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Published February 1, 2015 | Published
Journal Article Open

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=250K, XRD and SAED results reveal forbidden (001) 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(ℏω), the product of the phonon density of states F(ℏω) and the transport electron/phonon coupling strengthα^(2)_(tr)(ℏω), 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.

Additional Information

© 2015 American Physical Society. Published 2 February 2015; received 12 December 2014. The authors thank Sebastian Wimmer, Dr. Kenneth E. Gray, Professor G¨oran Grimvall, Professor Tai-Chang Chiang, Professor Igor A. Abrikosov, and Professor John F. Zasadzinski for valuable discussions. The financial support of the Swedish Research Council (VR) program 637-2013-7296 as well as Grants No. 2014-5790, No. 2009-00971, and No. 2013-4018, and the Swedish Government Strategic Research Area Grant in Materials Science (Grant No. SFO Mat-LiU 2009-00971) on Advanced Functional Materials is greatly appreciated. Supercomputer resources were provided by the Swedish National Infrastructure for Computing (SNIC). This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Published - PhysRevB.91.054101.pdf

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August 20, 2023
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