Ion-Exchange Synthesis, Crystal Structure, and Physical Properties of Hydrogen Titanium Oxide H2Ti3O7

Hydrogen titanium oxide H2Ti3O7 was prepared from Na2Ti3O7 as a parent compound via Na+/H+ ion exchange in acidic solution at 333 K. It crystallizes in the monoclinic system, space group C2/m, and the lattice parameters of a = 16.0380(8) A, b = 3.7533(1) A, c = 9.1982(3) A, and β = 101.414(3)°. The crystal structure of H2Ti3O7 was refined to the conventional values of Rwp = 2.60% and Rp = 1.97% with a fit indicator of GOF = Rwp/Re = 1.90 by Rietveld analysis using powder neutron diffraction data. The basic (Ti3O7)2– framework in H2Ti3O7 was changed from that in the parent Na2Ti3O7. The atomic coordinate of hydrogen atoms were determined by this study for the first time. The hydrogen site in the layer space was refined with a strict H1–O3 distance of 0.80(2) A and H2–O4 distance of 0.86(2) A in H2Ti3O7, respectively. The structural stability of H2Ti3O7 was confirmed by bond valence sums. From these results, protons were suggested as the ordered occupation in the crystal structure.