Structure of crystalline and amorphous materials in the NASICON system Na_1+xAl_xGe_2-x(PO₄)₃

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na_1+xAl_xGe_2-x(PO₄)₃ with x = 0, 0.4, and 0.8 was investigated by combining (i) neutron and x-ray powder diffraction and pair-distribution function analysis with (ii) ²⁷Al and ³¹P magic angle spinning (MAS) and ³¹P/²³Na double-resonance nuclear magnetic resonance (NMR) spectroscopy. A Rietveld analysis of the powder diffraction patterns shows that the x = 0 and x = 0.4 compositions crystallize into space group-type R3̄, whereas the x = 0.8 composition crystallizes into space group-type R3̄c. For the as-prepared glass, the pair-distribution functions and ²⁷Al MAS NMR spectra show the formation of sub-octahedral Ge and Al centered units, which leads to the creation of non-bridging oxygen (NBO) atoms. The influence of these atoms on the ion mobility is discussed. When the as-prepared glass is relaxed by thermal annealing, there is an increase in the Ge and Al coordination numbers that leads to a decrease in the fraction of NBO atoms. A model is proposed for the x = 0 glass in which super-structural units containing octahedral Ge⁽⁶⁾ and tetrahedral P⁽³⁾ motifs are embedded in a matrix of tetrahedral Ge⁽⁴⁾ units, where superscripts denote the number of bridging oxygen atoms. The super-structural units can grow in size by a reaction in which NBO atoms on the P⁽³⁾ motifs are used to convert Ge⁽⁴⁾ to Ge⁽⁶⁾ units. The resultant P⁽⁴⁾ motifs thereby provide the nucleation sites for crystal growth via a homogeneous nucleation mechanism.