SEQUENCE OF DIELECTRIC ANOMALIES AND HIGH-TEMPERATURE RELAXATION BEHAVIOR IN NA1/2BI1/2TIO3

Measurements of relative permittivity (epsilon=epsilon'-iepsilon'') in the complex perovskite crystal Na1/2Bi1/2TiO3 were carried out as functions of frequency (0.02-300 kHz) and temperature (300-900 K) in both heating and cooling processes. We have found three anomalies separating four different temperature regions. In order of decreasing temperature, the first anomaly centered near 800 K is a smooth but rapid increase in low-frequency (< 50 kHz) permittivity to very high values of order 1 X 10(5), associated with the cubic to tetragonal transition. This high permittivity shows considerable dispersion and decreases quite rapidly as temperature drops from 860 to 640 K. We attribute this large permittivity to superparaelectric clusters, and their slow relaxation to random electric fields from randomly placed Na+ and Bi3+ ions which redistribute and allow cluster reversal with the rhoepsilon0epsilon(dc)' time constant determined by the conductivity and permittivity. The other anomalies, seen previously by others, are a high-frequency permittivity (50 to 300 kHz) which shows no dispersion and increases slowly down to a peak of 3 X 10(3) near 640 K, where a trigonal phase exhibiting antiferroelectric and/or incommensurate characteristics is reported to set in. It then drops and develops weak low-frequency dispersion below 550 K which is the upper limit reported for spontaneous polarization decay.