Coexistence of monochalocogen and dichalocogen ions in BiSe₂ and BiS₂ crystals prepared at high pressure

A single crystal of bismuth diselenide, BiSe₂, containing both monochalcogen (Se^2-) and dichalcogen (Se₂^2-) ions, was prepared at a high pressure of 5.5 GPa. Its crystal structure, substitution chemistry, and physical properties were investigated. X-ray analysis showed that BiSe₂ is in a monoclinic system (space group C2/m) with the following lattice parameters: a = 16.740(3) Å, b = 4.1410(11) Å, c = 12.027(3) Å, and β = 127.658(13)°. A crystal structure of BiSe₂ can be viewed as a layered structure with stacks of neutral BiSe₂ blocks along the c-axis, or alternatively as a quasi-one-dimensional structure with double chains of BiSe₅ pyramids along the b-axis. Each Bi is coordinated with three Se^2- ions and two Se₂^2- ions, and the bond valence analysis indicated that Bi was trivalent. BiSe₂ and BiS₂ form a solid solution in the whole range while retaining the same structure, and the partial substitution of Sb for Bi is also achieved at 10%. All the compounds show a semiconducting property and diamagnetism that can be attributed to the closed-shell ion core. In spite of the compositional analogy with Bi₂Se₃, BiSe₂ is proven by the first-principles calculations not to be a topological insulator.