The high temperature structure of β-SnS and β-SnSe and the B16-to-B33 type λ-transition path

Abstract

Abstract The high temperature modifications β-SnS and β-SnSe crystallize in the orthorhombic TlI-type structure [Cmcm, No. 63; SnS at 905 K: a = 4.148(2), b = 11.480(5), c = 4.177(2) Å; SnSe at 825 K: a = 4.410(6), b = 11.705(7), c = 4.318(4) Å]. For the λ-type phase transition α → β from the GeS-type (B16) to the TlI-type (B33) the atomic shifts were determined from about 200° below the critical temperature to Tc (878 K and 807 K). They show a continuous transition from threefold to fivefold bonded atoms. This change in topology and bond character corresponds to the model of a SN2 chemical reaction. There is an indication of a possible first-order transition near Tc .