Heat capacity study of the doping effect of paramagnetic impurity in organic spin-Peierls system: p-CyDOV radical crystal

Heat capacity studies were performed for the doped verdazyl radical crystals, (p-CyDOV)_1-x(p-CyDTV)_x (x = 0, 0.01, and 0.07), to clarify the effect of magnetic impurity (p-CyDTV) on the spin-Peierls (SP) transition (T_SP = 15.0 K) of 3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl (p-CyDOV) radical crystal, where the > C=O group in p-CyDOV is replaced by the > C=S group in p-CyDTV. The antiferromagnetic (AFM) transitions were observed at T_N = 0.135, 0.290, and 0.164 K for the crystals with x = 0, 0.01, and 0.07, respectively, indicating the coexistence of AFM order and SP state in the systems with x = 0 and 0.01, and the appearance of the single phase of AFM order in the system with x = 0.07 below T_N = 0.164 K. However, the heat capacity anomaly associated with the SP transition was not found at T_SP = 15.0 K for the pure p-CyDOV crystal (x = 0), whereas the anomalous broad peak in the heat capacity was observed at T′_SP = 5.6 K. The entropy consumption in the above broad peak was about 3% of the total spin entropy R ln 2, suggesting that this transition may relate to SP transition. The pressure induced by the solidification of the Apiezon-N grease, which was used as the thermal binder for the heat capacity measurement, at low temperature may affect the shift of the SP transition temperature. The doping effects for the organic SP system (p-CyDOV), which include the temperature (T: T_SP and T_N - impurity (x) phase diagram, are similar to those for the inorganic SP system (CuGeO₃), although the one-dimensionality, i.e., |zJ′/ J| ≅ 6 × 10^-4, in the organic p-CyDOV system is much higher than that (∼10^-1) in the inorganic CuGeO₃ system, where zJ′ and J are inter- and intra-chain exchange interactions. This work is the first report of the heat capacity study of the doping effect on the organic SP transition system.