Thermodynamic characterization of the low-temperature phase transformations in KOH and KOD

Abstract

The heat capacities of KOH and its deuterated analog KOD have been measured over the temperature range 17<T<342 K and 32<T<295 K, respectively. The thermodynamics of the phase transformations to the low-temperature antiferroelectric phases have been characterized: Ttr (KOH)=226.7±0.2 K; Ttr (KOD)=253.1±0.1 K; ΔStr (KOH)=0.121±0.002 R; ΔStr (KOD)=0.1258±0.0007 R. The thermodynamic changes are consistent with tunneling of the hydroxide (deuteroxide) ions between two minima in a double-well potential.

Why is there no low-temperature phase transition in NaOH?

“…16 The transition entropies are nearly identical for both compounds, i.e., 1.0 J K Ϫ1 mol Ϫ1 . 72 Thus, it would be expected that excess heat capacity should not exist for KOH at low temperatures. Indeed, the lowtemperature heat capacities of KOH and KOD show normal isotopic behavior, with the heat capacity of KOH slightly less than that of KOD.…”

“…Indeed, the lowtemperature heat capacities of KOH and KOD show normal isotopic behavior, with the heat capacity of KOH slightly less than that of KOD. 72 The existence of excess heat capacity at low temperatures seems to be a common feature of all substances exhibiting deuterium-induced phase transitions. 73 and ͓N͑H,D͒ 4 ͔ 2 PdCl 6 82,83 also shows excess heat capacity for the hydrogenated form at low temperatures, thus providing a mechanism for entropy removal in lieu of a phase transition.…”

Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals

“…At ambient pressure, proton ordering phase transitions occur in all the alkali hydroxides with the exception of LiOH/LiOD and NaOH. 11 − 25 The critical transition temperature, T c , is between 150 and 310 K, and a conventional isotope effect T c (AOD) > T c (AOH) of about 30 K is observed for K, Rb, and Cs. Furthermore, pressure can promote similar transitions even when these are forbidden at ambient conditions.…”

“…In this regard, alkali hydroxides AOX (A = Li to Cs, X = H, D) display stretched and weak HBs. At ambient pressure, proton ordering phase transitions occur in all the alkali hydroxides with the exception of LiOH/LiOD and NaOH. − The critical transition temperature, T c , is between 150 and 310 K, and a conventional isotope effect T c (AOD) > T c (AOH) of about 30 K is observed for K, Rb, and Cs. Furthermore, pressure can promote similar transitions even when these are forbidden at ambient conditions. − Thus, the phase diagram of AOX crystals is determined by a complex interplay between the temperature, the metal cation A + , the hydrogen isotope X, and the pressure.…”