Kinetic effects associated with the low-temperature phase transitions in lawsonite

Abstract

The specific heat of natural and deuterated lawsonite, CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O, has been measured in the temperature ranges 77 K-310 K, using ac calorimetry, and 230 K-320 K, using differential scanning calorimetry. Anomalies due to the C mcm - P mcn ( T c = 273 K) and P mcn - P 2 1 cn ( T c = 130 K in natural lawsonite, 143 K in deuterated lawsonite) transitions have been observed and analysed. By comparing these data with the results of earlier conduction calorimetry experiments, the kinetic behaviour of these two transitions was studied. For the C mcm - P mcn transition, the excess entropy only correlates with other measures of the order parameter (Δ S ∝ Q 2 ) in the situation where an ordered lawsonite is heated very slowly. However, the total excess entropy associated with the phase transition does not appear to vary significantly with the rate or direction of the temperature ramp. Extrapolation of the entropy vs . temperature curves to absolute zero implies a maximum excess entropy of 6 J K −1 mol −1 , consistent with the configurational entropy of a two-site mixing model, with a small excess. For the P mcn - P 2 1 cn transition, the various experiments give mutually consistent results, implying that the transition behaviour is limiting second order without kinetic anomalies.