Thermodynamics of the anion ordering transitions in (TMTSF)2ReO4 and (TMTSF)2BF4.

We have studied the anion ordering transitions in the Bechgaard salts, (TMTSF) 2 ReO 4 and (TMTSF) 2 BF 4 using ac calorimetry, differential scanning calorimetry, and the vibrating-reed technique. An analysis of the sensitivity of ac calorimetry to (distributed) molar latent heat l(T) is presented. For the ReO 4 salt, the transition is weakly first order, exhibiting no sluggishness or hysteresis. The effective specific heat measured using ac calorimetry, i.e., c p +dl/dT, exhibits a λ-like anomaly, but the entropy of the transition is sample dependent and generally less than the expected value, R ln(2)