The temperature dependence of static electrical conductivity sigma(dc)(T) of LiNbO3 single crystal (sp. gr . R3c), oriented along the crystallographic c axis, has been studied using impedance spectroscopy. The impedance was measured under temperature effect in vacuum in a heating-cooling cycle. The sigma(dc)(T) dependence with activation energy E-a = 1.22 eV, caused by migration of lithium vacancies V-Li', was obtained upon heating. Upon cooling, the sigma(dc)(T) curve can be divided into three regions with activation energies of 0.66, 0.915, and 0.285 eV. After the measurements in vacuum, color centers containing oxygen vacancies V((O))( )over dot and electrons were formed in the crystal bulk. A thermal effect in vacuum on a LiNbO3 crystal changes the mechanism of electrical conductivity from ionic to mixed ionic-electronic. At 673 K, the ionic conductivity upon cooling is 3.2 x 10(-6) S/cm, which exceeds the a dc value upon heating by a factor of about 4. At 500 K, the electronic conductivity upon cooling is 4.3 x 10(-7) S/cm, which exceeds the sigma(dc) value upon heating by a factor of about 515 (extrapolation).
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