Phase relations of NASICON materials and compilation of the quaternary phase diagram Na₂O-P₂O₅-SiO₂-ZrO₂

A short overview is given on existing phase relations in the four related ternary diagrams, setting the frame for a quaternary phase diagram. On the basis of published data the three-dimensional phase region of NASICON materials is constructed and phase relations to ternary and binary systems as well as to single oxides are presented. To date, the NASICON region can be described as a compressed tetrahedron within the tetrahedral phase diagram. However, the three-dimensional presentation clearly elucidates that few reported compositions exist outside this compressed tetrahedron indicating that the phase region of NASICON materials may be larger than the solid solutions known so far. The three-dimensional representation also better elucidates the regions connecting the edges of the NASICON tetrahedron with ternary and binary compounds as well as single oxides, i.e., ZrO₂ and ZrSiO₄, Na₃PO₄, sodium silicates and sodium zirconium silicates and gives a better understanding of phase formations during the processing of the ceramics. The implications of the formation of secondary phases and glass-ceramic composites are discussed in terms of technological applications