Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

Abstract

The metal-insulator transition (MIT) of VO 2 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 TiO 2 -VO 2 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 VO 2 and another MIT compound NbO 2 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. It is also shown that the two compounds of the rutile form do not follow the general trends in structure observed for the other rutile compounds, giving clear evidence of an inherent structural instability present in the two compounds. The MITs of VO 2 and NbO 2 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 MoBr 3 , NbCl 4 , Ti 4 O 7 , and V 4 O 7 , 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 t 2g 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 t 2g electrons in crystals.2

Spinodally decomposed nanostructures in a TiO2–VO2 crystal

“…This is also the reason why the SD occurs with the composition modulation only along the c r axis: a smaller elastic strain energy is achieved at the c r plane. Moreover, this gives a persuasive explanation why the MI transition can take place in such thin lamellas in SD samples 12,13 and also in ultrathin VO 2 layers in artificial superlattice films. 24 It is intriguing to notice that the diffraction spots in Fig.…”

“…[19][20][21][22][23] Very recently, one of the authors has proposed that the driving force has nothing to do with electron correlations nor Peierls instability as previously believed but is a structural instability to form a "molecular orbital crystal" by generating a direct metal-metal bonding. 13 In the present paper, we report on microstructures generated by SD in a single crystal of Ti 0.4 V 0.6 O 2 . A mega stack of alternate Ti-and V-rich layers with a period of approximately 33 nm along the c axis is observed in the mm-size crystal.…”

“…12,13 A phase diagram with a miscibility gap was determined in a series of polycrystalline samples. We are interested in this system because of distinct differences in their chemical and physical properties: TiO 2 without d electrons is a wide band-gap insulator and shows a photocatalytic activity, 14,15 while VO 2 with one d electron is a metal at high temperatures and becomes an insulator at low temperatures; a metal-insulator (MI) transition takes place at T MI = 342 K upon heating.…”

Multi-nanolayered VO2/Sapphire Thin Film via Spinodal Decomposition

“…The borderline of the unstable region to the stable region, often called the binodal curve, was first reported in TVO by Zamma and Ueda in 1998 19 from calculations based on the common tangent construction of the free-energy diagram. Below the bimodal curve, there is a spinodal region, which was re-plotted from the literature data obtain from annealing experiments on the crystalline solid solution of Ti 0.4 V 0.6 O 2 at four temperatures (200, 300, 400 and 500 °C) 19,30 . For compositions within the spinodal, a homogeneous solution is unstable against infinitesimal fluctuations in density or composition 31 .

Figure 1Phase diagram of the VO 2 -TiO 2 system.

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Structural, electrical, and optical properties of polycrystalline NbO2 thin films grown on glass substrates by solid phase crystallization

“…At temperatures above 800 °C, the insulating phase undergoes an insulator–metal transition with an associated structural transformation. Such behavior is analogous to that in VO 2 , and has attracted considerable research attention . Indeed, early studies on NbO 2 thin films were triggered by interest in the conductivity of the amorphous state .…”