Dynamic and structural stability of cubic vanadium nitride
Artikel i vetenskaplig tidskrift, 2015

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 proportional to (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 alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h 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.


A. B. Mei

University of Illinois

O. Hellman

Linköpings universitet

California Institute of Technology (Caltech)

Nils Wireklint

Student vid Chalmers

C. M. Schleputz

Argonne National Laboratory

D. G. Sangiovanni

Linköpings universitet

B. Alling

Linköpings universitet

A. Rockett

University of Illinois

L. Hultman

Linköpings universitet

I. Petrov

Linköpings universitet

University of Illinois

J. E. Greene

University of Illinois

Linköpings universitet

Physical Review B - Condensed Matter and Materials Physics

24699950 (ISSN) 24699969 (eISSN)

Vol. 91 5 054101





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