Specific-heat studies of the mixed-valence tetrathiafulvalene (TTF) salts: (TTF)${\mathrm{Br}}_{0.72}$, (TTF)${\mathrm{Cl}}_{0.67}$, and (TTF)${\mathrm{}\left(\mathrm{SCN}\right)\mathrm{}}_{0.57}$

Specific-heat measurements have been made on the mixed-valence tetrathiafulvalene (TTF) salts (TTF)${\mathrm{Br}}_{0.72}$, (TTF)${\mathrm{Cl}}_{0.67}$, and (TTF)${\mathrm{}\left(\mathrm{SCN}\right)\mathrm{}}_{0.57}$, at low temperature (2-20 K) and near room temperatures (100-400 K). At low temperatures both positive and negative linear contributions to the specific heat are seen which we attribute to non-Debye phonon spectra. At higher temperatures no specific-heat anomalies are seen at either the metal-insulator or tetragonal-to-monoclinic transitions in (TTF)${\mathrm{}\left(\mathrm{SCN}\right)\mathrm{}}_{0.58}$. A broad bump in the specific heat of (TTF)${\mathrm{Cl}}_{0.67}$ at 250 K is attributed to the ordering of the chlorine sublattice. This is the first such observation of an order-disorder transition in these materials. A simple configurational model of this transition is presented.