Calorimetric Study of the Glassy State. XIII. Thermodynamic Properties of Normal and Deuterated Orthoboric Acid Crystals

Abstract

Abstract Thermal properties of normal and deuterated orthoboric acids were studied through the measurements of heat capacity in the temperature range from 13 to 370 K by using an adiabatic calorimeter and differential thermal analysis curves above room temperature. For both crystals, the heat capacity anomaly was found around 290 K in the heat capacity values dependent upon the thermal history of the specimen; i.e., the endothermic or exothermic enthalpy relaxation was observed in this temperature range. This behavior is of characteristic to the glass transition and is considered to be ascribed to the freezing-in phenomenon of the motion of rearrangement of the protons in hydrogen bondings. The enthalpy relaxation curves were analyzed with the exponential law and the characteristic time constant toward the equilibrium state was longer for enthalpy-excessive side than for enthalpy-deficient side. The glass transition temperature at which the endothermic relaxation time becomes 1 ks is 296.6 K for normal orthoboric acid and 298.2 K for deuterated orthoboric acid, respectively. The activation enthalpies were estimated to be 88±5 and 91±5 kJ mol−1 for the endothermic processes of normal and deuterated orthoboric acid, respectively. The melting points of normal and deuterated orthoboric acids were determined from the differential thermal analysis curves to be 169.9 and 167.4 °C, respectively.