The phase transition sequence of a K 2 ZnBr 4 crystal was investigated by DTA and dielectric measurements. It was found that the crystal undergoes at least two phase transitions, at -118°C and around 200°C, on heating. Ferroelectricity was observed below the lower transition point.

New Ferroelectric K2ZnBr4

Die Systeme AJ/CoJ2 (A = Cs, Rb, K, Tl, Na, Ag) wurden differenzthermoanalytisch untersucht. Die Systeme mit NaJ und AgJ sind rein eutektisch, in den anderen Systemen existiert je eine 2:1-Verbindung A2CoJ4 · Cs2CoJ4 kristallisiert im β-K2SO4-Typ. Fur Rb2CoJ4 wurde erstmals in der Gruppe der Tetrahalogenometallate(II) durch Einkristalluntersuchungen der monokline Sr2GeS4-Typ (C–P21/m) nachgewiesen. K2CoJ4 und Tl2CoJ4 sowie die Doppelbromide T-K2CoBr4 und T-Tl2CoBr4 sind isotyp. In beiden Strukturen liegen isolierte CoX42−-Tetraeder vor, die KZ fur die Alkali-Ionen sinkt aber von 9/10 im Cs2CoJ4 auf 6(+2) im Sr2GeS4-Typ. Reflexionsspektren und magnetische Suszeptibilitaten lassen sich mit der Kristallfeldtheorie interpretieren.



Lucken in der Kenntnis der Systeme des CoBr2 und CoCl2 werden geschlossen.



The Systems AI/CoI2 (A = Alkali Metal, Tl, Ag) and the Crystal Chemistry of the Double Halides AnCoX(n+2) with X = Cl, Br, I



The systems AI/CoI2 (A = Cs, Rb, K, Tl, Na, Ag) were investigated by differencethermal analysis. The systems of NaI and AgI are found to be eutectical. A compound A2CoI4 always exists in the other systems. Cs2CoI4 crystallizes in the β-K2SO4 type with a coordination number (C.N.) for Cs equal to 9/10. Results obtained with single crystal technique reveal for the first time that among the double halides Rb2CoI4 is of the monoclinic Sr2GeS4 type (C. N. for Rb = 6(+2)). The compounds K2CoJ4, Tl2CoJ4, T-K2CoBr4, and T-Tl2CoBr4 are isotypic. Both structure groups are characterized by isolated CoX42− tetrahedra. Reflectance spectra and magnetic susceptibilities can be explained on the basis of crystal field theory. – Our results close presently existing gaps in the knowledge on systems of CoBr2 and CoCl2 too.

Über die Systeme AJ/CoJ2 und die kristallchemischen Beziehungen in der Gruppe der Doppelhalogenide AnCoX(n+2) mit X = Cl, Br, J [1]

Investigation of phase diagrams by DTA and X-ray methods: The systems AX/CoX2 (A = Na-Cs, TI;X = Cl, Br, I)

The structural phase transitions of K 2 ZnBr 4 and K 2 CoBr 4 are investigated by means of X-ray scattering. The transition from the room-temperature α phase to the high-temperature β phase takes place at about 200°C. It is found that the β phase is an incommensurate one with the modulation wave vector q =(1/3-δ) c * , which is followed by a normal phase of Pmcn above 288°C (Zn-salt) or 282°C (Co-salt). If the β phase is supercooled to below room temperature, then the crystal is a commensurate phase with q =1/3 c * . With increasing temperature, the commensurate-incommensurate phase transition is observed at 5°C in Zn-salt. The transition sequence of normal, incommensurate and commensurate phases is quite common in A 2 X Y 4 -type incommensurate materials, though the α phase is very different from others.

Incommensurate-Commensurate Transition in K2ZnBr4 and K2CoBr4

In A2BX4 structures which are isostructural to β-K2SO4 (with A being a monoatomic cation) there are two crystallographically independent cations surrounded by 11 and 9 X-atoms. The 11-coordinated cation is less firmly bound and the arrangement of its five closest neighbours is irregular. One of these contacts is approximately parallel to the pseudohexagonal axis of the structure and is often shorter than the sum of the corresponding ionic radii. A survey of available structural data indicates that the low-temperature structural instability of a good number of these compounds is related to the coordination of this 11-coordinated cation and especially to the bonding strength of this short bond, which is often the shortest cation-anion contact in the structure. Typically, the relative contribution of this contact to the bond-valence sum of the 11-coordinated cation is larger in the compounds which undergo phase transitions at lower temperatures. The presence of this short contact is correlated with ...

I. Al-Khudair

Papers

Über die systeme alkalimetallbromid/kobalt(II)-bromid