Effect of Nanoscale Defects on the Physical Properties of Lithium Niobate and Lithium Tantalate Crystals

The elastic, ferroelectric, and transport properties of congruent lithium niobate and lithium tantalate crystals are studied in the temperature range of 77–450 K, depending on the conditions for recovery annealing. Significant changes in the elastic moduli and electrical conductivity that correlate with an increase in the displacement of the off-center Nb 5+ (Ta 5+ ) ions along the trigonal C ¯ axis of the oxygen octahedra NbO 6 (TaO 6 ) are found in the interval 120 to 300 K as a result of more detailed studies. The attenuation of acoustic waves is suppressed as the temperature falls, which can be explained by an increase in the degree of ordering of NbO 6 (TaO 6 ) clusters. It may be assumed that the strong change in electrical conductivity correlates with the concentration of point nanoscopic defects (antisite defects Nb Li 5+ (Ta Li 5+ ), coupled polarons Nb Li 4+ (Ta Li 4+ ), and bipolarons).