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≤x≤1.0) have been successfully prepared through solid-state reactions at 1223–1423 K under inert argon atmosphere. XRD measurements showed that they crystallize in α-NaFeO2 type (space group R3m) for 0.72<x≤1 and in β-RbScO2 type structure (P63/mmc) for 0.60≤x≤0.72, respectively. The structural transition around x=0.72 is characterized by the change of coordination state of Na+ from octahedron (x>0.72) to trigonal prism (x≤0.72). Rietveld refinement results for x=0.90 and 0.67 suggest that the covalency is strengthened in (Ni,Ti)–O and weakened in Na–O through the competition (Ni,Ti)–O–Na as x decreases, and consequently the lattice becomes more two-dimensional. In Na0.67Ni0.33Ti0.67O2 sodium ions have been found to be distributed at two different prismatic sites, 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 considered to be correlated with the composition parameters such as x, the bond natures and the positive charge distribution. Comparison of lattice energies of two structure types suggests that the entropy change should also play an important role in the transition.