Thermodynamic properties and crystal structures of nonstoichiometric niobium selenides NbSe/x/ for x = 1.48-2.00

The pressure-composition-temperature relations for niobium selenides NbSe(x) were determined by annealing the sample under controlled selenium vapor pressure in the composition range x = 1.48-2.00. There exist five phases: Nb2Se3, 2H(a), 3R, 2H(beta) and 4H. The 2H(a) and 3R phase are known to be stable over a wide range of nonstoichiometric composition; the 2H(beta) and 4H phases have an extremely narrow range at composition x = 2.00. The defect structures of 2H(a) and 3R phases were analyzed using statistical thermodynamics.