Thermodynamic properties of the mixed crystals of galvinoxyl radical and its precursory closed shell compound: The large entropy cooperating with the spin system

The heat capacities of the 9:1 and 6:1 mixed crystals of the galvinoxyl radical (4-[[3,5-bis(1,1-dimethylethyl)-4-oxo-2,5-cyclohexadien-1-ylidene]methyl]-2, 6-bis(1,1-dimethylethyl)- phenoxyl) and its precursory closed shell compound, hydrogalvinoxyl, are measured between 13 and 300 K. A heat capacity anomaly due to the phase transition from the diamagnetic low-temperature (LT) phase to the paramagnetic high-temperature (HT) phase is found at 71 K in the 9:1 mixed crystal. The entropy of transition is determined to be ΔS=10.11 J K−1 mol−1, which should include the surplus change of entropy in addition to the magnetic contribution. Furthermore, it is found that the phase transition is easily quenched in the 9:1 mixed crystal by rapid cooling and the supercooled HT phase is left stable below about 50 K. The entropy of the supercooled HT phase is estimated as a function of temperature from the temperature dependence of the heat capacities of the annealed and quenched samples. The difference in entropy between the LT and the supercooled HT phases decreases monotonically with decreasing temperature from 71 K and becomes about 4 J K−1 mol−1 at 14 K, which is comparable to the magnetic contribution from the spin system still effective in the supercooled HT phase. This suggests that the surplus entropy appearing in the phase transition is lost gradually with decreasing temperature in the supercooled HT phase. In the 6:1 mixed crystal, the phase transition is no more present and only the HT phase appears.