Structure of crystalline and amorphous materials in the NASICON system Na1+xAlxGe2-x(PO4)(3)

摘要

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na1+xAlxGe2-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) Al-27 and P-31 magic angle spinning (MAS) and P-31/Na-23 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 R3, whereas the x = 0.8 composition crystallizes into space group-type R3c. For the as-prepared glass, the pair-distribution functions and Al-27 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. Published under an exclusive license by AIP Publishing.