Low temperature specific heat of molecular rings: a study on the effects of the internal guest substitution and on the lattice contribution

The effects of the internal guest substitution within hexa-iron molecular r ings (Na:Fe6 and Li:Fe6) have been investigated by means of low temperature specific heat (LTSH) measurements. By changing Na to Li as central metal i on, the Schottky anomaly shifts towards lower temperatures. The data analys is is supported by the study of the LTSH of the non-magnetic Na:Ga6 compoun d which has the same core structure as the iron rings. For the non-magnetic Na:Ga6, significant deviations from the simple C similar to T-3 Debye law were found and the use of conventional C vs. T fitting curves is critically discussed. The singlet-triplet energy gap (E-1/k(B) = 12.1 +/- 0.5 K) and its zero-field splitting (D-1/k(B) = 1.5 +/- 1 K) evaluated from the magnet ic contribution of the Li:Fe6 LTSH are compared to values estimated by high field torque magnetometry.