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 rings (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 ion, the Schottky anomaly shifts towards lower temperatures. The data analysis is supported by the study of the LTSH of the non-magnetic Na:Ga6 compound which has the same core structure as the iron rings. For the non-magnetic Na:Ga6, significant deviations from the simple $C \sim T^3$ Debye law were found and the use of conventional C vs. T fitting curves is critically discussed. The singlet-triplet energy gap ($E_1/k_{\rm B}=12.1 \pm 0.5$ K) and its zero-field splitting ($D_1/k_{\rm B} =1.5 \pm 1$ K) evaluated from the magnetic contribution of the Li:Fe6 LTSH are compared to values estimated by high field torque magnetometry