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

Abstract

The specific heat (C_p) 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.