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The crystal structures of four Cs 3 MI 5  (M = Mn, Cd, Hg) compounds have been determined. Cs 3 MnI 5  ( A ) is tetragonal,  I 4/ mcm , with  a  = 10.187(1) Å and  c  = 16.574(1) Å. It has the Cs 3 CoCl 5  structure type. At room temperature there are two orthorhombic modifications ( B  and  C ) of Cs 3 CdI 5  crystals, both grown from water solution of CsI and CdI 2 . In addition a compound with composition Cs 3 (Cd 0.52 Hg 0.48 )I 5  ( 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 (NH 4 ) 3 ZnCl 5 -type structure, while  C  is very closely related to  D  and the Cs 3 HgI 5  type. All four structures can be described as built from MI 4  2−  tetrahedra and units of Cs 3 I 2+  with I surrounded octahedrally by six Cs. In  A  there is a rotationally distorted ReO 3  type framework of corner-sharing ICs 6  octahedra.  B ,  C  and  D  have columns of face-sharing ICs 6  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 BaNiO 3 . 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 ICs 6  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 Cs 3 CdI 5  polymorphism is discussed. The cell volume per formula unit of the Cs 3 (Cd, Hg)I 5  obeys Vegard's law.

Crystal structures of four Cs 3 MI 5 (M = Mn, Cd, Hg) phases and structural relationship among Cs 3 MI 5 compounds