Dynamic and structural stability of cubic vanadium nitride

Abstract

Structural phase transitions in epitaxial stoichiometric $\mathrm{VN}/\mathrm{MgO}(011)$ thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below ${T}_{c}=250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, XRD and SAED results reveal forbidden $(00l)$ reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior $I\ensuremath{\propto}\phantom{\rule{0.16em}{0ex}}{({T}_{c}\ensuremath{-}T)}^{1/2}$. Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function ${\ensuremath{\alpha}}_{\mathrm{tr}}^{2}F(\ensuremath{\hbar}\ensuremath{\omega})$, the product of the phonon density of states $F(\ensuremath{\hbar}\ensuremath{\omega})$ and the transport electron/phonon coupling strength ${\ensuremath{\alpha}}_{\mathrm{tr}}^{2}(\ensuremath{\hbar}\ensuremath{\omega})$, is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at $T>{T}_{c}$. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.