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In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, theemergence of exotic properties associated with rattling motion of Cs isexpected owing to its characteristic \beta-pyrochlore-type structure. However,a hurdle for crystal growth hampers elucidation of detailed properties andmechanisms of the MIT. Here we report on the epitaxial growth of\beta-pyrochlore-type CsW2O6 films and their electronic properties across theMIT. Using pulsed-laser deposi-tion technique, we grew single-crystallineCsW2O6 films exhibiting remarkably lower resistivity compared with apoly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistanceand positive Hall coefficient were found, which became pronounced below 200 K.The valence-band and core-levels photoemission spectra indicated the drasticchanges across the MIT. In the valence band photoemission spectrum, the finitedensity of states was observed at the Fermi level in the metallic phase. Incontrast, an energy gap appeared in the insulating phase. The split of W 4fcore-level spectrum suggested the charge disproportionation of W5+ and W6+ inthe insulating phase. The change of spectral shape in the Cs 4d core levelsreflected the rattling motion of Cs+ cations. These results strongly suggestthat CsW2O6 is a novel material, in which MIT is driven by the chargedisproportionation associated with the rattling motion.

Electronic properties across metal-insulator transition in \beta-pyrochlore-type CsW2O6 epitaxial films