QUASI-2-DIMENSIONAL ANTIFERROMAGNETISM IN FE[S2CNC4H8]2I

Recent studies of the magnetic susceptibility of powdered Fe[S2CNC4H8]2I confirm that Fe(III) in this salt is in the unusual spin state S=3/2 and that the system behaves as a quasi-2D antiferromagnet. Analysis of the data indicates that the [+/- 1/2] ground doublet is separated by DELTA/k = 18.7 K from an excited \+/-3/2] doublet. We have now measured the heat capacity of polycrystalline Fe[S2CNC4H8]2I between 0.5 and 80 K in a liquid He-3-led calorimeter. C(p) exhibits a lambda-type peak at T(N)=2.18+/-0.01 K. The critical entropy, S(T(N))=0.25R ln2, indicates that most of the spin order is of a short-range, low-dimensional nature. The contribution of lattice vibrations to Cp was estimated using the method of Sorai and Seki [J. Phys. Soc. Jpn. 32, 382 (1972)], which assumes a Schottky contribution from two doublets whose separation DELTA was determined from the magnetic data. At the same time, the cooperative peak was reasonably represented by the results for the 2D S=1/2 XY model with J(xy)/k= -2.18 K. In order to obtain a self-consistent fit of the lattice heat capacity, however, it was necessary to use a larger splitting (DELTA/k=32 K) than that found from susceptibility data. This discrepancy between the doublet-doublet separations indicated by chi and C(p) measurements may suggest a possible temperature dependence of DELTA.