Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4+(TCNQ)− showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH4-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH4-TCNQ. The enthalpy and entropy of transition are ΔtrsH=(667±7) J mol−1 and ΔtrsS=(2.19±0.02) J K−1 mol−1, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as ΔtrsSmag=0.61 J K−1 mol−1 and the lattice contribution as ΔtrsSlat=1.58 J K−1 mol−1. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K−1 mol−1), a majority of the magnetic entropy (∼4.6 J K−1 mol−1) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.