非化学量論化合物NbSe_x (x=1.48∼2.00)の熱力学的性質と結晶構造

抄録

The pressure-composition-temperature relations for niobium selenides NbSe_x were determined by annealing under controlled selenium vapor pressure in the composition range x=1.48∼2.00. Five phases were confirmed. Single crystals of these phases were grown by the chemical transport method, and their structures were determined by X-ray diffraction. Their ranges of stability are:
(1) Hexagonal 4 H phase: above 5 Torr Se₂ partial pressure at 900°C for x=2.00.
(2) The phase below 5 Torr Se₂ partial pressure for x=1.99∼2.00 is a 2 H structure but probably different from 2 H (b) structure proposed by F. Jellinek, designated as 2 H (β).
(3) Rhombohedral 3 R phase: 1.80<x<1.86 at 1000°C and 1.79<x<1.91 at 900°C.
(4) Hexagonal 2 H (a) phase: 1.68<x<1.82 at 1100°C and 1.66<x<1.76 at 1000°C.
(5) Monoclinic Nb₂Se₃ phase: 1.48<x<1.52 at 1100°C and 1000°C.
Within the stability range of the 3 R and 2 H (a) phases, the composition varies with Se₂ pressure. These variations can be explained on statistical thermodynamic grounds by assuming that excess Nb atoms randomly occupy the octahedral interstitial sites of a stoichiometric NbSe₂ structure.