A series of NASICON type compounds A1−xZr2−xNbx(PO4)3 (A=H, Li; x=0, 0.02, 0.1, 0.2) have been studied by 1H, 7Li, 31P NMR and impedance spectroscopy. Niobium substitution leads to cation mobility enhancement and favors the triclinic–rhombohedral phase transition. The phase transition and a local lithium motion are highly increased by vacancy formation, while cation conductivity is increased by interstitials formation in LiZr2(PO4)3 structure.