An apparatus to obtain low-temperature thermodynamic information under high pressures for a tiny single crystal of molecular compounds was developed based on the ac technique. To detect small temperature oscillation of a sample inside the cramp-type pressure cell, we have used a small ruthenium oxide chip sensor as a thermometer. The adoption of the four-terminal method by the ac resistance bridge has made high-resolution detection of thermal anomaly possible in the low-temperature region. The constructed high-pressure thermodynamic system was mounted on a 3He refrigerator and we have succeeded to detect the thermal anomaly in relevant to magnetic order of single crystal sample of Mn4-cluster complex up to 1.05 GPa. A distinct peak of the heat capacity and its upward shift with increasing pressures was observed using a tiny crystal of about 100 microg. The high-pressure behavior of the discontinuity of heat capacity at the superconductive transition of 6 mg of metal indium has also been detected by this apparatus. The details and performance of the technique are reported.

Low temperature heat capacity measurements under pressure were performed for tiny single crystals of two-dimensional coordination-networked compounds consisting of [Mn(4)] single-molecule magnets (SMMs) by the ac temperature modulation technique. Systematic variations of the peak temperature and the peak width of the thermal anomalies produced by pressure were clearly detected using two ruthenium oxide resistance chips in the Cu-Be clamp-type cell. A linear increase of the Néel temperature for an ordering set of large SMM spins with S=9 was observed in [Mn(4)(hmp)(6){N(CN)(2)}(2)](ClO(4))(2), while a non-monotonic variation of the peak temperatures and peak shapes was observed in [Mn(4)(hmp)(4)Br(2)(OMe)(2){N(CN)(2)}(2)](ClO(4))(2)·2THF·0.5H(2)O (hmp(-)=2-hydroxymethylpyridinate; N(CN)(2)(-)=dicyanamide as the linker among SMMs). We also report on thermodynamic behavior produced by changing external pressures and magnetic fields. The results are discussed in terms of the tilting angle of Ising axes in the two-dimensional plane.

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