X-ray characterization of the new nasicon compositions Na 3 Zr2_ x /4Si2- x P 1+ x0 12 with x= 0.333, 0.667, 1.000, 1.333, 1.667 M. Lucco-Borlera, D. Mazza, L. Montanaro, A. Negro, and S. Ronchetti Dipartimento di Scienza dei Materiali e Ingegneria Chimica—Politecnico di Torino (Received 5 March 1997; accepted 10 May 1997) It is known that solids with composition Na3Zr 2 Si2PO 12 heated at 1200 °C crystallize in the nasicon structure. This material shows a high ionic conductivity that represents an interesting improvement in the field of solid electrolytes. Our experimental results allow to establish for the first time that nasicon structures are stable along the compositional join Na 3 Zr 2 -^/4Si2- x Pi + x ®\2 w i m x extending from 0 to 1.667. These structures are characterized by a Zr underoccupation of octahedral sites and a constant number of Na + ions. This fact envisages a possible application of these materials in the field of ceramic sensors and ionic conductors. © 1997 International Centre for Diffraction Data. [S0885-7156(97)00403-X] Key words: nasicon, ionic conductors, zirconium INTRODUCTION The discovery of nasicon (Na superionic conductor) rep- resented a substantial improvement in solid electrolytes de- velopment. Its ionic conductivity surpasses indeed that of two-dimensional networks, like beta-alumina. It is well known, from the pioneer work of Hong (1976), that along the compositional range Nai + ^Zr 2 Si^P 3 __ c O 1 2 with 0<x <3, solids heated at 1200 °C crystallize in the nasicon struc- ture. The same author showed that nasicon structure has rhombohedral symmetry (space group R3c; no. 167) along all the compositional range, except in the interval 1.8<x <2.2, where a small distortion to monoclinic symmetry oc- curs (e.g. C2/c). From the structural determination it results that zirco- nium ions have sixfold coordination with oxygen, while sili- con and phosphorus are randomly distributed on sites with tetrahedral fourfold coordination. Sodium ions are located in two positions, one with sixfold and the other with eightfold coordination, both positions having Na-O distances exceed- ing the normal values typical in sodium oxides. The Zr and (Si,P) coordination polyhedra are firmly con- nected by sharing all the oxygen corners, forming there from a three-dimensional framework. This framework can be eas- ily described by defining a structural building unit (SBU) formed by two overlaying ZrO 6 octahedra, connected by three (Si,P)O 4 tetrahedra, each sharing a corner, with the two above octahedra (Fig. 1). From this SBU it is inferred that the stoichiometry of the phase is 2-(ZrO 6 ) + 3 (SiyP^yO^ or considering that each oxygen atom is shared between two polyhedra, simply Zr 2 Si 3) ,P 3 _ 3> ,O 12 . The negative charge imbalance of this formula is compensated by the Na + ions, lying in the interpolyhedral locations that re- sult within the framework, leading therefore to the classical nasicon formulation Na,+^Zr 2 Si x P 3 _ x O 12 where x now stands for 3v. The compositions showing the highest conductivity cor- respond to x = 2 and therefore to three Na ions per formula unit. Kohler et al. (1983) and later Rudolf et al. (1985) suc- ceeded in preparing Zr-deficient monoclinic compositions, in which the relevant structural data were deduced from X-ray and neutron diffraction studies. Na Na Figure 1. Schematic representation of two overlaying structural building unit (SBU). 171 Powder Diffraction 12 (3), September 1997 0885-7156/97/12(3)/171 /4/$6.00 i 1997 JCPDS-ICDD 171