Dynamic and structural stability of cubic vanadium nitride

Abstract

Structural phase transitions in epitaxial stoichiometric VN/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 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 ∝ (T c − 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 α 2 tr F ( ω), the product of the phonon density of states F ( ω) and the transport electron/phonon coupling strength α 2 tr ( ω), 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.

Kohn Anomaly and Phase Stability in Group VB Transition Metals

“…The schematic representation of a cubic → tetragonal phase transition in VN (the phonon dispersion curves with a distinct region of the imaginary frequencies and the related displacements of atoms) is offered by Rehák et al [115] (Figure 5). Recently, Mei et al [116] have utilized ab initio MD to prove that VN can be stabilized by anharmonic atomic vibrations in the B1 structure at~250 K. Indeed, it has been discovered from XRD measurements by Kubel et al [117] that, at room temperature, VN in the B1 structure is strongly anharmonic with atomic displacements randomly shuffled along the <111> crystallographic directions with respect to the ideal B1 lattice sites. These characteristics have been recently linked by Zheng et al [118] to electron-phonon coupling effects in the VN lattice thermal conductivity.…”

Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN

“…Kinetic properties are extrapolated to finite temperatures using transition state theory (TST) [44] by assuming essentially fully harmonic lattice vibrations, or employing quasiharmonic approximations [45,46]. These approaches, however, are not applicable to crystal phases which are unstable at 0 K (e.g., Group-VB TM nitrides [47,48]) and may yield inaccurate predictions when the role of anharmonic lattice vibrations becomes relevant [49][50][51]. Quantitatively reliable evaluation of kinetically controlled properties at given temperatures and pressures of interest necessarily requires the use of computer simulations reproducing atomic trajectories.…”

Reflection thermal diffuse x-ray scattering for quantitative determination of phonon dispersion relations

“…Phonons, elementary quanta of lattice vibrations, are well known to control hightemperature transport and structural properties of crystalline solids [1][2][3][4][5][6][7]. As a result, the ability to spectrally resolve phonon energies has proven central to the understanding of fundamental materials properties.…”