The Ba_1-xSn_xCl_1+yF_1-y Solid Solution

Abstract

A tetragonal distortion (P4/nmm) of the β-PbF₂ fluorite type is obtained by 50% substitution of F by Cl, and it gives the PbClF structure. In PbClF, both anions contribute to the conduction, however it is a poor conductor, but it is purely ionic. We have found that one can substitute up to 25% Ba by Sn and, up to 15% Cl by F (y<0) or up to 25% F by Cl (y>0), to give a Ba_1-XSn_xCl_1+yF_1-y solid solution (0 ≤ x≤ 0.25) which has the same structure as BaCIF (PbCIF type), with full Ba/Sn disorder, whereas the anion sites remain ordered like in unsubstituted BaClF. However, there is disorder between -yF and (1+y)Cl on the Cl site if y<0, or between yCl and (1-y)F on the F site if y>0. Surprisingly, tin(II) is present in the ionic Sn²⁺ stannous ion form instead of being covalently bonded. This is established from the Mössbauer parameters (δ≈ 4.05mm/s, Δ≈0). This also makes it possible to have a significant electron conductivity from the non-hybridized tin (II) lone pair.