Investigation of the Soft Mode of SbSBrxI1−x Crystals

Abstract

The behaviour of the potential energy V(z) of the B 1u (A 1 ) symmetry soft mode vibrations of atoms in the direction of the z(c) axis of SbSBrxI 1−x crystals (x = 0, 0.2, 0.75, 1) upon the normal coordinate (relative displacements of atoms) was investigated in the region of the phase transition temperatures. For this B 1u (A 1 ) symmetry soft mode, the potential energy barrier height ΔV, the potential energy V(z), its harmonic and anharmonic terms were studied as functions of the mixture composition x and temperature T. The dependence of the soft mode frequency ωS upon the composition x and temperature has been established assuming the double-well shape of the potential energy (per atom) V(z). By employing A 1u (A 2 ) symmetry coordinates, the force constants C of interaction between molecular chains have been obtained. The type of the phase transition has been established by calculating the Rhodes–Wohlfarth factor R. To calculate this factor, the ratio /ΔV depending upon the mixture composition was employed. It has been found that for mixed crystals with x < 0.8, the phase transition is intermediate between order–disorder and displacive types, and for crystals with x > 0.8, the phase transition is of displacive type.