Heat Capacity Measurements of
(BPDT-TTF)2ICl2 around
the Spin-Peierls Transition

Heat capacity measurements of a charge transfer complex; (BPDT-TTF)2ICl2 has been performed by relaxation and AC technique. This compound is a one dimensional dimer-Mott system. A broad anomaly related to paramagnetic to nonmagnetic transition was observed around 25 K. Although the theoretical transition entropy should be as large as Rln2, the actual transition entropy appeared to be only a few percent of Rln2. It is a common feature of dimer-Mott systems that electronic transitions are moderate and have broad thermodynamic anomalies. The aspect of delocalized nature of electron spins over a dimer site is an important point for studies of organic charge transfer complexes. We will make further research to evaluate the energy gap caused by the transition and investigate the response against the external fields.

(by G. Y. Guan & Y. Nakazawa)

Fig. 1. Crystal structure of (BPDT-TTF)2ICl2.

Fig. 2. Heat capacity of (BPDT-TTF)2ICl2 measured by AC calorimetry. Inset shows magnetic susceptibility under 1, 3 and 5 T. The broad anomaly of heat capacity is associated with the spin-Peierls transition appeared as the decrease of magnetic susceptibility around 25 K.

Fig. 3. Heat capacity of (BPDT-TTF)2ICl2 measured by relaxation technique.