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

Lawrence V. D. Gammond, Henry Auer, Rita Mendes Da Silva, Anita Zeidler, Jairo F. Ortiz-Mosquera, Adriana M. Nieto-Munoz, Ana Candida M. Rodrigues, Igor d'Anciaes Almeida Silva, Hellmuth Eckert, Chris J Benmore, Philip Salmon

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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) 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 R¯3, whereas the x = 0.8
composition crystallizes into space group-type R¯3c. 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.
Original languageEnglish
Article number074501
Number of pages23
JournalJournal of Chemical Physics
Early online date16 Aug 2021
Publication statusE-pub ahead of print - 16 Aug 2021

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