STRUCTURE AND NATURE OF THE LOW-TEMPERATURE PHASE-TRANSITION IN M3H(XO4)2-TYPE CRYSTALS WITH A ZERO-DIMENSIONAL H-BOND NETWORK

A review is given of the studies of M3H(XO4)2-type crystals (M = K, Rb, Cs; H = H, D; X = S, Se) with a zero-dimensional (0-D) H-bond network, focusing on the characteristic features of their structures and the nature of the low-temperature phase transition. It is shown that a linear relation between the transition temperature T(c) and H-bond distance R(O...O) is valid as in the 1-D to 3-D cases (T(c) = 4154 (R-R0), where T(c) is in K, R in angstrom, and R0 = 2.495 angstrom). The large value of R0 is also discussed. The (possible) antiferroelectric, higher-order and order-disorder characteristics of the transition are described. The origin of the disappearance of the transition in three hydrogenated crystals is discussed in connection with the quantum effect of the H-bond. The need to obtain a unified picture of the isotope effect including the geometric and the quantum aspects is emphasized.