Electrical conductivity of non-stoichiometric LiNbO3 single crystals

Single crystals of LiNbO3 with different melt composition, (Li/Nb)(m) = 0.945, 1.0, 1.1 and 1.2, are grown by slow cooling of the melt. The crystals subjected to chemical analyses reveal a marked difference in starting/melt and crystal composition. Practically, evaporation losses of initial ingredients during the crystal growth are found to be responsible for nonstoichiometry in LiNbO3 single crystals. Partial replacement of monovalent Li+ by excess pentavalent Nb+5 creates additional cation vacancies to attain electro-neutrality in crystal. Complex impedance spectroscopy technique is applied to resolve the displacement current and ionic current through the crystals with P-O2 >1 atm. Oxygen partial pressure (>1 atm.) dependent conductivity, along the c-axis, obeys the Arrhenius law in the temperature range from 750 to 1000degreesC. The strong dependence of O-2 conductivity on crystal composition is understood to be because of defects. Although, the dc conductivity of LiNbO3 single crystals is very low compared to ionic, but it follows the Arrhenius behavior. The results are discussed in the light of defect chemistry.