A novel inorganic solid electrolyte with a layered framework structure stable up to 1043 K, Na 14.5 [Al(PO 4 ) 2 F 2 ] 2.5 [Ti(PO 4 ) 2 F 2 ] 0.5 (NATP), has been hydrothermally prepared and characterized by single-crystal and powder X-ray diffraction techniques, X-ray fluorescence (XRF) analysis, IR spectroscopic measurement, thermogravimetric and differential thermal analysis (TGA and DTA). NATP crystallizes in the acentric hexagonal space group P3 with a=10.448(2), b=10.448(2), c=6.589(3)Å, Z=1, containing a large number of Na + cations in the interlamellar space and the cavities of its framework. There are six different crystallographic Na + cationic sites, in which 8% Na(5) and 12% Na(6) sites are vacant. Electrical conductivity measurements show that Na + cations exhibit a high mobility with two domains for the electrical conductivity versus temperature.