Glass-to-Crystal Transition in Li1+xAlxGe2–x(PO₄)₃: Structural Aspects Studied by Solid State NMR

The structural aspects of the glass-to-crystal transition in the technologically important ion conducting glass ceramic system Li 1+ x Al x Ge 2– x (PO 4 ) 3 (0 ≤ x ≤ 0.75) have been examined by complementary multinuclear solid state nuclear magnetic single and double-resonance experiments. In the crystalline state, the materials form solid solutions in the NASICON structure, with additional nanocrystalline AlPO 4 present at x values ≥0.5. Substitution of Al in the octahedral Ge sites results in a binomial distribution of multiple phosphate species, which differ in the number P–O–Al and P–O–Ge linkages and can be differentiated by 31 P chemical shift and 31 P{ 27 Al} rotational echo adiabatic passage double resonance (REAPDOR) spectroscopies. The detailed quantitative analysis of these data, of complementary 27 Al{ 31 P} rotational echo double resonance (REDOR) and of homonuclear 31 P– 31 P double quantum NMR studies suggest that the AlO 6 coordination polyhedra are noticeably expanded compared to the GeO 6 sites in the NASICON-type LiGe 2 (PO 4 ) 3 (LGP) structure. While the glassy state is characterized by a significantly larger degree of disorder concerning the local coordination of germanium and aluminum, dipolar solid state NMR studies clearly indicate that their medium range structure is comparable to that in NASICON, indicating the dominance of P–O–Al and P–O–Ge over P–O–P and Al–O–Ge connectivities.