Thermodynamic studies of the anion-radical salt system [Ni(bipy)3](TCNQ)4·(CH3) 2CO, where TCNQ is 7,7 ,8, are reported. The anion-radical salt systems based on TCNQ belong to a material class in which the arrangement of the anion-radical salt has considerable impact on the charge transfer and magnetic properties. The crystal structure of the studied compound consists of [Ni(bipy)3]2+ cations containing Ni 2+ ions and four types of crystallographically independent anion-radicals TCNQ ·− (A, B, C and D). These TCNQ ·− radicals form two different types of TCNQ ·− stacks (AABB and CCDD), where a strong exchange interaction is expected. The temperature dependence of the specific heat of a single crystal was studied in magnetic fields up to 5 T and in the temperature range from 0.4 K to 30 K. The temperature dependence of specific heat displays a broad Schottky-like maximum above 0.4 K. Using a single-ion approximation, the analysis of the temperature dependence of the specific heat below 10 K yields values for the anisotropy parameters, D/kB = −1.95 K and E/kB = 0.3 K. These results suggest that the observed maximum in the specific heat originates from Ni 2+ ions while the exchange interaction between the transition metal ions and the TCNQ is negligible.
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