J. Grannec

Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

Sequences of phase transitions (PTs) occurring in (AI)2BIMIIIF6 elpasolites have been studied via X-ray diffraction, differential scanning calorimetry (DSC) and adiabatic calorimetry at low temperature (LT). The substitution of NH4+ ions for alkaline ions in sites with either 6 or 12 coordination number considerably affects both transition temperature and entropy change associated with the transition(s). Two main mechanisms are responsible for the PTs: displacive transitions, as for the Rb2KMIIIF6 series, in which the transitions are associated with small octahedral tilts and are followed by a rather small entropy change; order-disorder transitions of rigid octahedra, as for (NH4)2NH4MIIIF6. In this latter case, the fluoride atoms located at the apices of the octahedra are disordered onto several possible positions in the cubic...

On a low-temperature form of KBiF4

Abstract The existence of a low-temperature α form of KBiF 4 has been shown by differential thermal analysis and microcalorimetry as well as by electrical measurements. Properties of α-KBiF 4 are compared to those of K 1− x Bi x F 2 x +1 (0.50 ≤ x ≤ 0.70) with fluorite-type structure and KBi 3 F 10 .

Anionic conductivity of some bismuth fluorides with fluorite-type structure

Abstract The transport properties of fluorite-type Na 1−x Bi x F 1+2x (0.60⩽ x ⩽0.70), K 1−x Bi x F 2+x (0.50⩽ x ⩽0.70), Rb 1−x Bi x F 1+2x (0.50⩽ x ⩽0.60) and Pb 1−x Bi x F 2+x (0⩽ x ⩽0.50) solid solutions have been studied. They are all anionic superconductors. Some of them could be used as electrolytes in solid galvanic cells.

Proprietes magnetiques des hexafluorures Na3MeF6 (Me = Co, Ni, Cu)

Resume Les hexafluorures de type cryolithe Na3MeF6 (Me = Co, Ni, Cu) ont ete prepares en vue detudier leurs proprietes magnetiques. Le cobalt et le cuivre (d6 et d8) comportent une structure electronique a spin fort. Le nickel (d7) adopte en revanche une structure a spin faible a basse temperature qui evolue vers une structure a spin fort a haute temperature. Lenergie dactivation de la transition a ete etudiee.

On the polymorphism of RbBif4

Two low-temperature forms of the fast ionic conductor RbBiF4 have been identified in addition to the already known high temperature form (CaF2-type structure). The investigations used X-Ray diffraction, D.T.A., microcalorimetry and electrical measurements. The electrical properties of RbBiF4 and of the formerly studied KBiF4 are compared.

Thermodynamic properties of the mixed elpasolites Rb2KGaxSc1−xF6 (x=0.6–1.0)

Measurements of the heat capacity of the elpasolite Rb2KGaF6 and the solid solutions Rb2KGaxSc1−xF6 with a gallium content in the range x=0.6–0.95 are performed. The nature of the influence of the chemical composition of the compounds and the hydrostatic pressure on the temperature and entropy of the ferroelastic phase transitions is established.

Transitions de phases dans les composés Na3MX6 (M = Ti, Co, In; X = O, F)

Abstract Structural phase transitions were detected in Na 3 TiOF 5 , Na 3 CoF 6 and Na 3 InF 6 by micro-DTA and X-ray diffraction. Optical measurements were performed on single crystals. Ferroelastic domains were observed for the low-temperature phases. The three compounds are isostructural with phases of general formula Na 3 MF 6 (M = 3d transition or IIIb elements). At high temperature, all these compounds exhibit the cubic Fm—m symmetry which is the prototype phase. The ferroelastic Curie temperature was related to the size and the electronic configuration of the cation M n + .

Le fluorure Sr2ZnGaF9, premier membre d'une nouvelle famille antiferroelectrique

Abstract Sr 2 ZnGaF 9 is a member of the new tetragonal family Sr 2 M II M III F 9 (M II ≡Mg, Mn, Fe, Co, Ni, Zn; M III ≡Al, Cr, Fe, Ga) with unit cell parameters a =20.04(2) A, c =14.3(1) A, space group I 4 1 22. Two phase transitions have been found in Sr 2 ZnGaF 9 at T =245 K and T =650 K, using X-ray diffraction, thermal, optical and dielectric measurements. The room temperature phase is probably antiferroelectric.

Phase transition, ferroelasticity and ferroelectricity in Li3InF6

Abstract The ternary fluoride Li 3 InF 6 exhibits three allotropic forms, α, β and γ. Single crystals of the α and γ modifications have been obtained by the Bridgman technique. The X-ray diffraction study shows the low-temperature α-form to be monoclinic and the high-temperature γ-form to be orthorhombic. On the basis of optical and dielectric measurements, simultaneous ferroelastic and probably ferroelectric behaviours are assigned to this compound.

Some physical properties of d-transition metal fluorides in unusual oxidation states

Abstract Unusual oxidation states can be obtained in d-transition metal fluorides by way of various experimental procedures: high-pressure elementary fluorine, high pressure in the solid state temperature, etc … Relationships between structures and physical properties are described in some selected examples.