Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH 4+(TCNQ) − showing spin-Peierls transition

Abstract

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH 4-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 NH 4-TCNQ. The enthalpy and entropy of transition are Δ trs H=(667±7) J mol −1 and Δ trs S=(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 Δ trs S mag=0.61 J K −1 mol −1 and the lattice contribution as Δ trs S lat=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.