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Dynamic and structural stability of cubic vanadium nitride

A. B. Mei ; O. Hellman ; Nils Wireklint (Institutionen för teknisk fysik) ; C. M. Schleputz ; D. G. Sangiovanni ; B. Alling ; A. Rockett ; L. Hultman ; I. Petrov ; J. E. Greene
Physical Review B. Condensed Matter and Materials Physics (1098-0121). Vol. 91 (2015), 5,
[Artikel, refereegranskad vetenskaplig]

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.

Denna post skapades 2015-03-04. Senast ändrad 2015-03-04.
CPL Pubid: 213388


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