Titanium phosphate glasses: Beyond tetrahedral network structures

The structure of titanium phosphate glasses (TiO2)x(P2O5)1-x with 0.70 ≤ x ≤ 0.75 was investigated by combining neutron and high-energy x-ray diffraction with solid-state 31P nuclear magnetic resonance (NMR) and Raman spectroscopy. The results were interpreted with the aid of an analytical model that delivers the composition dependence of the structural motifs. The structure of these materials was also simulated using ab initio molecular dynamics. A detailed 31P magic-angle spinning (MAS) NMR lineshape analysis, aided by the results obtained from double-quantum coherence spectroscopy, indicates the presence of P-O-P-connected network forming units at a level decreasing from 23% to 11% with increasing x. The diffraction results show a Ti-O coordination number of 5.32(7) at x = 0.715 that increases to 5.49(7) at x = 0.750. The findings demonstrate the prevalence of five- and six-coordinated titanium atoms and the coexistence of both two-coordinated oxygen atoms, O(II), and three-coordinated oxygen atoms, O(III). The Ti-centered polyhedra contribute to a network in which the phosphate groups form P-O(II)-Ti and P-O(III)-2Ti connections, with signatures that are evident in the 31P MAS NMR spectra. The results suggest that structural variability is a key factor in promoting vitrification in this atypical glass-forming system. The findings provide a benchmark for investigating the structure of other glass-forming materials based on networks of higher-coordinated polyhedral units.