New Metastable Phase of Bismuth (III) Selenide: Crystal Structure and Electrical Properties

Using high‐pressure–high‐temperature treatment at P = 4–7.7 GPa; T = 1373–1473 K with subsequent quenching, a new metastable phase of bismuth selenide (m‐Bi₂Se₃) is synthesized and its crystal structure, electrical resistivity, and annealing at heating are investigated. Using X‐ray powder diffraction, and analogy with high‐pressure structures of the rare‐earth element sesquichalcogenides, the crystal structure of the new metastable phase of m‐Bi₂Se₃ is determined as an orthorhombic distorted cation‐deficient structure of the Th₃P₄ type with the Fdd2 space group and Z = 10.66. The unit‐cell dimensions are: a = 13.4660(7) Å, b = 12.7772(7) Å, c = 9.0896(5) Å. The density of m‐Bi₂Se₃ (7.47 g cm⁻³) is slightly less than the initial rhombohedral phase of Bi₂Se₃, but the coordination number (CN) = 8 is higher than CN = 6 of the initial phase. The period of stability under ambient conditions of the new m‐Bi₂Se₃ phase is only about 2 months. After 300 °C annealing, m‐Bi₂Se₃ completely returns to the initial layered structure, and differential scanning calorimetry confirms that the reverse transformation as well as the direct transition occur through an amorphous state. The new phase is a narrow bandgap semiconductor with the energy gap of about 80 meV.