"Lead-free" piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, (Cu Nb - V O ) and (V O - Cu Nb - V O ) . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric (V O - Cu Nb - V O ) defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric (Cu Nb - V O ) defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric "hardening.

Cu(2+)-doped Pb[Zr(0.54)Ti(0.46)]O(3) (PZT) and Cu(2+)-doped [K(0.5)Na(0.5)]NbO(3) (KNN) ferroelectrics with a dopant concentration of 0.25 mol% were investigated by means of multi-frequency and multi-pulse electron paramagnetic resonance (EPR) spectroscopy. Through the use of high magnetic fields and pulsed microwave fields an enhanced resolution was achieved yielding valuable information about the structural distortion at the dopant site. The results obtained suggest that Cu(2+) substitutes for both systems as an acceptor centre for the perovskite B-site. For reasons of local charge compensation, different kinds of defect associates invoking one and two oxygen vacancies are formed. These two kinds of extended defects differ in their electric and elastic properties. The results obtained are analyzed in order to characterize differences of the local structure in the Cu(2+)-defect center for morphotropic phase boundary compositions between the two systems. In particular, it is found that Cu(2+)-doping in KNN creates 50% more oxygen vacancies than the same amount of copper in PZT. Furthermore, local differences in covalent and ionic bonding are monitored.

The defect structure resulting from copper doping of potassium niobate ( KNbO 3) ceramics was investigated by a combined analysis of electron paramagnetic resonance (EPR) spectroscopy and first-principles calculations based on density functional theory (DFT). The results indicate that under atmospheric oxygen partial pressure, Cu preferentially substitutes on the Nb sites where it can trap one or two oxygen vacancies. Correspondingly, for 0.25 mol% Cu doped KNbO 3 ceramics, two types of defect associates are formed — [Formula: see text] and [Formula: see text]. The association of Cu impurities and oxygen vacancies lowers the defect formation energy by 1.0 eV to 2.7 eV, depending on the Fermi level. Owing to the opposite charges of these two types of defects, overall charge neutrality is theoretically possible by mutual compensation of the defect associates, without formation of additional non-associated oxygen vacancies.

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HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Defect structure and formation of defect complexes inCu2+-modified metal oxides derived from a spin-Hamiltonian parameter analysis Rüdiger EichelTo cite this version:Rüdiger Eichel. Defect structure and formation of defect complexes in Cu2+-modified metal oxides derived from a spin-Hamiltonian parameter analysis. The nearest neighbor oxygen octahedron about copper functional centers in metal oxides has been systematically studied by means of electron paramagnetic resonance (EPR) spectroscopy. In particular, the determined g ,zz and A Cu ,zz spin-Hamiltonian parameters were analyzed, finding linear dependences as function of chemical bonding and local distortion of the oxygen octahedron. Moreover, through the introduction of a dimensionless coordination parameter ξ, different defect structures with respect to the number of coordinated oxygen vacancies may be distinguished. This allows for a distinct assignment of defect complexes between the copper functional center with one or two oxygen vacancies.