X-ray diffraction and differential thermal analysis data obtained in the Cu_1.95Ni_0.05S phase-transition region are analyzed. It is established that the low-temperature rhombic α phase in Cu_1.95Ni_0.05S transforms to the hexagonal β phase at temperatures of 370–390 K and to the cubic γ phase at temperatures of 740–765 K according to the scheme $$\alpha \to \mathop {\alpha + \beta }\limits_{370 - 390K} \to \mathop {\alpha + \gamma }\limits_{740 - 765K} \to \gamma $$ α → α + β 370 − 390 K → α + γ 740 − 765 K → γ . It is determined (using the temperature dependence of differential thermal analysis) that the transition α → β is accompanied by heat absorption while the transition β → γ is accompanied by heat release. It is found that both transitions are allowed and belong to the reconstructive type. Both transitions are found to occur in a fluctuation volume of ~10^–20 cm^3 at temperature rates of 0.11 and 0.08 K^–1. It is demonstrated that the transition α → γ is accompanied by alternation of the structures passing through the intermediate β phase, which is incommensurate with respect to the α and γ phases.

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