Composition and Electrical Property Relationships in Polycrystalline Inorganic Materials.

摘要

The goal of this program is to establish the defect chemistry of compounds whose applicability depends on the types and amounts of ionic and electronic defects. Attention was focused in this project on LiNbO3, a material being studied for a variety of electro-optic device applications. The main experimental approach was the measurement of the equilibrium electrical conductivity of LiNbO3 single crystals as a function of temperature (900-1100 C), oxygen partial pressure (10 to the minus 15th power - 100,000 Pa or 10 to the minus 20th power - 1 atm), Li/Nb ratio (0.92-1.00), and impurity additions (up to 8.9 mole % MgO and 5.7 mole % TiO2). The conductivity was characterized by an electronic component that varied as P sub O2 to the minus 1/4 power for P sub O2, less than 10 Pa and an ionic contribution tending toward P sub O2 independence at higher P sub O2. Both components increased slightly (less than double) with increasing Li2O-deficiency, but were virtually unaffected by impurity additions. The crystal density increased on reduction proving that oxygen vacancies are not the major product of reduction, as commonly assumed. A defect model is proposed that involves only electrons, lithium vacancies, and excess niobium defects. The major conclusion is that LiNbO3 has a very large degree of intrinsic ionic disorder, amounting to several percent above 900 C, and that this controls the defect chemistry except for the most extreme limits of Li2O-deficiency or reduction.