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We report heat capacity measurements of the pnictide materialsSmFeAsO$_{1-x}$F$_x$, NdFeAsO, LaFeAsO$_{1-x}$F$_x$ and LiFeAs. ForSmFeAsO$_{1-x}$F$_x$, with x close to 0.1, we use 3 He measurements todemonstrate a transfer of entropy from the peak at TN to a previouslyunidentified ~2 K feature which grows with increasing doping. Our results onthe Sm samples are compared with a similarly doped La sample to elucidate thecrystal field levels of the Sm3+ ion at 0, 23, and 56 meV which lead to aSchottky-like anomaly, and also show that there is a significant increase inthe Sommerfeld coefficient $\gamma$ when La is replaced by Sm or Nd. Thelattice contribution to the heat capacity of the superconducting oxypnictidesis found to vary negligibly with chemical substitution. We also present a heatcapacity measurement of LiFeAs showing the feature at Tc, which issignificantly rounded and much smaller than the BCS value.

Heat capacity measurements on FeAs-based compounds: a thermodynamic probe of electronic and magnetic states