Monoclinic hydroxyapatite was prepared by a wet method and followed by heating in air at 1473K for 1 hour, and its phase transition was investigated by a high temperature X-ray powder diffractometry and a differential scanning calorimetry. The prepared hydroxyapatite was 98% monoclinic. The Rietveld refinement results revealed that the low temperature phase had monoclinic symmetry, space group P2₁/b and the high temperature phase had hexagonal symmetry, space group P6₃/m. In monoclinic hydroxyapatite, two different sizes of oxygen triangles existed. The rotation of phosphate tetrahedra at about 6° and the change of the hydroxyl order to disorder arrangement occurred due to the phase transition. The phase transition took place reversibly at 480.5(1)K in heating process and 477.5(1)K in cooling process. The enthalpy value was found to be 130(1) J mol^-1. Further, the transition was observed to resolve into two steps that were attributed to the rotation of phosphate tetrahedra and the order to disorder arrangement of hydroxyls.

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An ac calorimeter was constructed for measurements below room temperature. A cryostat was designed while modifying that for adiabatic calorimetry for highly stable thermal condition. Temperature control comparable to that in adiabatic calorimetry was achieved. Chopped-light was introduced to the cryostat through optical fiber, and lead to the sample through a fused-quartz rod. The cryostat is applicable to general physical measurements keeping its capability for thermal control.

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DCNQI-Cu系のリエントラントな金属-絶縁体転移に興味をもち,特に低温比熱,転移近傍の比熱の跳びおよび潜熱が転移を理解する上で重要な情報を与えると考え,その転移の熱的な特性研究をおこなってきた。測定は極低温から低温転移領域(40K)までは熱緩和法,低温転移領域から高温転移領域(100K)までは断熱法,転移領域では示差熱測定(DTA)を用いた。金属状態のゾンマーフェルト定数γの圧力依存性,絶縁相におけるCu²⁺の反強磁性転移の比熱の温度依存性,リエントラント転移の転移における潜熱の測定は転移の機構を理解する上で最も重要な情報を与えてくれる。これらの結果を踏まえて,両相の自由エネルギーを見積もった結果,(絶縁相の)Cu²⁺のスピン系の自由度と(金属相の)フォノンに助けられた自由電子の自由度の競合でリエントラントな転移が生まれることが明らかになった。

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