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

¹ Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, Apartado 1065, E-41080 Sevilla, Spain
² Instituto de Ciencia de Materiales de Aragon, CSIC - Universidad de Zaragoza, Pza. San Francisco, E-50009 Zaragoza, Spain
³ Center for Materials Science, University of Oslo, N-0349 Oslo, Norway
⁴ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom

^* e-mail: sah21{at}esc.cam.ac.uk., Present address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom.

The specific heat of natural and deuterated lawsonite, CaAl₂Si₂O₇(OH)₂·H₂O, 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-P2₁cn (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 (S Q²) 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-P2_1cn transition, the various experiments give mutually consistent results, implying that the transition behaviour is limiting second order without kinetic anomalies.

Key-words: lawsonite, phase transitions, H ordering, calorimetry, kinetics..

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