DEFECT CLUSTERS AND SUPERSTRUCTURES OF ZR4-XO( DISSOLVED NI1)

Ni1-xO (x< 0.001) powders, pure and mixed with pure ZrO2 or yttria-par tially stabilized zirconia (Y-PSZ), were sintered and then annealed at 1573 and 1873 K for up to 300 h to investigate the dopant dependence of defect clustering in the Ni1-xO lattice. Transmission electron micr oscopic observations coupled with energy X-ray analysis indicated that the dissolution of Zr4+ (ca. 2.0 mol% with or without co-dopant Y3+< 0.3 mol%) but not Ni3+ caused defect clustering, which was more rapid at 1873 than 1573 K and which preferred to nucleate at interfaces and dislocations. The paracrystalline distribution of defects was found to be nearly 3.5 and 2.5 times the lattice parameter of Ni1-xO for Zr-do ped and (Zr,Y)-codoped Ni1-xO, respectively. The predominantly dissolv ed Zr4+ cations, in octahedral sites with charge- and volume-compensat ing nickel and oxygen vacancies (i.e., Zr(oct)square(n)O(6-m)square(m) ), could create local domains in which Ni3+ should be expelled and, th us, in the vicinity the paracrystalline state and then the spinel Ni3O 4 could precipitate in local domains. The spinelloid, a superstructure of spinel with a relatively high Zr4+ content (ca. 3.5 mol%), appeare d only for the Ni1-xO particles located at Y-PSZ grain boundaries. (C) 1998 Academic Press.