Investigation of electrical properties inside double phosphates Na8-2xM4-x2+(P2O7)(4) containing simultaneously the Na+ ion and a divalent cation (M2+=Mn2+, Co2+, Ni2+) is undertaken by impedance spectroscopy. Ionic conductivity in these materials is due to Na+ ion mobility in tunnels along the c-axis centred on the (0,0,0) position. The best performance are obtained for the Go-phase. Correlations between structure and electrical properties are established.