Anomalous heat capacity and phonon density of states of superhard nanostructured cubic boron nitride

Nanostructured polycrystalline cubic boron nitride, an advanced superhard and refractory covalent solid with hardness approaching that of diamond, exhibits extreme mechanical properties due to high chemical and phase purity, ideal crystallinity and high intergranular adhesion. It has been produced by direct phase transformation of pyrolytic turbostratic BN at high pressure – high temperature conditions (20 GPa, 2500 K) using a multianvil apparatus. The non-trivial Raman fingerprint of nano-cBN shows both bulk and surface/grain boundary modes in the phonon density of states. The anomalous heat capacity of nano-cBN is comparable to that of nanostructured metal bulks and shows universal behavior with a maximum at 20-25 percent of the Debye temperature. The intergrain boundaries and their intrinsic stacking faults structure is the only plausible cause of excessive heat capacity. We show that nanostructured cubic boron nitride hosts grain-boundary phonon modes that give rise to an anomalous excess heat capacity, a universal effect previously known only in metals. These findings establish grain boundaries as active thermodynamic degrees of freedom in covalent nanoceramics, opening perspectives for tuning vibrational and thermal properties of ultrahard materials.