POTENTIALITY OF AN AC CALORIMETRIC METHOD IN THE STUDY OF PHASE-TRANSITIONS

To manifest potentiality of an ac calorimetric method, successful appl ication of a light-irradiation ac calorimetric method is introduced in the studies of phase transitions of quartz, ferroelectric BaTiO3, and two-dimensional antiferromagnetic K2NiF4-family. In quartz, the alpha -to-incommensurate and the incommensurate-to-beta phase transitions, w hich Lie in a narrow temperature range, were clearly separated by this method. In BaTiO3, the heat capacity at first-order transition was me asured with high precision and, therefore, it was found that the detai led behavior at the first-order transition is well-expressed in terms of the free energy expansion. As a result, in a stress-clamped crystal in which the phase transition becomes second order, the discontinuity of the heat capacity at the phase transition was estimated to be 0.19 k(B). This result stimulated further theoretical investigation. From this study, it was revealed that BaTiO3 is classified as a typical dis placement-type ferroelectric. In order to study crossover phenomenon f rom a two-dimensional Ising system to a two-dimensional Heisenberg sys tem in the K2NiF4 family, the critical amplitude of the heat capacity was measured as a function of the strength of Ising-like anisotropy. B ased upon the analysis for the results, the existence of an asymptotic transition temperature in a two-dimensional Heisenberg system is prop osed. (C) 1997 Elsevier Science B.V.