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The heat capacities of nanocrystalline Ni3Fe and control materials with larger crystallites were measured from 0.4300 K. The heat capacities were integrated to obtain the enthalpy, entropy, and Gibbs free energy and to quantify how these thermodynamic functions are altered by nanocrystallinity. From the phonon density of states (DOS) measured by inelastic neutron scattering, we find that the Gibbs free energy is dominated by phonons and that the larger heat capacity of the nanomaterial below 100 K is attributable to its enhanced phonon DOS at low energies. Besides electronic and magnetic contributions, the nanocrystalline material has an additional contribution at higher temperatures, consistent with phonon anharmonicity. The nanocrystalline material shows a stronger increase with temperature of both the enthalpy and entropy compared to the bulk sample. Its entropy exceeds that of the bulk material by 0.4 kB/atom at 300 K. This is insufficient to overcome the enthalpy of grain boundaries and defects in the nanocrystalline material, making it thermodynamically unstable with respect to the bulk control material. 

Engineering the thermal properties in solids is important for both fundamental physics ( e.g. electric and phonon transport) and device applications ( e.g. thermal insulating coating, thermoelectrics). In this paper, we report low thermal transport properties of four selenide compounds (BaAg 2 SnSe 4 , BaCu 2 GeSe 4 , BaCu 2 SnSe 4 and SrCu 2 GeSe 4 ) with experimentally-measured thermal conductivity as low as 0.31 ± 0.03 W m −1 K −1 at 673 K for BaAg 2 SnSe 4 . Density functional theory calculations predict κ < 0.3 W m −1 K −1 for BaAg 2 SnSe 4 due to scattering from weakly-bonded Ag–Ag dimers. Defect calculations suggest that achieving high hole doping levels in these materials could be challenging due to monovalent ( e.g. , Ag) interstitials acting as hole killers, resulting in overall low electrical conductivity in these compounds.