SPONTANEOUS TRANSITION BETWEEN RELAXOR AND FERROELECTRIC STATES IS LANTHANUM-MODIFIED LEAD-ZIRCONATE-TITANATE-(6-7)/65/35

Dielectric and light scattering measurements are performed in lanthanum modified lead zirconate titanate (PLZT) (6-8)/65/35 hot-pressed ceramics in order to refine the phase diagram. A relaxor-ferroelectric phase transition takes place in PLZT (6-7)/65/35 at temperature lower than the temperature of the dispersive dielectric constant maximum. At this temperature the size of polar regions ''a'' essentially increases up to a>lambda (=0.63 mu m). The appearence of ferroelectric long-range order in PLZT with decreased concentration of La ions is explained by larger coupling between ferroelectrically active BO6 octahedrons due to the smaller concentration of defects and higher dielectric constant.