Structural analysis of the dominant axial Fe3+ center in LiNbO3 crystal by electron nuclear double resonance

Abstract

Measurements of the angular dependencies of Electron Nuclear Double Resonance (ENDOR) were used for the analysis of the substitution site and local environment of the dominant axial Fe3+ center in LiNbO3 crystals. All components of hyperfine and quadrupole tensors for several shells of Li and Nb nuclei were determined. Since the found isotropic part of the hyperfine interaction of the iron electrons with the Li nuclei was small, it was justified to approximate the anisotropic part by the dipole-dipole interaction of electron and nuclear spins. From the comparison of the calculated and observed ENDOR spectra of the Li nuclei, it was derived that the Fe3+ ion occupies a Li site but is shifted from the regular Li position by 0.009 nm toward the octahedral structural vacancy. There is no evidence for the presence of any charge compensators in the nearest Li and Nb shells, which indicates a distant mechanism of excess charge compensation. ENDOR of Nb nuclei shows that there is a large isotropic hyperfine interaction due to the significant indirect transferred interaction of Fe3+ with nearest off axis Nb nuclei via O2− and polarization of Nb inner electron shells.

Structures of Impurity Defects in Lithium Niobate and Tantalate Derived from Electron Paramagnetic and Electron Nuclear Double Resonance Data

“…It was concluded also that Fe 3+ ions can be shifted from regular Li-site; however, the obtained values for the shift were very different: 0.006 nm [21] or 0.05 nm from octahedron center [24], less than 0.012 nm [25], about 0.01 nm [26], 0.018 ± 0.007 nm [28], less than 0.005 nm [39]. Fitting observed ENDOR data in LN crystals grown from Li-rich melt (xm ≈ 55%) was obtained with the shift 0.009 nm from Li site [217].…”