STRUCTURAL RELATIONSHIPS AMONG Cs₃MI₅ PHASES. Rune Sjövall and Christer Svensson, Dept. of Inorganic Chemistry 2, Lund University, P.O. Box 124, S-221 00 Lund, Sweden.

Compounds Cs₃MI₅ with M a d-element crystallize in a limited number of type structures. We have investigated structures with separate MI₄^2- ions and infinite nets of composition (Cs₃I)²⁺.

The main structure types are those of Cs₃CoCl₅ [1] represented by Cs₃MnI₅, space group I 4/mcm with a = 10.187(1), c = 16.574(1) Å, and (NH₄)₃ZnCl₅ [2] represented by Cs₃CdI₅, space group Pnma with a = 10.036(1), b = 11.852(1), c = 14.850(1) Å.

There is also the Cs₃HgI₅ type [3] in space group Pbca with a = 18.789(3), b = 18.433(3), c = 10.106(2) [4]. A mixed Cd-Hg compound with composition Cs₃(Cd_0.52Hg_0.48)I₅ crystallize in the same type with a = 18.771(2), b = 18.439(3), c = 10.118(2) Å. Further, we have found a new superstructure for another form of Cs₃CdI₅ with space group Pbca and a = 18.893(1), b = 37.015(4), c = 10.085(1) Å.

The Cs₃MnI₅ structure is a distorted antitype of perovskite with corner shared octahedra of Cs⁺ around central I^- ions, and MI₄^2- ions corresponding to the large cations of perovskite. The other structures in space groups Pnma and Pbca are antitypes of BaNiO₃ with different orientations of the tetrahedral ions corresponding to the Ba²⁺ ions and slightly different displacements of the infinite chains of Cs⁺ octahedra (around I^- ions) sharing opposite faces.

References:

[1] H.M. Powell and A.F. Wells (1935) J. Chem. Soc. 1935, 359-362.

[2] H.P. Klug and L. Alexander (1944) J. Amer. Chem. Soc. 66, 1056-1064.

[3] P.M. Fedorov, V.I. Pakhomov and I.N.Ivanova-Korfini (1975) Koordin. Khim. 1, 1569-1571.

[4] R. Sjövall and C. Svensson (1988) Acta Crystallogr. C44, 207-210.