Investigating the thermal properties of CrN: Theoretical insights and real ceramics

Abstract

Ab-initio calculations of lattice heat capacity and phonon thermal conductivity have been performed for CrN of rock-salt structural type (paramagnetic) and for the orthorhombic antiferromagnetic (AFM) phase below TN ≈ 285 K. The comparison with experimental data below ≈30 K unveils that in contrary to the theoretically calculated heat capacity following the βT3 term with Debye temperature ΘD = 830 K, a large excess of experimental heat capacity in a form of δT2 is observed. This additional heat capacity is ascribed to magnons in the specific layered-type AFM arrangement of CrN. The experimental thermal conductivity shows a similar low-temperature excess of T2-dependence. Considering that mean free path of heat carriers is essentially limited by the grain size and calculated phonon conductivity itself is very low below ≈30 K, such additional contribution can be ascribed again to AFM magnons though theoretical model of such conductivity is still to be assessed. In spite of this complexity, our model of phonon thermal conductivity reproduces well the maximum of 7.2 Wm−1K−1 at 150 K followed by a decline tied to phonon-phonon scattering (Umklapp process). Nonetheless at higher temperatures, the experimental data evidence sudden drop at TN to ≈2 Wm−1K−1 with only slightly varying thermal conductivity at elevated temperatures, which reminds rather a behavior of amorphous or defective solids. It is proposed that atomic displacements that stabilize spin-parallel and spin-antiparallel Cr pairs in the orthorhombic AFM phase persist partially as fluctuating ones in the cubic paramagnetic state above TN, which gives rise to dynamic strain fields that cause strong phonon scattering.