Hydrogen in nonstoichiometric cubic titanium monoxides: X-ray and neutron diffraction, neutron vibrational spectroscopy and NMR studies

First-principles study of hydrogen dynamics in monoclinic TiO

“…As was correctly anticipated in Ref. [20], the low-energy ( 60 meV) peak in their neutron scattering spectra (cf. Fig 4 and 5 of the work cited) is due to optical vibrations of oxygen atoms.…”

“…For additional insight, and also in order to offer some discussion of interesting neutron energy loss spectra published by Skripov at al. [20], we calculated density of vibration modes for H-doped (one atom per supercell) monoclinic TiO. The QE calculation used density functional perturbation method, and Siesta -frozen phonon calculation.…”

“…It was emphasized in Ref. [20] that "... the volume of the voids formed by oxygen vacancies is too large for H atoms, so that a hydrogen atom can be easily displaced from the geometrical center of the vacancy". The diversity of such displacements was considered as a possible reason for the broadness of the H-related vibration spectra.…”

“…A different choice, that of disordered B1 lattice, would make the results too much dependent on the particular model(s) of disorder used. An ambition of the present work is to challenge the conclusions formulated by Skripov et al [20], on the basis of first-principles calculations, and/or to give these qualitative conclusions a numerical expression. Specifically, we probe adsorption energies of hydrogen at different sites, calculate vibration modes of the crystal doped with hydrogen, identify plausible diffusion paths and esimate corresponding energy barriers.…”

“…Titanium monoxide enters the picture as Swaminathan et al [19] reported that strongly reduced titania (with nominal composition TiO 1.23 , the crystal structure of which reveals X-ray diffraction peaks of disordered TiO) exhibits enhanced HER activity. Recently, Skripov et al [20] investigated hydrogen absorption in substoichiometric TiO (TiO 0.72 H 0.30 and TiO 0.96 H 0.14 ) by a combined use of X-ray and neutron diffraction, neutron vibrational spectroscopy and nuclear magnetic resonance. This work led to the following conclusions: (i) in both nearly stoichiometric and strongly substoichiometric (oxygendeficient) samples, hydrogen atoms resided exclusively at O vacancy sites; (ii) the hydrogenation of nearly stoichiometric, originally B1 disordered, phase provokes an emergence of an ordered (Ti 5 O 5 monoclinic) phase coexisting with the disordered (B1-structure) TiO; (iii) hydrogen diffusion seems to be insignificant; (iv) vibrations of hydrogen atoms occur throughout a broad range of frequencies, presumably as a manifestation of different environments (a possible symmetry-lowering off-center displacement of the trapped hydrogen; a completeness or not of its coordinating Ti octahedron) occurring at different O vacancy sites.…”