Paracrystal Formation from Ni1−xO and CaO upon Interdiffusion

Abstract

Ni1−xO and CaO powders 1:4 in molar ratio were sintered and annealed at 1200°C for 300 h or 1400°C for 1–80 h to investigate interdiffusion-induced defect clusters in the two rock salt-type oxides by analytical electron microscopy. Paracrystalline array of defect clusters ca. 3.2 and 1.6 times the lattice spacing of the average structure formed, respectively, from 8 mol% Ca dissolved Ni1−xO particles and 10 mol% Ni dissolved CaO matrix grains. The formation of paracrystal in the two oxides can be explained by the substitution of isovalent but size-mismatched dopant for cation in octahedral sites and resultant Ni3+ in tetrahedral interstitial sites to generate a considerable number of volume- and charge-compensating defects. The undersized dopant, i.e., nickel cation in CaO, may also directly reside in interstitial tetrahedral site; hence there is a higher concentration of defect clusters than the case of Ca2+-doped Ni1−xO. The paracrystal formed predominantly at the Ni1−xO/CaO interface due to interdiffusion kinetics and a beneficial lower strain and interfacial energy.