Structure of amorphous materials in the NASICON system Na1+xTi 2SixP3-xO12.

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system Na1+xTi 2Si x P3-xO 12 with x= 0.8 and x= 1.0 was explored by combining neutron and high-energy x-ray diffraction with 29Si, 31P and 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The 29Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si 4units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The 31P{ 23Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the 31P MAS NMR line shape originates from four-coordinated P n units, where n= 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their 31P- 23Na dipolar coupling strengths. The results support an intermediate range order scenario of a phosphosilicate mixed network-former glass in which the phosphate groups selectively attract the Na +modifier ions. Titanium takes a sub-octahedral coordination environment with a mean Ti-O coordination number of 5.17(4) for x= 0.8 and 4.86(4) for x= 1.0. A mismatch between the P-O and Si-O bond lengths of 8% is likely to inhibit the incorporation of silicon into the phosphorus sites of the NASICON crystal structure.