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Register a new userStructural instability of the rutile compounds and its relevance to the metal-insulator transition of VO2
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Zenji Hiroi
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The metal-insulator transition (MIT) of VO2 is discussed with particular emphasis on the structural instability of the rutile compounds toward dimerization. Ti substitution experiments reveal that the MIT is robust up to 20% Ti substitutions and occurs even in extremely thin V-rich lamellas in spinodally decomposed TiO2-VO2 composites, indicating that the MIT is insensitive to hole doping and essentially takes on a local character. These observations suggest that either electron correlation in the Mott-Hubbard sense or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a broad perspective of crystal chemistry on the rutile-related compounds, it is noted that VO2 and another MIT compound NbO2 in the family eventually lie just near the borderline between the two structural groups with the regular rutile structure and the distorted structures characterized by the formation of dimers with direct metal-metal bonding. The MITs of VO2 and NbO2 are natural consequences of structural transitions between the two groups, as all the d electrons are trapped in the bonding molecular orbitals of dimers at low temperatures. Such dimer crystals are ubiquitously found in early transition metal compounds having chain-like structures, such as MoBr3, NbCl4, Ti4O7, and V4O7, the latter two of which also exhibit MITs probably of the same origin. In a broader sense, the dimer crystal is a kind of molecular orbital crystals in which virtual molecules made of transition metal atoms with partially-filled t2g shells, such as dimers, trimers or larger ones, are generated by metal-metal bonding and are embedded into edge- or face-sharing octahedron networks of various kinds. The molecular orbital crystallization opens a natural route to stabilization of unpaired t2g electrons in crystals.
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Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2.
First-order phase transitions in solids are notoriously challenging to study. The combination of change in unit cell shape, long range of elastic distortion, and flow of latent heat leads to large energy barriers resulting in domain structure, hysteresis, and cracking. The situation is still worse near a triple point where more than two phases are involved. The famous metal-insulator transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. Even though VO2 is one of the simplest strongly correlated materials, experimental difficulties posed by the first-order nature of the MIT as well as the involvement of at least two competing insulating phases have led to persistent controversy about its nature. Here, we show that studying single-crystal VO2 nanobeams in a purpose-built nanomechanical strain apparatus allows investigation of this prototypical phase transition with unprecedented control and precision. Our results include the striking finding that the triple point of the metallic and two insulating phases is at the transition temperature, T_tr = T_c, which we determine to be 65.0 +- 0.1 C. The findings have profound implications for the mechanism of the MIT in VO2, but in addition they demonstrate the importance of such an approach for mastering phase transitions in many other strongly correlated materials, such as manganites and iron-based superconductors.
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