P-T Phase Diagram and Structural Study of the Co–Se System

Abstract

Heat-treatment in selenium atmospheres was used to control defect concentration in the Co–Se system. In the composition range of hitherto reported γ-CoSe, three different regions were demonstrated in discontinuous vapor pressure-composition relationships. Each region may be expressed in the following representative compositions: (CoSe_1.03), (CoSe_1.13) and (CoSe_1.30). A solid solution range was not detected in any of the three regions within the limit of error of chemical analysis.The stable region of each phase is shown in the P-T phase diagram.
X-ray and electron diffraction patterns were obtained for single crystals grown by the Bridgman method or by chemical transport reaction. The (CoSe_1.03) phase has the NiAs type structure. The (CoSe_1.13) phase is consistent with a superlattice of the 3c-type Fe₇Se₈ structure, which is closely related to the NiAs structure by ordering of metal vacancies. The magnitude of splitting and the intensities of superlattice reflections change depending upon the controlled selenium pressure. (CoSe_1.30) is a monoclinic phase of the 2c-type Fe₃Se₄ structure, which can be regarded as a slight distortion of the NiAs structure. The precession X-ray patterns were explained in terms of multiple twinning around an axis perpendicular to the basal plane.
All specimens in these three phases show metallic conductivity, the order of 10⁴ mho/cm at room temperature. The magnetic susceptibility is weakly paramagnetic and nearly independent of temperature.