Phase transition of the doubly disordered Ba_1−x Sn _x Cl_1+y F_1−y solid solution at high temperature

Abstract

The Ba_1−x Sn _x Cl_1+y F_1−y solid solution has the BaClF structure, with full disorder of the cations and partial disorder of the anions. It can be prepared either by the wet method or the dry method. Most samples prepared by the aqueous route give a tin(II) single line at ca. 4.1 mm/s characteristic of the Sn²⁺ stannous ion, and therefore of ionic bonding. In some rare cases, a very weak quadrupole doublet indicates the presence of a small amount of covalently bonded tin(II). The solid solution prepared by the dry method has different unit-cell parameters, and it contains a larger amount of covalently bonded tin, except at very low x values or highly positive y values, and the Sn²⁺ recoil-free fraction is much smaller. In the present work, a study of the phase transition that takes place when precipitated Ba_1−x Sn _x Cl_1+y F_1−y is heated has been undertaken. A dramatic increase of the amount of covalently bonded tin(II) occurred on heating.