Specific heat of bis(tetrathiotetracene) triiodide in the vicinity of the metal-insulator phase transition

Abstract

Specific heat measurements have been made on pressed pellet samples of highly disordered bis(tetrathiotetracene) triiodide (TTT2I3) ln the temperature range 20-100 K. A specific heat anomaly in the form of a broad bump is observed at 50 K. The anomaly is coincident with a previously observed phase transition in the magnetic susceptibility and occurs in the vicinity of peaks in the electrical conductivity and thermoelectric power. The specific heat anomaly provides the first direct evidence of a phase transition in TTT2I3. Comparison of the magnitude of the discontinuity in the specific heat in the absence of broadening with a mean field model corrected for fluctuations indicates that the phase transition is a Peierls transition from a metallic to a nonmetallic state with a critical temperature of 50 K. The broadening of the transition indicates the importance of both fluctuations and disorder in TTT2I3.