Structural studies of CsKBi mixed chlorides relation to the crystal structures of A2BMX6, A3MX6, and A2MX6

Abstract

The room-temperature crystal structures of Cs 3BiCl 6(I) and CsK 2BiCl 6(II) have been determined from X-ray single-crystal data. These compounds are both monoclinic, space group C2 c , with eight formula units per cell and cell parameters a = 27.017(17), b = 8.252(8), c = 13.121(10) Å, β = 99.70(11)° (I), a = 25.653(13), b = 7.799(5), c = 12.874(9) Å, β = 99.24(8)° (II). The crystal structures have been refined to final R values of 0.069(I) and 0.044(II) from 1706(I) and 2008(II) independent reflections and 93 variable parameters. The BiCl 6 octahedra are slightly distorted (average d BiCl = 2.699 Å) and the alkaline cations are coordinated with eleven or eight chlorine atoms. These structures are better described using the notion of “layers of close-packed octahedra” and can therefore be easily compared with the A 2 BMX 6 (elpasolite type), A 3 MX 6 (cryolite type), and A 2 MX 6 (K 2PtCl 6 type) crystal structures. In these compounds, the “layers” of isolated octahedral MX 6 entities ( X = F, Cl, Br, O) define “tetrahedral” and “octahedral” holes, entirely or partially occupied by the A and/or B cations.