Preparation and structural properties of layer-type oxides NaxNix/2Ti1-x/2O2 (0.60 <= x <= 1.0)

Layer-type oxides NaxNix/2Ti1-x/2O2 (0.60 less than or equal to x less than or equal to 1.0) have been successfully prepared through solid-state react ions at 1223-1423 K under inert argon atmosphere. XRD measurements showed t hat they crystallize in alpha-NaFeO2 type (space group R (3) over bar m) fo r 0.72 < x less than or equal to 1 and in beta-RbScO2 type structure (P6(3) /mmc) for 0.60 less than or equal to x less than or equal to 0.72, respecti vely. The structural transition around x = 0.72 is characterized by the cha nge of coordination state of Na+ from octahedron (x > 0.72) to trigonal pri sm (x less than or equal to 0.72). Rietveld refinement results for x = 0.90 and 0.67 suggest that the covalency is strengthened in (Ni,Ti)-O and weake ned in Na-O through the competition (Ni,Ti)-O-Na as x decreases, and conseq uently the lattice becomes more two-dimensional. In Na0.67Ni0.33Ti0.67O2 so dium ions have been found to be distributed at two different prismatic site s, having the occupation ratios, 0.198 at (0, 0, 1/4) and 0.472 at (1/3, 2/ 3, 1/4), respectively. The occurrence of structural transition is considere d to be correlated with the composition parameters such as x, the bond natu res and the positive charge distribution. Comparison of lattice energies of two structure types suggests that the entropy change should also play an i mportant role in the transition. (C) 2000 Elsevier Science B.V. All rights reserved.