Theoretical analysis of H/D isotope effect in K3H(SO4)2 and its influence on phase transition temperature

Abstract

SO 4 ) 2 (KHS) is a typical zero-dimensional hydrogen-bonded dielectric material. KHS remains in a paraelectric state as the temperature decreases from room temperature. In contrast, its deuterium-substituted counterpart, K 3 D(SO 4 ) 2 (DKHS), undergoes a phase transition to an antiferroelectric state at a critical temperature of 84 K. In this study, to clarify the mechanism underlying this isotope effect, path integral molecular dynamics simulations are performed on both KHS and DKHS systems, taking into account both thermal and nuclear quantum effects. The results show that quantum effects lower the free energy barrier, facilitating the distribution of hydrogen atoms between the two stable structures. In other words, quantum effects reduce the degree of order in the arrangement of hydrogen atoms and favor the paraelectric state, consistent with experimental results. Furthermore, quantum effects shift the hydrogen atom distribution toward the central position between two oxygen atoms, thereby drawing the oxygen atoms closer and shortening the oxygen-oxygen distance. This trend is consistent with experimental results, which indicate that the oxygen-oxygen distance in KHS is shorter than that in DKHS.