Specific heat measurements of the lattice contribution and the spin-density-wave and anion-ordering transitions in the (TMTSF)(2)ClO4 salt (TMTSF equivalent to tetramethyltetraselenafulvalene)

The specific heat of the quasi-one-dimensional organic compound (TMTSF)(2)C lO4 was measured between 1.9 and 35 K for two stares corresponding to the m etallic/superconducting ground state and the insulating spin-density-wave ( SDW) ground state at low temperature, by varying the cooling conditions thr ough the anion-ordering (AO) transition at 24 K. These are the first thermo dynamical measurements of the SDW ground state. Like in the previously inve stigated compounds (TMTSF)(2)X, with X = PF6 and AsF6, and (TMTTF)(2)Br (wh ere TMTTF equivalent to tetramethyltetrathiafulvalene), the lattice specifi c heat shows strong deviations from a Debye behaviour, which can be explain ed by the quasi-one-dimensional structural character Bumps in C/T-3 occur a t T = 3.5-4 K and 5 K for the quenched SDW and relaxed metallic states resp ectively; these are in good agreement with the frequencies of low-tying mod es detected by infrared spectroscopy for the metallic sample. In addition, analysis of the lattice contribution suggests a T-alpha (alpha = 2.7) acous tic contribution for T greater than or similar to 2 K, ascribed to the low dimensionality of the structure. The SDW transition was detected at T-C = 4 .5 K, as expected from the high value of the quenching rate (of 3.3 K s(-1) ) through the AO transition (the amplitude of the anomaly represents about 3% of the total heat capacity). The AO transition was investigated either b y decreasing the temperature for the slow-cooled sample or by reheating, at a similar rate, for the quenched sample; the specific heat anomalies are i dentical for the two procedures, due to reordering annealing effects on reh eating, a phenomenon previously investigated by x-ray experiments. Surprisi ngly, the entropy under the AO anomaly is R ln (4/3), much lower than the R ln 2 value expected for the two possible configurational orientations of t he perchlorate ion in the disordered state. Similar low values of the entro py have been previously measured for the perrhenate (ReO4) and BF4 salts.