Structural analysis of the orthorhombic room- and low-temperature phases of [N(CH₃)₄]₂ZnI₄

Abstract

Bis(tetramethylammonium) tetraiodozincate (TMAZnI4) exhibits two successive structural transitions of first and second order at 218 and 255 K, respectively. The crystal structures of phase I (room temperature) and of phase III (150 K) have been determined by X-ray diffraction. The high-temperature phase I has a fl-K2SO4-type structure with an orientational disorder of the TMA groups. In phase III, the crystal structure is commensurately modulated by an ordered displacement of the rigid ZnI4 tetrahedra and TMA groups. The modulation direction is perpendicular to the pseudohexagonal axis, an orientation which is specific to this compound. Crystal data of phase I: Mr = 691.0, Pmcn, a=9-639(9), b = 16.722 (4), c = 13.254 (6)/~, V = 2163 (3) ,~3, Z = 4, Dx = 224 g cm -3, A(Mo Ka) = 0.7107 A, /.~ = 70-2cm-~, F(000)=1312, R=0.045 for 1276 observed reflections. Crystal data of phase III: Mr = 721-28, Pbc2~, a = 9-574(3), b= 33-020(6), c= 13-106 (3) ,~, V = 4143 (3) ,~3, Z = 8, Ox = 2-31 g cm -3, MMo Ka) = 0-7107/~, ~t = 71-01 cmF(000) = 2624, R = 0.041 for 2886 observed reflections.