Sodium-containing zirconium oxide samples (Na ⩽ 5.6%), prepared by contacting hydrous zirconium oxide with an Na⁺ solution at a given pH, have been studied by various techniques (XRD, DTA, XPS, surface area determinations and chemical analysis) to elucidate the effect of sodium on surface and bulk properties of zirconium oxide. Both dried (383 K) and thermally treated (⩽ 773 K in air) samples have been investigated.

The data show that sodium ions are adsorbed on the hydrous zirconium oxide in different amounts depending on the pH value. For the samples dried at 383 K the adsorbed Na⁺ are atomically dispersed on the hydrous zirconium oxide surface and cause a sintering of the material. For the thermally treated specimens, sodium slows down the crystallization process and affects the crystalline phases. Sodium is present in different locations: (i) atomically dispersed on the surface, (ii) as Na₂O crystals and (iii) incorporated in solid solution. The relative concentration of sodium in these different locations depends on the overal alkaline-ion content. The metastable tetragonal and/or metastable cubic zirconium oxide formation is discussed in terms of different interactions between the sodium species and the oxide matrix.