Spécificités des ions NbIV et MoIVdans les monophosphates de type Nasicon

The complete solid solutions NaMIVVVVx (Ti, Zr)2-x(PO4)3 with M = Nb, Mo and 0 ≤ x ≤ 1 have been studied in order to state precisely their crystal chemical, electrical and magnetic characteristics. Substitutions of Ti and Zr by Nb or Mo involve local deformations without space group modification and, correlatively, increase the ionic conductivity by motion of Na1 cations. Results of magnetic and spectroscopic studies characterize M transition elements having the oxidation number IV, except for NaNbTi(PO4)3 where the two states TiIII-NbV and TiIV-NbIV coexist, in agreement with electronic conductivity values and redox potentials of the corresponding elements.