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

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

Postprint urn:nbn:de:0011-n-6417411 (5.7 MByte PDF)
MD5 Fingerprint: 96326f5a63e4a24e94e5d1a3bf522315
Copyright AIP
Erstellt am: 3.12.2021

The Journal of chemical physics 155 (2021), Nr.7, Art. 074501, 24 S.
ISSN: 0021-9606
Zeitschriftenaufsatz, Elektronische Publikation
Fraunhofer IKTS ()
NASICON; solid electrolyte; neutron diffraction; x-ray diffraction; glass ceramic

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 R3̄ , whereas the x = 0.8 composition crystallizes into space group-type R3̄ 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.