Anomalous low-temperature heat capacity in antiperovskite compounds

The low-temperature heat capacities are studied for antiperovskite compounds AXM₃ (A=Al, Ga, Cu, Ag, Sn, X=C, N, M=Mn, Fe, Co). A large peak in (C-γT)/T³ versus T is observed for each of a total of 18 compounds investigated, indicating an existence of low-energy phonon mode unexpected by Debye T³ law. Such a peak is insensitive to the external magnetic field up to 80 kOe (1 Oe=79.5775 A·m^-1). For compounds with smaller lattice constant, the peak shifts towards higher temperatures with a reduction of peak height. This abnormal peak in (C-γT)/T³ versus T of antiperovskite compound may result from the strongly dispersive acoustic branch due to the heavier A atoms and the optical-like mode from the dynamic rotation of XM₆ octahedron. Such a low-energy phonon mode may not contribute negatively to the normal thermal expansion in AXM₃ compounds, while it is usually concomitant with negative thermal expansion in open-structure material (e.g., ZrW₂O₈, ScF₃).