Solid State Communications 149 (2009) 1818–1821 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc Electron-nuclear double resonance study of V 4+ ions in VO 2 crystals P.H. Bunton a,∗ , D.B. Baker a , D.E. Engquist a , M. Klemm b , S. Horn b , Shan Yang c , S.M. Evans c , L.E. Halliburton c a Department of Physics, William Jewell College, Liberty, MO 64068, USA b Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany c Department of Physics, West Virginia University, Morgantown, WV 26506, USA article info Article history: Received 1 July 2009 Accepted 9 July 2009 by A.H. MacDonald Available online 15 July 2009 PACS: 76.30.Fc 76.70.Dx 61.72.-y 71.30.+h Keywords: A. Semiconductors C. Point defects D. Electronic states (localized) E. Electron paramagnetic resonance abstract Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize V 4+ ions in a bulk single crystal of vanadium dioxide (VO 2 ). These S = 1/2 defects are observed in the as-grown crystal because an adjacent nonmagnetic M 4+ impurity, e.g., a Si 4+ ion, has destroyed the normal antiferromagnetic coupling associated with the close pairs of V 4+ ions that occur in the low-temperature monoclinic phase of VO 2 . EPR spectra taken near 5 K with the magnetic field along the [110] and [001] axes show resolved hyperfine patterns due to one 51 V nucleus. ENDOR spectra taken at 5 K with the magnetic field parallel to the [001] axis have large nuclear electric quadrupole splittings as a result of a significant electric field gradient at the 51 V nucleus. Spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions are reported. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction The metal–insulator transition in vanadium dioxide (VO 2 ) has been widely studied, both theoretically and experimentally, since its discovery in 1959 [1–3]. These continuing efforts are developing a more complete understanding of the underlying fundamental physical mechanisms and, at the same time, are introducing a wide range of advanced applications (e.g., smart windows and ultrafast optical switching). In general, VO 2 is a transition-metal oxide with narrow d-electron bands, and thus represents a strongly correlated electron system with physical responses dependent on the interplay of long-range bandlike behavior and very strong short-range electron–lattice and electron–electron correlations. Below the 340 K transition temperature, VO 2 has a monoclinic structure and is semiconducting with an energy gap of about 0.6 eV. Above the transition, it is rutile and metallic. Both Mott- Hubbard and Peierls instabilities have been advanced to describe the thermodynamics and energetics of the semiconducting and metal phases of VO 2 [4], and various band and localized approaches have been used to describe the transition (see the recent band- theoretical approach by Eyert [5]). ∗ Corresponding author. E-mail address: buntonp@william.jewell.edu (P.H. Bunton). The low-temperature semiconducting phase of VO 2 is well suited for study by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Below the metal–insulator transition, the V 4+ ions (each with one d electron) form pairs that couple antiferromagnetically to give S = 0, and EPR signals are not expected unless impurities or native defects are present. Thus far, only a few EPR studies of VO 2 crystals have been reported [6–11] and no ENDOR studies have been reported. D’Haenens et al. [7,8] used the EPR spectra of substitutional Cr 3+ ions with a V 5+ neighbor to investigate the M 1 , T, and M 2 phases [12] that form in chromium-doped VO 2 crystals. These investigators showed that the T and M 2 phases in V 1−x Cr x O 2 involved intermediate pairing and tilting interactions along the vanadium chains and interpreted their EPR results as evidence of electron localization in the form of a Mott insulator. More recently, Misra et al. [11] fully characterized the temperature dependence of the EPR spectra assigned to substitutional Fe 3+ ions with a V 5+ neighbor. In the present paper, we describe a low-temperature EPR and ENDOR study of defect-associated V 4+ ions in the monoclinic phase of as-grown VO 2 crystals. These V 4+ ions, with S = 1/2 due to the single d electron, are not strongly coupled to another V 4+ ion. A nonmagnetic M 4+ impurity, most likely a Si 4+ ion, substitutes for a V 4+ ion and leaves the remaining V 4+ ion of the original pair without an adjacent electron spin with which to couple and form 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.012