THE CRYSTAL-STRUCTURE AND CRYSTAL-CHEMISTRY OF CS2ZNI4 AT ROOM-TEMPERATURE INCLUDING A COMPARISON WITH THE STRUCTURE TYPE OF A(3)BX(5) (A =NH4+, CS B = CO, ZN, HG X = CL, BR, I) COMPOUNDS

Cs2ZnI4 belongs to the family of A(2)BX(4) compounds which at room tem perature have a structure isomorphous to beta-K2SO4 At low temperature s it is known to undergo a sequence of phase transitions. The room tem perature structure crystallizes in space group Pnam with lattice param eters a = 10.812(11) Angstrom, b = 14.461 (5) Angstrom, c = 8.306(2) A ngstrom. The structure was refined to a final R(F) value of 5.68% for 1104 reflections. As beta-K2SO4 the structure is pseudohexagonal. The [ZnI4](2-) tetrahedra are only slightly distorted. Cs1(+) ions have ni ne nearest I- neighbours, Cs2(+) have 11 nearest neighbours. Structure s of type A(2)BX(4) (beta-K2SO4) are closely related to compounds of t ype A3BX5 (A = CS, NH4+; B = Co, Zn, Hg; X = Cl, Br, I) yet phase tran sitions have only been observed in the first. A detailed comparison of the two structure types shows that strong ''underbonding'' of one of the A cations, short X-X distances and the size of the cavities in whi ch the A ions are incorporated are parameters closely connected to the occurrence of phase transitions at low temperatures.