Geometric and quantum aspects of phase transition and isotope effect in hydrogen-bonded ferroelectrics and related materials

Abstract

Structural phase transitions in hydrogen-bonded (H-bonded) ferroelectrics and related materials are reviewed from the geometrical viewpoint of H-bond system. It is pointed out that the isotope effect in the transition temperature Tc has a correlation with the symmetry of H-bond and its geometric isotope effect. It is shown that among isomorphous members, a liner relation between Tc and O c O distance R is valid in three dimensionally (3-D) H-bonded tetragonal KH2PO4-, 2-D squaric acid (H2C4O4)-, 1-D CsH2PO4-, 1-D PbHPO4-, 0-D M3H(SeO4)2-type crystals. Implication of the linear relation as negative evidence for proton-tunnelling model is discussed. It is emphasised that the geometric characteristic is responsible even for the low-temperature phase transition in M3H(XO4)2-type crystals of which the three members have no phase transition and are quantum ferroelectrics. The disappearance of the transition is discussed as a manifestation at low temperature of isotopic downward shift of the transition temperatu...