The crystal structure and crystal chemistry of Cs2ZnI4 at room temperature including a comparison with the structure type of A3BX5 (A = NH4+, Cs; B = Co, Zn, Hg; X = Cl, Br, I) compounds
Abstract
Cs2ZnI4 belongs to the family of A2BX4 compounds which at room temperature have a structure isomorphous to β-K2SO4. At low temperatures it is known to undergo a sequence of phase transitions. The room temperature structure crystallizes in space group Pnam with lattice parameters a = 10.812(11) Å, b = 14.464(5) Å, c = 8.306(2) Å. The structure was refined to a final R(F) value of 5.68% for 1104 reflections. As β-K2SO4 the structure is pseudohexagonal. The [ZnI4]2− tetrahedra are only slightly distorted. Csl+ ions have nine nearest I− neighbours, Cs2+ have 11 nearest neighbours.
Structures of type A2BX4 (β-K2SO4) are closely related to compounds of type A3BX5 (A = Cs, NH4+; B = Co, Zn, Hg; X = Cl, Br, I) yet phase transitions 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 which the A ions are incorporated are parameters closely connected to the occurrence of phase transitions at low temperatures.
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