Energy-Gap Structure of (BPDT-TTF)₂ICl₂ at Low Temperatures

Thermodynamic investigation of a radical cation salt consisting of a donor molecule of BPDT-TTF (bispropylenedithiotetrathiafulvalene) and a linear anion of ICl₂⁻ is reported. In order to discuss on low-temperature character below the transition temperatures, thermal relaxation calorimetry by single crystal sample was performed. This compound has a layered structure similar to numerous BEDT-TTF compounds and the donor molecules form dimer-based arrangement in the layer similar to κ-, β-type BEDT-TTF radical cation salts. A step like anomaly around 23 K in the temperature dependence of T⁻¹ of which entropy is evaluated as only few % of Rln2 was reported. The low temperature heat capacity shows clear gapped nature evidenced by the absence of T-linear contribution in the low-temperature heat capacity. The magnetic fields up to 5 T do not affect on the heat capacity, which demonstrates that rigid gap structure due to spin singlet formation like spin-Peierls type ordering or charge ordering (CO) state. The possibility of charge fluctuations above the transition temperature is suggested in this system.

(by G. Guan & Y. Nakazawa)

Fig. 1. Molecular structure of BPDT-TTF (bispropylenedithiotetrathiafulvalene).

Figure 2. Temperature dependence of the magnetic susceptibility of (BPDT-TTF)₂ICl₂ obtained by 1, 3, and 5 T. The paramagnetic to non-magnetic transition is observed around 25 K, suggesting that a kind of spin-Peierls type transition occurs around this temperature.

Figure 3. Low temperature heat capacity of (BPDT-TTF)₂ICl₂ at 0 T, 3 T, and 5 T shown in C_pT ⁻¹ vs T² plot. The absence of finite term demonstrates that the ground state is a kind of spin singlet state with distinct gap in thermal excitations.