Electrical Conductivity of Non-Stoichiometric LiNbO₃ Single Crystals

Abstract

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 1000°C. 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.

Keywords:

• Lithium niobate
• ferroelectrics
• single crystals
• defects
• complex impedance spectroscopy
• oxygen ion transport

Acknowledgment

Author is thankful to Dr. S. S. Bhoga, Dr. S. Upadhyaya and Mrs S. Acharya for help rendered during the experimental work and providing some useful data.

Notes

^*Indicate charge distribution in completely filled MO

^**Indicate charge distribution in half filled MO