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.

中文翻译：

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金红石化合物的结构不稳定性及其与 VO2 的金属-绝缘体转变的相关性

讨论了 VO2 的金属-绝缘体转变 (MIT)，特别强调了金红石化合物对二聚化的结构不稳定性。Ti 取代实验表明 MIT 高达 20% 的 Ti 取代是稳健的，即使在旋节线分解的 TiO2-VO2 复合材料中极薄的富含 V 的薄片中也会发生，表明 MIT 对空穴掺杂不敏感并且基本上具有局部特征。这些观察结果表明，Mott-Hubbard 意义上的电子相关性或 Peierls（费米表面）不稳定性对 MIT 的影响很小。通过对金红石相关化合物的晶体化学的广泛视角，值得注意的是，VO2 和家族中的另一种 MIT 化合物 NbO2 最终位于两个结构组之间的边界附近，具有规则的金红石结构和以形成具有直接金属 - 金属键合的二聚体为特征的扭曲结构。VO2 和 NbO2 的 MIT 是这两组之间结构转变的自然结果，因为所有 d 电子在低温下都被困在二聚体的键合分子轨道中。这种二聚体晶体普遍存在于具有链状结构的早期过渡金属化合物中，例如 MoBr3、NbCl4、Ti4O7 和 V4O7，后两者也表现出可能具有相同来源的 MIT。在更广泛的意义上，二聚体晶体是一种分子轨道晶体，其中由过渡金属原子组成的虚拟分子具有部分填充的t2g壳，如二聚体、三聚体或更大的，通过金属-金属键合产生并嵌入边缘或面- 共享各种八面体网络。分子轨道结晶为稳定晶体中未配对的 t2g 电子开辟了一条自然途径。