J.P. Laval

Structural characterization of NaMgH2F and NaMgH3

The direct reaction of hydrogen on a mixture of Na + Mg or NaF + Mg allowed for synthesis of NaMgH3 and NaMgH2F, respectively. Both phases were indexed with an orthorhombic unit cell with dimensions a = 546.34, b = 770.30, c = 541.08 pm for NaMgH3 and a = 547.59, b = 769.68, c = 540.31 pm for NaMgH2F. The crystal structures were refined by X-ray Rietveld refinement. They derive from the perovskite structure type by a Pmm → Pnma distortion. In the hydridofluoride, H− and F− anions are equally distributed in the two anionic sites in a disordered way.

Defect structure of anion-excess fluorite-related Ca1−xYxF2+x solid solutions

Abstract On the basis of the transformation of a cube within a fluorite-type matrix into an archimedean antiprism of the kind found in numerous ordered anion-excess fluorite-related superstructures, a new polyhedral cluster, labeled 4:4:3, [according to B. T. M. Willis, Proc. Br. Ceram. Soc. 1, 9 (1964) and A. K. Cheetham, B. E. F. Fender, and M. J. Cooper, J. Phys. C 4, 3107 (1971) ], is proposed to explain the defect structure and short-range order in Ca1−xYxF2+x solid solution. In agreement with the spectroscopic, dielectric, and electric experiments, this new structural model fits perfectly well the measured occupation numbers for normal F and interstitial F′ and F″ fluorine atoms for the whole range of compositions without requiring the too short F′ue5f8F′ distances generated by the previously proposed 2:2:2 or 3:4:2 clusters. Such 4:4:3 clusters and nearly identical 4:4:4 or 4:4:5 ones, are probably present in the other highly and moderately doped Ca1−xLnxF2+x solid solutions; they could be precursors for the largest clusters, i.e., ordered microdomains, observed for samples annealed for a long time.

Short-range order in heavily doped CaF2:Ln3+ fluorites: A powder neutron diffraction study

Abstract The Ca0.68Ln0.32F2.32 series (Ln=La, Nd, Tb, Ho, Er, Yb, Lu) of anion-excess fluorite-related disordered phases has been studied by a time-of-flight powder neutron diffraction technique. All the structure refinements taking into account the information provided by Fourier-difference maps analysis, clearly indicate a steady evolution of the defect structure. With increasing dopant cation size, F interstitials ( 1 2 , u, u; u≅0.37) prominent for low-size cations (Lu, Yb) are progressively replaced by F″ ones (v, v, v; v≅0.41) and relaxed normal fluorine F″″ atoms (w, w, w; w≅0.31). These results can be explained by the presence for high-size dopant cations (La→Tb) of small 1 : 0 : 3 cluster involving two ninefold coordinated Ln3+ cations, and for low- and medium-size dopant cations (Ho→Lu) of a mixture of small 1 : 0 : 3 clsters and large cuboctahedral 8 : 12 : 1 ones, the number of the latter increasing with the decrease of the dopant cation radius.

Short-range order in the anion-excess fluorite-relatedCa0.68Ln0.32F2.32solid solutions: EXAFS study of theLn3+environment

Abstract Anion-excess fluorite-typeCa0.68Ln0.32F2.32series and Ca2LnF7-related low-temperature superstructures were studied by EXAFS. This analysis, completed with a structure refinement of Ca2LuF7, shows the narrow analogy between clustering in ordered and disordered states. It confirms the change in short-range order whenLn3+size increases, as previously evidenced by a neutron diffraction study.

Neutron diffraction study of the anion-excess fluorite-related Ca1−xThxF2+2x solid solution

Abstract The defect structure of Ca1−xThxF2+2x solid solution (0 ≤ x ≤ 0.18) has been examined by powder neutron diffraction. Two kinds of 〈xxx〉 interstitials whose respective numbers increase linearly with increasing dopant concentration, can be distinguished: one labeled F″ ( x ⋍ 0.41 ) is a true interstitial anion, the other noted F‴ ( x ⋍ 0.30 ) can be considered as a relaxed normal F anion. Two models of defect clusters, compatible with the observed distribution of F anions between normal and interstitial sites, and containing, respectively, one and two Th4+ cations, are proposed. In both clusters, Th4+ is 10-fold coordinated. Such clusters, which seem characteristic of large-size dopant cations, are suggested as possible defect-models for the Pb1−xThxF2+2x solid solution.

