The relation between the Mg concentration threshold effect and the defect structures of the congruent Mg:LiNbO/sub 3/

Congruent Mg:LiNbO/sub 3/ with Mg concentration of 0, 1.0, 2.7, 5.0, and 6.0 mo 1% has been prepared. A set of defect models consistent with density measurement is proposed. When the Mg concentration (Mg) exceeds a critical value (Mg)/sub c/, Nb-site Mg in the form of Mg/sub Li/Mg/sub Nb(1/3/)Nb/sub Nb(2/3)/O/sub 3/ appears in the crystal. The (Mg)/sub c/ varies as the Li/Nb ratio of the crystal and should be 5.30 mo 1% for the congruent crystal. It is suggested that the optical absorption by the reduced crystals is due to defects comprising both F centers, formed under reduction, and different types of dipolarons. When (Mg) exceeds (Mg)/sub c/, (Mg/sub Li/-Mg/sub Nb/)-F-(Mg/sub Li/-Nb/sub N/) occurs in the reduced crystal, which causes an abrupt change in the optical absorption of the crystal.<
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