We report specific-heat data of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and its derivatives in the temperature range of 1.8-40 K. Our results reveal that TTF-TCNQ and tetraselenafulvalene TCNQ behave as elastically isotropic three-dimensional solids at low temperatures. A large contribution arising from the population of energetically low-lying optical-phonon modes is observed, with no evidence of thermal excitation over a small energy gap. Additional low-temperature specific-heat measurements of related derivatives are reported.

• Received 2 October 1978

DOI:https://doi.org/10.1103/PhysRevB.20.5090