Etude par diffractions X et neutronique en temps de voi de la solution solide de type fluorine excedentaire en anions Pb1−xZrxF2+2x (0 ≤ x ≤ 0,18)

Abstract Anion-excess fluorite-type solid solution Pb 1− x Zr x F 2+2 x (0 ≤ x ≤ 0.18) has been studied by time of flight neutron diffraction techniques. These experiments have been completed by some low-temperature measurements and by single crystal X-ray diffraction study of a partially ordered phase ( x = 0.175). The distribution of fluorine atoms between normal and interstitial sites as a function of the substitution rate is given. The important formation of F″ interstitials and of vacancies observed for low doping concentrations ( x ≤ 0.05) is followed by a linear increase of the numbers of interstitials and vacancies. A “relaxed 4:4:2” cluster, based upon the same structural principle as the 4:4:3-one observed for the homologous Ca 1− x Y x F 2+ x solid solution, i.e., transformation around each substituted Zr atom of a fluorite cube into a square antiprism, is proposed. Mainly because of the high difference in size between Pb 2+ and Zr 4+ cations, the formation of this cluster involves high relaxation of both interstitials F i and surrounding normal F atoms. With increasing Zr concentration, a linear aggregation of these 4:4:2-clusters occurs, leading to more complex clusters containing 2, 3, 4, or 5 Zr 4+ cations; the highly relaxed F atoms are then pushed to the two ends of these new clusters. Such clusters could be precursors for the infinite monodimensional (Pb 4 Zr 2 F 16 ) ∞ clusters observed in the anion-excess fluorite-related superstructure Pb 3 ZrF 10 , and which are probably present in the fully ordered Pb 5 ZrF 14 phase ( x = 0.167).

Short-range order in the Ca1−xLaxF2+x solid solution: 1: 0: 3 or 1: 0: 4 clusters?

Abstract The defect structure of the Ca 1− x La x F 2+ x solid solution (0 ≤ x ≤ 0.38) has been examined at room temperature by powder neutron diffraction. Two kinds of 〈 xxx 〉 interstitial anions, whose respective numbers increase linearly with increasing dopant cation concentration, have been found: one labeled F″ ( x ⋍ 0.41 ) is a true interstitial; the other labeled F‴ ( x ⋍ 0.31 ) can be considered a relaxed normal anion. Two 1:0: n defect clusters are compatible, within the experimental errors, with these results: the 1:0:3 (1 V F , 0F′, 3F″, 2La 3+ ) and the 1:0:4 (1 V F , 0F′, 4F″, 3La 3+ ) clusters. Charge balance considerations and comparisons with the homologous Ca 1− x M IV x F 2+2 x solid solutions ( M IV = Th, U) allow us to think that the less dense 1:0:3 cluster is present for the whole domain of both kinds of solid solutions.

Une nouvelle structure ordonnee derivee de la fluorine : Pb3ZrF10

Abstract Pb3ZrF10 crystallizes with orthorhombic symmetry, space group : Cmcm and unit-cell parameters : a = 10,713(3) A , b = 12,817(4) A , c = 5,909(2) A (Z = 4). Its structure has been solved and refined to a conventional R = 0,041 for 350 independent reflexions. It can be described as an ordered intergrowth of (Pb2F4)n fluorite like monodimensional units and (Pb4Zr2F16)n columnar clusters built of two isolated square antiprisms sharing faces with four [PbF11] complex polyhedra. A structural mechanism based on the transformation of edgesharing fluorite cubes to isolated square antiprisms is proposed to explain the change from fluorite to the anion-excess fluorite like Pb3ZrF10 structure. Analogies with homologous structures are discussed.

Cationic ordering in tysonite type structures I: crystal structure of BaTeF6

Abstract BaTeF 6 crystallizes with orthorhombic symmetry (space group Fdd2) and the unit-cell parameters: a xa0=xa01505.9(3) pm, b xa0=xa01520.6(3) pm, c xa0=xa0900.5(2) pm, Z xa0=xa016. Its structure was solved and refined with SHELXL93 to R 1 xa0=xa00.030 and wR 2 xa0=xa00.077 on the basis of 1429 independent reflections recorded from a single crystal with an automatic 4-circle diffractometer. The Te(IV) atoms are 5-fold coordinate and their lone pair E is stereochemically active. The Ba atoms are 11-fold coordinate. This structure derives from the tysonite type (LaF 3 ) by a cationic ordering between BaF 11 Edshammar polyhedra and TeF 5+6 polyhedra resulting from the distortion of similar Edshammar polyhedra under the influence of the stereochemically active lone pair of Te 4+ .

Structural characterization of new hydridofluorides KCaH3−xFx with x=1; 1.5; 2; 2.5

Abstract The direct reaction of hydrogen on a mixture KF, CaH 2 and CaF 2 in stoichiometric proportions allowed to synthesize a series of discrete phases KCaH 3− x F x with x =1, 1.5, 2 and 2.5. All phases were indexed with an orthorhombic unit cell [e.g.: a =623.47(5), b =881.05(8), c =620.83(6) pm for KCaHF 2 ]. Their crystal structures are isotypic with KCaF 3 and have been refined by a X-ray Rietveld study. They derive from perovskite type by a Pm -3 m → Pnma distortion. Contrary to the homologous NaMgH 2 F in which H − and F − anions are equally distributed in a disordered way in the two anionic sites 4c and 8d, in the KCaH 3− x F x series, the hydride ions preferentially occupy the 8d site and begin to occupy the 4c site only for the two compositions richer in hydrogen.