Heat Capacity of Potassium p-Chloranil Anion Radical Salt between 13 and 330 K

The heat capacity measurement for an organic free-radical salt, potassium p-chloranil, revealed a first-order phase transition at Tc=260.01 K. In spite of a first-order nature, the pre- and the post-transitional effects are remarkable. The enthalpy and the entropy of transition were determined to be ΔH=2796 J mol−1 and ΔS=11.06 JK−1 mol−1, respectively. The magnetic contribution to ΔS is only 2.23 JK−1 mol−1 when a dimeric configuration with a singlet-triplet spin equilibrium is assumed in the low temperature phase and a monomeric behavior of radicals with spin doublet is assumed above Tc. The present phase transition bears a close resemblance to a spin-Peierls transition in a linear chain system. The infrared spectrum at 120 K indicates a possible formation of dimers in the low temperature phase of this salt. A phenomenological model for the present phase transition is presented.