For CuInP2S6 layered crystal the temperature anomalies of the heat capacity and entropy at the ferrielectric ordering are explained based on the theoretical studies which predict the appearance of a critical endpoint and a bicritical endpoint on the phase diagram. Thus, the splitting of the anomaly of the heat capacity in Ag0.1Cu0.9InP2S6 mixed crystal can be interpreted as a transformation of the ferrielectric first-order phase transition, that is observed for CuInP2S6, into the sequent paraelectric–ferrielectric second-order phase transition and the isostructural first-order transition with a sharp change of the polarization in the ferrielectric phase.

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