NMR Investigations in Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 Ceramics. Part I: Structural Aspect
Résumé
Because of its high Li+ conductivity, the family Li1+xAlxTi2–x(PO4)3 has already been widely studied; previous structural characterizations reported that aluminum occupied two types of sites in the NASICON framework: the first one in octahedral coordination corresponding to Ti/Al substitution; the second one in tetrahedral coordination corresponding to P/Al substitution. In this work we show that it is possible to synthesize samples presenting only the Ti/Al substitution in the octahedral site, which is more consistent with the formulation. The static local properties of our samples were characterized by multinuclear nuclear magnetic resonance (NMR) and X-ray diffraction. The MAS NMR aluminum spectrum is characterized by a strong parameter of asymmetry (ηQ = 0.9), indicating that aluminum ions are situated in sites which lost their axial symmetry. This loss of symmetry is accompanied by an increase of the number of chemical sites of the phosphorus, among which some are characterized by broad lines. The strong asymmetry quadrupolar parameter, together with the strong broadening of the 31P lines assigned to phosphorus with three Ti4+ and one Al3+ are marks of the M2(IV)PO4 skeleton’s distortion. The multinuclear NMR experiment also allowed us to analyze the abnormal behavior of the lithium quadrupolar parameter νQ in relation with the flexibility of the M2(IV)PO4 skeleton characteristic of the NASICON family.