STRUCTURES AND PHASE TRANSITIONS OF HALOGENOMETALLATES(II) OF GERMANIUM AND TIN WITH MONOVALENT COUNTERCATION. B.R. Serr, G. Wittenburg, D.G. Ebling, H.W. Rotter and G. Thiele, Institut f. Anorg. & Analyt. Chemie und Materialforschungszentrum der Albert-Ludwigs-Universität, D-79104 Freiburg i. Br., Germany

Systematics of structures and phase transitions of the title compounds were studied by X-ray diffraction, Raman spectroscopy, DTA/DSC and impedance spectroscopy. A set of 45 structures at ordinary temperature and 27 high temperature or high pressure phases was used to study the effects of size and shape of the countercation (Rb, Cs, MeNH₃, Me₂NH₂, Me₃NH and Me₄N), the used halogen (Cl, Br or I) and above all to characterize and quantify the influence of the stereochemically active lone pair of divalent germanium and tin [1,2]. Besides other compositions, the main part of the title compounds crystallize in perovskite type structures AMX₃. Normally, M(II) is coordinated by a halogen octahedron. While regular coordination is found among the tin compounds, the environment of Ge(II) is always [3+3] distorted, due to the effect of the lone pair. Raman spectroscopy shows, that Ge(II) and Sn(II) are disordered in the hexagonal packed structures of AMI₃ with A= Me₄N, while they are fixed for A = Me₂NH₂. Many of the studied compounds are polymorphous and transform to the cubic modification at elevated temperatures. The studied phase transitions show reconstructive or dilatative / displacive mechanisms, the HT-phases are strongly disordered due to cation libration and disordering of M(II) in its X₆-octahedron. The transitions are accompanied by drastic changes of the dielectric properties. The characterisation of the thermal behaviour proofs impedance spectroscopy as a splended tool for the examination of phase transitions.

[1] B.R. Serr, G. Heckert, D.G. Ebling, H.W. Rotter and G. Thiele, J. Mol. Struct. 348 (1995) 95-98.

[2] U. Schwarz, H. Hillebrecht, M. Kaupp, K. Syassen, H.G. v. Schnering and G. Thiele, J. Solid State Chem. 118 (1995) 20-27.