Spectroscopic Studies on Mechanism of Metal-Insulator Transition of Titanium and Vanadium Oxides

最近，Tanaka提出了Ti_2O_3和VO_2[1]金属-绝缘子过渡（MIT）机制的一个新的统一模型。在Ti_2O_3的情况下，MIT发生在450 K左右，从低温的绝缘相到高温相的金属相。在他的模型中，根据MIT， Ti离子对的Ti三维电子构型从（a_<1g>↑；a_<1g>↓）变为（a_<1g>↑；e_g（π）↑），这一变化导致Ti_2O_3的MIT。在此基础上，对VO_2的MIT机理进行了解释。他还指出，在Ti 2p-3d x射线吸收光谱及其线性二色性（XAS/LD）中可以清楚地观察到这种电子构型的变化。本研究在广岛同步辐射中心HiSOR的BL14束流线上生长Ti_2O_3单晶，并在300 K（绝缘相）和520 K（金属相）之间进行了Ti 2p-3d XAS/LD实验。XAS光谱显示出明显的LD，反映了Ti三维态的各向异性。与300 K相比，520 K时的LD强度降低了60%，表明Ti三维态的各向异性有所降低。对XAS/LD光谱进行多重分析，得到（a_<1g>↑；a_<1g>↓）：（a_<1g>↑；e_g（π）↑）分别= 0.55:0.45,0.15:0.85。LD强度仅在MIT温度附近发生变化，表明Ti三维电子构型的变化与MIT有关。这些结果支持了Tanaka[1]提出的MIT模型。我们还从ols XAS光谱中发现了Ti三维态的变化。此外，ti2p软x射线发射光谱仅在金属相呈现弹性峰A. Tanaka， J. Phys。Soc。中国科学院学报（英文版），2004。

Recently, Tanaka proposed a new unified model for a mechanism of Metal-Insulator Transition (MIT) for Ti_2O_3 and VO_2 [1]. In case of Ti_2O_3, MIT takes place around 450 K from an insulating phase at low temperatures to a metallic phase at high temperature phase. In his model, in accordance with MIT, a Ti 3d electron configuration of the Ti ion pair changes from (a_<1g>↑ ; a_<1g>↓) to (a_<1g>↑; e_g(π)↑) and this change leads to MIT of Ti_2O_3. The mechanism of MIT for VO_2 is also interpreted based on the same scenario. He also pointed out that such a change of the electronic configuration is clearly observed in the Ti 2p-3d x-ray absorption spectra and its linear dichroism (XAS/LD).In the present study, we have grown Ti_2O_3 single crystals and carried out the Ti 2p-3d XAS/LD experiments between 300 K (insulating phase) and 520 K (metallic phase) on the beamline BL14 of HiSOR, Hiroshima Synchrotron Radiation Center. The XAS spectra show a remarkable LD, reflecting an anisotropy of the Ti 3d states. The LD intensity at 520 K is reduced to 60 % relative to that at 300 K, which indicates a decrease of the anisotropy of the Ti 3d states. The multiplet analyses for the XAS/LD spectra give that (a_<1g>↑; a_<1g>↓) : (a_<1g>↑ ; e_g(π)↑) = 0.55:0.45, 0.15:0.85 at 520 and 300 K, respectively. The LD intensity changes only around MIT temperature, indicating that the change of the Ti 3d electronic configuration is related to MIT. These results support the MIT model proposed by Tanaka [1]. We have also found the change of the Ti 3d states from the O ls XAS spectra. In addition, the Ti 2p soft x-ray emission spectra exhibit an elastic peak only at metallic phase.[1] A. Tanaka, J. Phys. Soc. Jpn. 73, 152 (2004).