CuO-doped NaNbO3 antiferroelectrics: Impact of aliovalent doping and nonstoichiometry on the defect structure and formation of secondary phases
Erunal, E; Jakes, P; Korbel, S; Acker, J; Kungl, H; Elsasser, C; Hoffmann, MJ; Eichel, RA
PHYSICAL REVIEW B, 2011, Band 84, Heft 18, Artikel Nummer 184113, DOI: 10.1103/PhysRevB.84.184113
The interplay between aliovalent CuO doping and nonstoichiometry on the development of defect structures and the formation of secondary phases of antiferroelectric NaNbO3 ceramics has been investigated by means of x-ray diffraction (XRD), first-principles calculations using density functional theory (DFT), and electron paramagnetic resonance spectroscopy. The results indicate that, for stoichiometric 0.25 mol% CuO-doped NaNbO3, as well as for 2.0 mol% Nb-excess sodium niobate, the Cu2+ functional centers are incorporated at the Nb site (Cu'''(Nb)). For reasons of charge compensation, two kinds of mutually compensating defect complexes (Cu'''(Nb) - V-O(center dot center dot))' and (V-O(center dot center dot) - Cu-Nb(center dot center dot) - V-O(center dot center dot))(center dot) are formed where, for the niobium-excess compound, additionally, V'(Na) contribute to the mechanism of charge compensation. In contrast, for 2.0 mol% Na-excess sodium niobate, a Na3NbO4 secondary phase has been detected by XRD, and only part of the Cu2+ forms these types of defect complexes. The major part of the Cu2+ is incorporated in a fundamentally different way by forming Cu2+-Cu2+ dimeric defect complexes.