Various composition of Na1+xSixZr2P3-xO12 (x from 1.6 to 2.4), Y-doped Nasicon (Na1+x+yZr2-yYySixP3-xO12, Na1+xZr2-yYy SixP3-xO12-y, where x = 2, y = 0.12) and Fe-doped Nasicon (Na3Zr2/3Fe4/3P3O12) were prepared by coprecipitating. Differential thermal analysis (DTA), thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy (IS) were used as experimental techniques. In order to obtain Nasicon materials free from ZrO2 admixture, the calcination of coprecipitates must be carried out in proper thermal conditions. The results of DTA, TG and XRD measurements allowed us to propose the best calcination conditions (to obtain mainly Nasicon phases - monoclinic or rhombohedral, depending on composition). Nasicon-type materials exhibit monoclinic to rhombohedral reversible structural transition, at transition temperature depending on composition (x). The influence of dopants was also studied. The DSC measurements in the temperature range RT-300degreesC allowed us to determine the temperatures of this structural transition in the case of Na1+xSixZr2P3-xO12, and Y-doped Nasicon. In the case of Fe-doped materials this transition was not detected. Additionally, the correlation between the composition, microstructure and electrical properties was studied. (C) 2004 Published by Elsevier B.V.