Anomalous low-temperature heat capacity in antiperovskite compounds**Project supported by the National Key Basic Research Program of China (Grant Nos. 2011CBA00111) and the National Natural Science Foundation of China (Grant Nos. 51322105, U1632158, 51301165, and 51301167).

Abstract

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).