Hydrogen motion and site occupation in Ti2CoHx(Dx): NMR and neutron scattering studies

Nuclear magnetic resonance measurements of the 1H and 2D spin-lattice relaxation rates R1 for Ti2Ni-type compounds Ti2CoH0.5, Ti2CoH0.9 and Ti2CoD1.0 have been combined with inelastic neutron scattering (INS) measurements of the H(D) vibrational spectra and neutron diffraction measurements for the same samples. The R1 results for Ti2CoH0.9 and Ti2CoD1.0 give evidence for the existence of a fast H(D) motion with the characteristic jump rate of about 3×108 s−1 at room temperature, whereas for Ti2CoH0.5 such a fast motional process is absent. The results of the INS and neutron diffraction measurements suggest that the appearance of the fast hydrogen jump process in Ti2CoHx(Dx) with x≈1 is related to the occupation of an additional type of interstitial sites by hydrogen atoms. While in Ti2CoH0.5, hydrogen atoms are found to occupy only the octahedral sites with Ti6 coordination, a partial occupation of the tetrahedral Ti3Co and Ti2Co2 sites by H(D) atoms (in addition to the nearly completely filled sublattice of the octahedral sites) has been revealed for Ti2CoH0.9 and Ti2CoD1.0. Possible paths of hydrogen jump motion in Ti2CoHx(Dx) are discussed on the basis of the distances between the sites partially occupied by H(D) atoms.