Structure of amorphous materials in the NASICON system Na1+xTi2SixP3-xO12

Abstract

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system Na1+xTi₂Si_xP3-xO₁₂withx= 0.8 andx= 1.0 was explored by combining neutron and high-energy x-ray diffraction with²⁹Si,³¹P and²³Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The²⁹Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si⁴units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The³¹P{²³Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the³¹P MAS NMR line shape originates from four-coordinated Pⁿunits, wheren= 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their³¹P-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) forx= 0.8 and 4.86(4) forx= 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.

Keywords: NASICON; amorphous materials; neutron and x-ray diffraction; solid-state NMR; structure.