On the phase transition in foreign phase inclusions in CsH2AsO4, KD2PO4, and KH2PO4 crystals

The low-frequency internal friction Q ^?1 and the shear modulus G in a paraelectric phase of CsH₂AsO₄, KD₂PO₄, and KH₂PO₄ ferroelectrics were studied using a reversed torsion pendulum method. Anomalies in the Q ^?1(T) and G(T) dependences were observed above the Curie temperatures of these crystals, at temperatures 308, 253, and 293 K, respectively. The anomalies were associated with a first-order phase transition $(\bar 42m \to mm2)$ occurring in the foreign phase inclusions.