Interstitial hydrogen in vanadium dioxide

It has long been recognized that VO$_{\mathrm{2}}$ has a monoclinic-to-rutile phase transition as the temperature increases through 340K, accompanied by a remarkable increase in conductivity from insulator to metal[1,2]. Recently it has been found that interstitial hydrogen can modify both the structural and the electronic phase transition[3]. The way hydrogen does this has only recently begun to be studied[4]. We are using infrared spectroscopy to study the properties of H$_{\mathrm{i}}$, and are also using the CRYSTAL06 code[5] with hybridized DFT Hamiltonian to determine equilibrium positions and vibrational frequencies. IR spectroscopy finds several OH vibrational lines in hydrogenated VO$_{\mathrm{2}}$ samples. Within the monoclinic structure of VO$_{\mathrm{2}}$ there are four inequivalent sites for H$_{\mathrm{i}}$ which may lead to distinct spectroscopic signatures. [1] F. J. Morin, Phys. Rev. Lett. \textbf{3}, 34 (1959). [2] A. Zylbersztejn and N. F. Mott, Phys. Rev. B \textbf{11}, 4383 (1975). [3] J. Wei \textit{et al.}, Nature Nanotechnology \textbf{7}, 357 (2012). [4] K. H. Warnick \textit{et al.}, Appl. Phys. Lett. \textbf{104}, 101913 (2014). [5] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual }(University of Torino, Torino, 2006).