Powdered NiO and CaO, in the molar ratio of 1:4, were sintered and annealed (1200C, 300h or 1400C, 1 to 80h). Paracrystalline arrays of defect clusters which had about 3.2 or 1.6 times the lattice spacing formed, respectively, from 8mol%Ca dissolved NiO particles and 10mol%Ni dissolved CaO matrix grains. The formation of paracrystals in the oxides could be explained in terms of the substitution of isovalent (but size-mismatched) dopants for cations at octahedral sites. The resultant Ni3+ in tetrahedral interstitial sites generated a considerable number of volume- and charge-compensating defects. An undersized dopant, such as Ni cations in CaO, could also reside directly in interstitial tetrahedral sites. Hence, there was a higher concentration of defect clusters than in the case of Ca2+-doped NiO. The paracrystals formed mainly at the NiO/CaO interface, due to the interdiffusion kinetics and a beneficial lower strain and interfacial energy.

Paracrystal Formation from NiO and CaO upon Interdiffusion. M.L.Jeng, P.Shen: Journal of Solid State Chemistry, 2000, 152[2], 421-7