion of the W n 2 nO n 7 n octahedral network has been analysed and compared to that observed in homologous Aurivillius phases.

Conductivite anionique des phases appartenant au systeme PbF2 ZrF4

The transport properties of the disordered fluorite-type solid solution Pb1−xZrxF2+2x (0 < x ⩽ 0.18) and of the ordered compounds of formulations Pb5ZrF14 (x = 0.167), Pb3ZrF10 and PbZrF6 have been studied. The disordered phase Pb0.90Zr0.10F2.20 is a fairly good anionic conductor. For x = 0.167, the disordered phase is shown to be a better conductor than the ordered one.

Equilibrium and non-equilibrium phase diagram within the TeO2-rich part of the TeO2-Nb2O5 system

The TeO n 2 n-rich part of the TeO n 2 n-Nb n 2 nO n 5 n system has been investigated by temperature programmed X-ray diffraction and differential scanning calorimetry. Three invariant equilibria have been observed: one eutectic reaction (8 mol% NbO n 2.5 n, T n E n=690±5u2006°C, L n E n TeO n 2 n+Nb n 2 nTe n 4 nO n 13 n), one peritectic reaction (incongruent melting, at 766±5u2006°C, of the Nb n 2 nTe n 4 nO n 13 n compound) and one congruent melting reaction (congruent melting, at 810±5u2006°C, of the Nb n 2 nTe n 3 nO n 11 n compound). A large glass-forming domain has been evidenced (0 to 25 mol% NbO n 2.5 n). The thermal behaviour of these glasses has been followed. The glass transition, crystallization temperatures and the nature of crystalline phases formed have been determined.

Crystal structure of the βBaZr2F10 compound. Relations with the ReO3-type and the fluorozirconate glasses

The high temperature βBaZr2F10 phase crystallizes with triclinic symmetry and unit-cell parameters a = 24.256(9) A, b = 15.383(6) A, c = 9.057(3) A, α = 90.00°(7), β = 112.98°(7), γ = 90.57°(5) (space group P1 or P1, Z = 16). In fact, this triclinic cell is a one-dimensional supercell (a × 4) of a basic pseudomonoclinic subcell with C2c or Cc possible space groups. With this monoclinic subcell, an average structure (space group C2c has been solved and refined to a conventional R = 0.056 for 411 independent reflections. The structure consists of a three-dimensional network of corner- and/or edge-sharing ZrF7 and BaF11 anionic polyhedra. It can also be described either as a regular succession along 0y of identical BaZr2F17 polyhedral layers sharing horizontal and oblique edges, or as a 2D network of interpenetrated zig-zag chains of ZrF7 pentagonal bipyramids, joined together by twisted Ba sheets. Structural relations with the ReO3 type and with the corresponding fluorozirconate glasses are considered.

La structure Pb3ZrF10: Base pour deux nouvelles series homologues de phases ordonnées derivant de la fluorine par exces d'anions

Abstract Examination of the fluorite-related Pb3ZrF10 structure revealed that anion excess is localized within infinite columns of independent square antiprisms [ZrF8]. This monodimensional B2X8 cluster, formed in other known anion-excess fluorite-related superstructures, can be used as a basic structural unit for an original homologous series, formulated AnB2X2n+8 (n ≥ 2) or more generally MmX2m+4, whose compounds BaZrF6 α (n = 2, highly distorted), K2ReF8 (n = 4), Pb3ZrF10 (n = 6), and probably Pb5ZrF14 (n = 10) are the first known members. Another new homologous series AnB2X2n+6 or MmX2m+2, with an infinite column B2X6 of independent [BX7] polyhedra as the basic structural unit, is proposed. K2NbF7 is described as the member n = 4 of this series, whose compounds Pb3TiF9, Ba9Al2F24, and Pb5GaF13 could be the members, respectively, n = 6, 9, 10. It is shown that monodimensional-clusters models can be used successfully for the description of any anion-excess fluorite-related compounds exhibiting cations with wide difference in size.

Dynamics and crystal chemistry of tellurites: 1. Raman spectra of thallium tellurites: Tl2TeO3, Tl2Te2O5 and Tl2Te3O7

Abstract Raman spectra of the three entitled crystals are analysed within the framework of a lattice-dynamical model treatment using preliminary obtained X-ray diffraction data. The short range atomic arrangement and spectrochemical peculiarities of these structures are jointly discussed, which is considered as an initial step for studying the nature of the glass phases in the x Tl 2 O+(1− x )TeO 2 system. The charged TeO 3 2− groups and the neutral TeO 2 quasi-molecules are proposed as the basic units forming the complex tellurite anions. However, no relevant characteristic frequencies can be indicated in the spectra since the interatomic separation in those units are highly variables and their vibrational states are mixed and delocalised.

Crystal structure of a new gallium tellurite: Ga2Te4O11

Abstract Ga2Te4O11 crystallises with triclinic symmetry (space group P1) and unit cell parameters: a = 5.125(1) A, b = 6.559(1) A, c = 8.173(2) A, α = 75.06(2), β = 89.25(2), γ = 69.62(2), Z = 1. Its crystal structure has been refined by a full matrix least-squares process to R1 = 0.023 and wR2 = 0.060 values, on the basis of 2931 independent single crystal X-ray reflections. It can be described as a three-dimensional polyhedral network of independent Te2O6 groups of TeO3 trigonal pyramids and TeO4 disphenoids sharing corners, and infinite (Te2O5)∞ quasi-linear chains of the same corner-sharing TeO3 and TeO4 units, linked one to the other via common corner or common edge by GaO4 and GaO5 polyhedra. The stereochemical activity of the lone pair of each Te atom has been analysed and a comparison is made with all the known other M2Te4O11 crystal structures.