In the search for new materials to be used as solid electrolytes, this paper discusses the substitution of phosphorus with silicon in the NaTi2(PO4)(3) NASICON (Na-Super Ionic Conductor) compound, giving rise to the Na1+yTi2SiyP3-yO12 series. In fact, the substitution of P+5 for Si+4 enables the increase in the Na+ charger carrier concentration. The solid electrolytes are synthetized by the glass-ceramic route, which consists in the controlled crystallization of a precursor glass subjected to specific heat treatment. Experimental results indicate that precursor glasses are successfully crystallized in compositions containing y <= 1.2. X-ray diffraction patterns show the formation of NASICON phase in the Na1+yTi2SiyP3-yO12 glass-ceramics for y-0.8. However, with further addition of silicon, the major crystalline phase obtained is the Na(TiO)(PO4) phase. Surprisingly, the electrical characterization reveals that the y = 1.0 sample, whose main phase is the non-NASICON Na(TiO)(PO4), exhibits the lowest activation energy (0.31 eV) and the highest ionic conductivity of 1.0 x 10(-4) S cm(-1) at room temperature and 1.7 x 10(-2) S cm(-1) at 300 degrees C. Rietveld refinement and electrical conductivity results suggest that the increased ionic conductivity in the Na(TiO)(PO4) phase is due to the inclusion of some Si+4 ions in its structure, thus forming a new and highly Na(TiO)((Si) PO4) conductive phase. (C) 2019 Elsevier Ltd. All rights reserved.