Electron paramagnetic resonance of lithium niobate heavily doped with chromium and lithium niobate codoped with magnesium and iron
Jorgensen, Jonathan David
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In this thesis, electron paramagnetic resonance (EPR) was utilized in measuring and characterizing the dopant ions in three samples of lithium niobate (LiNbO 3). The first sample was LiNbO 3 of congruent composition doped with 0.25% mol chromium (LiNbO 3:Cr). This sample was studied in detail using two microwave frequencies, 9.4 GHz and 34.4 GHz. It was also studied both at room temperature and at 10 K. Several centers including complexes of Cr-Cr pairs were observed in addition to the most prevalent axial Cr ³+ center. The other two samples were LiNbO 3:Mg:Fe, one of congruent composition and the other of stoichiometric composition. The congruent composition contained 6% mol Mg and 0.02% mol Fe, while the stoichiometric sample contained 0.45% mol Mg and 0.01% mol Fe. The stoichiometric composition contains all the same centers observed in the congruent material, plus two additional centers. Since the stoichiometric material provides EPR spectra of much higher resolution, those centers existing in both compositions were characterized more accurately from the stoichiometric material. A discussion of models for dopant center symmetries, dopant positions in the LiNbO 3 lattice, and the charge compensators required by each center is provided. It is shown that charge compensators play an important role in explaining the existence of the additional centers observed in the stoichiometric material.