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-Muñoz, Ana Candida M. Rodrigues, Igor D'Anciães Almeida Silva, Hellmut Eckert, Chris J. Benmore, Philip S. Salmon

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Abstract

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 R3¯, whereas the x = 0.8 composition crystallizes into space group-type R3¯c. 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
Pages (from-to)074501
Number of pages23
JournalJournal of Chemical Physics
Volume155
Issue number7
Early online date16 Aug 2021
DOIs
Publication statusPublished - 21 Aug 2021

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

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