Phase-Transition Behaviors of Crystalline Hypophosphorous Acid and a Feasible Incommensurate Wave-like Array of the Proton Displacements from Each Center of Hydrogen Bonds along the 1-Dimensional Chain

Abstract

Crystalline hypophosphorous acid, composed of strongly hydrogen-bonded chains, was investigated by adiabatic calorimetry in a temperature range between 20 and 306 K and by dielectrometry as a supplemental means. The heat-capacity curve showed subsequent phase transitions: a second-order one at 210.4 K followed by a first-order one at 159 K and the other two ones at 149 and 139.4 K with decreasing temperature. The total entropy of the transitions was assessed to be 0.35 J K–1 mol–1 by using the baseline heat capacities that were determined by means of a real-coded genetic algorithm. The small value was interpreted as the transitions were associated with displacements of the protons forming hydrogen bonds: namely, while they are located at the center between two oxygen atoms above 210.4 K, the protons are displaced close to one of the oxygen atoms with the bond length enlarged below it. Temperature dependence of dielectric constants displayed no sign, around 210 K, indicating the existence of (anti-)ferroelectricity. It was thus expected that the manner of off-center proton displacements along each hydrogen-bonded chain reveals a wave-like and incommensurate appearance below 210.4 K, referred to as the normal crystal lattice above it, and following locking to a wave-like but superlattice commensurate one at 159 K.