In our study, various fast lithium-ion conductors of composition Li-1 (+) xAlxGe2 -xP3O12 (LAGP, 0 <= x <= 1.2) have been synthesized following a sol-gel process with subsequent annealing. For the annealed, crystallized samples, the subtle structural changes upon exchange of Ge4+ for Al3+ and Li+ in LiGe2P3O12 were followed employing X-ray diffraction and solid-state NMR. Among the samples studied, Li1.4Al0.4Ge1.6P3O12 (x = 0.4) shows the highest Li mobility at room temperature with an activation energy of approx. 30 kJ mol(-1). Employing heteronuclear dipolar NMR techniques, i.e., Al-27{P-31}-REDOR NMR and P-31(Al-27)-REAPDOR NMR, allowed us to dearly assign the different P-31-MAS-NMR signals to P(OAl)(n)(OGe)(4 - n), species with 0 <= n <= 3 and to show that the tetrahedrally coordinated Al is incorporated in an extra AlPO4 phase, thus not participating in the NASICON structure. For Alrich samples with x >= 0.7, apart from crystalline impurity phases such as AlPO4, Li4P2O2 and GeO2, an additional phase Li9Al3(P2O7)(3)(PO4)(2) is observed, which replaces the LAGP phase as the dominant phase for x > 1.0, giving rise to a compositional limit of Al content as x approximate to 0.6 in LAGP by the sol-gel route. In addition, the effect of the annealing temperature on the nature of the resulting phases was examined. (C) 2015 Elsevier B.V. All rights reserved.