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 fun ctions of frequency (0.02-300 kHz) and temperature (300-900 K) in both heating and cooling processes. We have found three anomalies separati ng four different temperature regions. In order of decreasing temperat ure, the first anomaly centered near 800 K is a smooth but rapid incre ase in low-frequency (< 50 kHz) permittivity to very high values of or der 1 X 10(5), associated with the cubic to tetragonal transition. Thi s high permittivity shows considerable dispersion and decreases quite rapidly as temperature drops from 860 to 640 K. We attribute this larg e permittivity to superparaelectric clusters, and their slow relaxatio n to random electric fields from randomly placed Na+ and Bi3+ ions whi ch redistribute and allow cluster reversal with the rhoepsilon0epsilon (dc)' time constant determined by the conductivity and permittivity. T he other anomalies, seen previously by others, are a high-frequency pe rmittivity (50 to 300 kHz) which shows no dispersion and increases slo wly down to a peak of 3 X 10(3) near 640 K, where a trigonal phase exh ibiting antiferroelectric and/or incommensurate characteristics is rep orted to set in. It then drops and develops weak low-frequency dispers ion below 550 K which is the upper limit reported for spontaneous pola rization decay.