Heat capcities of κ-(BEDT-TTF)2Cu(NCS)2 and duterated κ-(BEDT-TTF)2Cu[N(CN)2]Y (Y = Cl, Br) were measured by chopped-light ac calorimetry around 1 Hz. While glass transitions due to freezing of the ethylene motion were detected around 100 K (at 80 K in daily scale) in duterayed κ-(BEDT-TTF)2Cu[N(CN)2]Y (Y = Cl, Br), no glass transition was observed in κ-(BEDT-TTF)2Cu(NCS)2. These results suggests that envilonment around the ethylene moeities is different between κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Y (Y = Cl, Br).
Heat capacity of β-(BEDT-TTF)2I3 was measured by chopped-light ac calorimetry. A higher-order phase transition toward the incommesurate phase was observed at 175 K which is arising from the ordering of the ethylene conformation. In the incommesurate phase, two phases are appeared depending on the cooling rate; βH (commensurete structure, Tc = 8 K) and βL (Tc = 1.5 K), where the former appears by annealing more than 24 h at 104 K and the latter by cooling on usual rate. Step like thermal anomalies were detected around 120 K in the both phases of βH and βL. Although the anomaly at 120 K seemed to be due to a glass transition, the temperature of the anomaly temperature did not show the frequency dependence which is characteristic to the glass transition. The excess entropy of the thermal anomaly at 100 ~ 200 K was estimated to be about 4.5 J K−1 mol−1 which was comparable to the expected entropy (Rln2 = 5.76 J K−1 mol−1) of one ethylene conformational disorder. Difference in thermal anomaly at 120 K was recognized between the βH and βL phases.
Heat capacities of two charge transfer complexes containing trans-Stilbene (STB) were measured by adiabatic calorimetry between 6 K and 320 K. Crystal structure of STB-TCNQF4, synthesized for the first time in this study, was determined by single-crystal X-ray diffractometry. The structure is of the mixed stack type and different from STB-TCNQ. In cace of STB-TCNQF4, a glass transition, relating to freezing of the intramolecular crankshaft motion of STB molecule, was detected at 240 K. STB-[trinitrobenzene (TNB)]2 also showed a similar thermal anomaly attributable to a glass transition around 140 K. The glass transition temperatures observed so far are different among STB-TCNQ, STB-TCNQF4, STB-(TNB)2 and the neat STB. The difference, a reflects hardness of the dynamic motion of STB molecule in the crystal, is discussed along with the difference of crystal structure and the degree of charge-transfer in each complex.
Heat capacities of halogen-bridged one dimensional binuclear-metal (the so-called MMX chain) complexes, Pt2(EtCS2)4I and Pt2(n-PenCS2)4I, were measured by adiabatic calorimetry and chopped-light ac calorimetry. For Pt2(n-PenCS2)4I, when measuring after cooling from a room temperature, a first-order phase transition was observed at 207.4 K. The entropy of this transition is as large as 50 J K−1 mol−1, which roughly corresponds to that on melting for the disordered pentyl chains reported in the crystallographic study. Another monotropic phase transition was observed at 320 K on heating. In the case of rapid cooling from 370 K, a first-order phase transition was detected at 220.5 K, and a broad anomaly probably due to a higher-order phase transition was detected at 180 K. A very small thermal anomaly was detected at 240 K by ac calorimetry, and is attributed to a metal-insulator transition.
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