Physical properties and phenomenological theory around the successive high-temperature phase transitions of (C2H5NH3)2CuCl4

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Abstract
Calorimetric, thermal expansion and optical measurements as well as optical observations confirm the sequence of structural phase transitions D4h17(I) to D2h18(II) to D2h12(III) in the perovskite layer compound (C2H5NH3)2CuCl4. For the I to II change the results show second-order character together with the appearance of ferroelasticity in the crystal whereas first-order character is found for the II to III change. A thermodynamic theory of the elastic and dielectric properties is developed. From the free-energy expansion compatible with both structural changes in the crystal the authors obtain the temperature dependence of the material constants which is in agreement with the experimental results in this work. The coupling of the order parameter as well as the origin of the spontaneous ferroelasticity are discussed.