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In order to obtain useful information on the transient process of phase transitions in ferroelectrics by various methods including calorimetry, the "mK-stabilized cell" of a small size has been developed. It is based on the heat flux differential scanning calorimeter (DSC) and has a temperature stability better than 1 mK. The "cell" can be used to change the temperature under nearly quasi-static conditions by an infinitesimally small rate not only on heating but also on cooling while passing through the transition points. It enables simultaneous measurements of endothermic heat and exothermic heat along with dielectric constants, displacement currents, etc. with a high degree of temperature resolution. X-ray diffraction measurements for sensing thermal anomalies are also possible by a minor modification of the "cell." Precise and simultaneous measurements of thermal, electrical, and acoustic properties were carried out at the 403 K phase transition in BaTiO 3 single crystal grown by the top-seeded solution growth method. It has been clarified that the exothermic heat at the transition on cooling has more useful information than the endothermic heat on heating; in the cooling process two thermal anomalies occur separately at T 1 and T 2 although the transition is in a narrow temperature range. It is recognized from other methods that the nature of the 1 1964 Kojima et al.transition on cooling is not of a single but of multiple steps. Resonant ultrasonic measurements using the "cell" were carried out, after developing a new excitation method. The sample does not have a simple softening approaching the transition point on cooling but has different elastic moduli for the two thermal anomalies at T 1 and T 2 . The dielectric constant also has an intermediate constant value between T 1 and T 2 . The crystal structure in the room temperature phase below the transition point has been determined by X-ray diffraction. In this region, tetragonal and monoclinic structures coordinating with each other exist.

An apparatus has been developed for the measurement of the ultrasonic resonances of a solid cube or parallelepiped, which involves a method of excitation and detection. The key point is that a small reentrant-type microwave cavity is used in the apparatus. The excitation of the sample vibration is due to the electrostatic force between a metal-coated sample surface and the outer surface of the cavity. The resonant frequency of the sample is measured by taking advantage of the amplitude modulation of the characteristic resonant frequency of the cavity. This modulation is caused by the capacitance change due to the sample vibration. The apparatus is so small that it is mounted in the “mK-stabilized cell” developed previously, which has a temperature stability and/or resolution of the order of 1mK. The usefulness of the apparatus combined with the “mK-stabilized cell” has been demonstrated by the precise measurement of the resonant frequency of BaTiO3 single crystal at its phase transition, which is relevant to the change of its elastic modulus at the critical point.