STRUCTURAL PHASE TRANSITION IN BISMUTH HIGH-T_c SUPERCONDUCTING MATERIALS UNDER SYMMETRY OF D¹⁷_4h(I4/mmm)

By applying the Landau's second-order phase transition theory, the symmetry changes due to the continuous phase transition from the phase of D¹⁷_4h symmetry were studied. The possible low symmetry phases were obtained by minimizing the thermodynamic potential expanded in the order parameters up to fourth order. Some related experimental results were discussed.