M. Chung et al., THERMODYNAMICS OF THE ANION ORDERING TRANSITIONS IN (TMTSF)2REO4 AND (TMTSF)2BF4, Physical review. B, Condensed matter, 48(13), 1993, pp. 9256-9263
We have studied the anion ordering transitions in the Bechgaard salts,
(TMTSF)2ReO4 and (TMTSF)2BF4 using ac calorimetry, differential scann
ing 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 ReO4 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 lambda-
like anomaly, but the entropy of the transition is sample dependent an
d generally less than the expected value, R ln(2). In comparison, the
transition is sluggish for the tetrafluoroborate salt, so that only a
lower limit on the entropy could be determined. The thermal anomalies
are compared to the anomalies observed using the vibrating-reed techni
que; the latter are affected by changes in the lattice constants as we
ll as the Young's moduli at the transitions.