Jean Pannetier

An approach to the structure of quasicrystals: A single crystal X-ray and neutron diffraction study of the R-Al5CuLi3 phase

Abstract The structure of the R-Al5CuLi3 cubic phase, expected as being related to the icosahedral T2-Al6CuLi3 quasicrystal, has been determined by single crystal X-ray and neutron diffraction. This structure is discussed with respect to the one of (Al, Zn)49Mg32. A description is made in terms of successive polyhedral shells with two triacontahedra whose sizes are in the ratio of the golden mean τ = (√5 + 1) 2 .

Nouvelle description de la structure pyrochlore le compose Cd2Nb2O6S

Abstract A new description of the A 2 B 2 X 6 X′ pyrochlore structure is given. The structure is visualized as two interpenetrating networks with cristobalite type like in Cu 2 O. One is formed of the A 2 X′ sublattice, the second network B 2 X 6 is described in the same manner, the B atoms having a tetrahedral arrangement. The centre of the tetrahedron is unoccupied and the X atoms, with variable positions, are near the middle of the tetrahedron edges. The new pyrochlore Cd 2 Nb 2 O 6 S is described as an arrangement of Cd 2 S and Nb 2 O 6 sublattices.

Icosahedral crystals: neutron diffraction tells you where the atoms are

Neutron diffraction data were obtained from a single-phase icosahedral powder of the system Al74Si5Mn21 and its modification by isomorphous substitution on the Mn sites. Amplitudes and phase differences of the partial structure factors (FAl, FMn) were determined. From their Qperpendicular to -dependences within a strip-projection approach, phases were reconstructed. Atomic densities were then calculated in the physical space and in the six-dimensional periodic lattice, resulting in the first experimentally deduced decoration of the quasi-periodic network. The Mackay icosahedron no longer appears as the necessary basic structural unit. In six dimensions the structure has a simple CsCl-like space-group symmetry.

About SnF2 stannous fluoride. I. Crystallochemistry of α-SnF2

Abstract The crystal structure of monoclinic stannous fluoride α-SnF 2 has been refined from single-crystal X-ray data. The unit cell contains four cyclic Sn 4 F 8 tetramers. The structure contains two types of Sn atoms: Sn(1) is surrounded tetrahedrally by three fluorine atoms and a lone pair, E , and Sn(2) is surrounded octahedrally by five fluorine atoms and a lone pair. The structure is examined within the framework of Galys and Browns models. Topological relationships to rutile are presented.

MSnF4 (M = Pb++, Ba++, Sr++) : Thermal expansion and phase transitions

Abstract The unit-cell parameters of PbSnF 4 , BaSnF 4 and SrSnF 4 were measured from liquid N 2 temperature up to 400° C. PbSnF 4 undergoes two phase transitions at ∼ 260° and 380°C. One of the new phases (β) can be quenched to room temperature. All three phases have structures related to that of fluorite. Most of our results conflict with recent publication (Mat. Res. Bull., 13 , 877 (1978)).

Ferroelectric and antiferroelectric materials with pyrochlore structure

Abstract Several compounds with the pyrochlore structure have been investigated. Dielectric and structural studies of the compound Cd2NB2O6S show the existence of three phase transitions at 106°C, 184°C and 282°C. The ferroelectric properties have been studied by several methods (x-Ray, DTA, dielectric and dilatometric methods, second harmonic generation). We also discuss some structural hypotheses. Bi2MIIIMvO7, oxides (MIII=Cr, Fe, In, Sc; Mv=Nb, Ta, Sb) with the cubic pyrochlore structure have been prepared. Some compounds exhibit broad maxima in the temperature dependence of the dielectric constant and the tangent of dielectric loss angle.

Structural prereactional transformations in Ca(OH)2

Abstract The nature of the lattice disorder generated by prereactional phenomena in Ca(OH)2 is investigated by single-crystal neutron diffraction. A contraction of the layers is observed and the resulting distortions along the c axis increase; above a critical concentration of defects, the distortions along the c axis can no longer increase and a disorientation of the layers occurs to accomodate the newly created defects. This disorientation causes the relaxation of the distortions. The characteristic temperatures at which the above mentioned phenomena occur are strongly dependent on the water pressure and take place far below the temperature of onset dehydration.

About SnF2 stannous fluoride. II. Crystal structure of β- and γ-SnF2

Abstract Two new structural modifications (β and γ) of SnF 2 have been prepared and their structures refined from X-ray powder data by analogy with high- and low-pressure TeO 2 . Both structures are described and discussed using Galys and Browns models. Topological relationships to rutile and cristobalite structures are outlined.

A neutron diffraction and119Sn Mössbauer study of PbSnF4 and BaSnF4

Abstract119Sn Mössbauer data have been recorded for α- and β-PbSnF4 and BaSnF4. A temperature dependent neutron diffraction study on PbSnF4 clearly indicates the phase transition temperatures. BaSnF4 is confirmed to have the tetragonal P4/nmm structure.

Neutron structure refinement of γ- and β-lithium lodate: Comparison between α, γ, and β phases

Abstract Neutron powder diffraction patterns of (γ + α) and β lithium iodate samples are used to refine the structure of γ (at 488 K) and β (at 513 K) LiIO 3 . All three modifications are built up from Li + and (IO 3 ) − pyramids but lithium is six-, five-, and fourfold-coordinated to oxygen in the α-, γ-, and β-phases, respectively. The study of IO 3 group displacements through the α → γ phase transition suggests that the γ phase structure is closely related to a mixed orthohexagonal description of the α structure where both enantiomorphic forms of the hexagonal unit cell are present. The existence of a new α′ modification is postulated in view of neutron diffraction and NMR results. A simulation of the structure of hexagonal α-LiIO 3 is also presented.