Thermal and dielectric studies of (NH₄)₃H(SeO₄)₂and its isotope effect

Abstract The specific heat (Cp) of (NH4)3H(SeO4)2 was measured from 90 K to 357 K, and the transition entropies for four phase transitions were estimated. Anomalous parts of Cp observed above and below the I-II transition point were fitted to the power law with the critical exponents α+ = 0. 58 and α = 0. 18. The crystal is ferroelastic in the phase III possessing monoclinic symmetry and is paraelastic in the phases II and I possessing trigonal symmetry. The Curie= Weiss constant increases markedly with increasing deuterium concentration.