Crystal structures of four Cs₃MI₅ (M = Mn, Cd, Hg) phases and structural relationship among Cs₃MI₅ compounds

Abstract

The crystal structures of four Cs₃MI₅ (M = Mn, Cd, Hg) compounds have been determined. Cs₃MnI₅ (A) is tetragonal, I4/mcm, with a = 10.187(1) Å and c = 16.574(1) Å. It has the Cs₃CoCl₅ structure type. At room temperature there are two orthorhombic modifications (B and C) of Cs₃CdI₅ crystals, both grown from water solution of CsI and CdI₂. In addition a compound with composition Cs₃(Cd_0.52Hg_0.48)I₅ (D) was prepared. B crystallizes in Pnma, a = 10.036(1) Å, b = 11.852(1) Å, c = 14.850(1) Å, and C and D in Pbca, with cell dimensions a = 18.893(1) Å, b = 37.015(4) Å, c = 10.085(1) Å, and a = 18.771(2) Å, b = 18.439(3) Å, c = 10.118(2) Å, respectively. The B compound has the (NH₄)₃ZnCl₅-type structure, while C is very closely related to D and the Cs₃HgI₅ type. All four structures can be described as built from MI₄²⁻ tetrahedra and units of Cs₃I²⁺ with I surrounded octahedrally by six Cs. In A there is a rotationally distorted ReO₃ type framework of corner-sharing ICs₆ octahedra. B, C and D have columns of face-sharing ICs₆ octahedra, extending in each case along the 10 Å axis. In this description A is an antitype of perovskite and the B, C and D structures are related to the antitype of BaNiO₃. The C and D structures can be generated from the B type by glide-reflection twinning in the octahedral planes parallel to (011) of B. D requires twinning in all planes, and C requires that every other plane is a twin plane. The tetrahedral Mn–I distances are 2.690(1) Å and the Cd–I distances are 2.748(1) Å–2.786(1) Å [B], 2.725(3) Å–2.785(3) Å [C], 2.726(2) Å–2.789(2) Å [D, average of Cd–I and Hg–I]. The Cs–I distances in the ICs₆ octahedra are in the range 3.848(1) Å–4.143(1) Å [A], 3.771(1) Å–4.019(1) Å [B], 3.778(3) Å–4.022(3) Å [C], and 3.794(2)Å–3.953(2) Å [D]. In the alternative, conventional, description the Cs⁺ ions are eight or nine coordinated by I in all four structures, except one of the two independent Cs⁺ in A which is ten coordinated. The origin of the Cs₃CdI₅ polymorphism is discussed. The cell volume per formula unit of the Cs₃(Cd, Hg)I₅ obeys Vegard's law.