The electronic potential of the V-VI-VII compounds in the region of phase transition

This paper deals with the dependence of the electronic potential on normal coordinates for SbSI, SbSBr, SbSeI, SbSeBr crystals at Sb, Bi, S,I, Se, Br sites in the paraelectric phase and for SbSBr crystal in the ferroelectric phase. In the paraelectric phase the normal made of symmetry Ri,, is studied. The large anharmonicity of electronic potential implies that SbSI and SbSBr exhibit intermediate behaviour between a displacive and order-disorder phase transition. The electronic potential of the B-1u mode of SbSeI, SbSeBr, BiSeI, BiSeBr, SbSCl, SbSeCl, BiSCl, BiSeCl at Sb, Bi sites is characterized by a relatively small anharmonicity within the temperature range 20-295 K. The non-ferroelectric nature of these crystals follows from the results. Because of the large atomic radius of Bi compared with Sb, a ferroelectric phase transition is also doubtful in BiSl and BiSBr crystals.