Pressure-induced ferroelectric to antiferroelectric phase transition inPb0.99(Zr0.95Ti0.05)0.98Nb0.02O3

Abstract

Zr-rich, Nb-doped lead zirconate titanate ceramic and powder samples with composition near ${\mathrm{Pb}}_{0.99}{({\mathrm{Zr}}_{0.95}{\mathrm{Ti}}_{0.05})}_{0.98}{\mathrm{Nb}}_{0.02}{\mathrm{O}}_{3}$ [PZT95/5(2Nb)] have been studied in the range of hydrostatic pressure 0--6.2 kbar and temperature 12--295 K by time-of-flight neutron powder diffraction and dielectric measurements. The combination of the two techniques has led to further insights into the properties and pressure-induced ferroelectric rhombohedral $R3c$ $({F}_{R(\mathrm{LT})})$ to antiferroelectric orthorhombic Pbam $({A}_{O})$ phase transition in this material, and the diffraction results have provided a detailed view of the ionic displacements induced by changes in pressure and temperature as well as the displacements accompanying the transition. At 295 K the diffraction results revealed a sharp transition at 2.1 kbar; at 200 K this transition occurs at 1.1 kbar. The transformation is incomplete: after the initial sharp drop in the ${F}_{R(\mathrm{LT})}$ content at the transition, 20 wt % of the sample remains in the low-pressure ${F}_{R(\mathrm{LT})}$ phase. Above the transition, the fraction of ${F}_{R(\mathrm{LT})}$, which exists as a minority phase in the high-pressure ${A}_{O}$ phase, continues to decrease, but even at our highest pressure of 6.2 kbar, $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{wt}\phantom{\rule{0.2em}{0ex}}%$ of the sample remains in the ${F}_{R(\mathrm{LT})}$ phase. The volume contraction at the ${F}_{R(\mathrm{LT})}$-to-${A}_{O}$ transition unexpectedly results in the retained minority ${F}_{R(\mathrm{LT})}$ being anisotropically ''clamped,'' with its $a$ axis slightly expanded and $c$ axis contracted at the transition. On pressure release to 1 bar at 295 K, only 26% of the ${F}_{R(\mathrm{LT})}$ phase is recovered, and this remains in the clamped state because of the surrounding majority ${A}_{O}$ phase. Heating the sample above 350 K at 1 bar followed by cooling to room temperature results in full recovery of the ${F}_{R(\mathrm{LT})}$ phase. The spontaneous polarization $({P}_{S})$ of the ${F}_{R(\mathrm{LT})}$ phase and its pressure and temperature dependences were determined from the ionic displacements. At 295 K, ${P}_{S}=38\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$---a value greater than the $31\text{--}32\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ commonly observed on ceramic PZT95/5(2Nb) samples. The difference is undoubtedly related to residual porosity in ceramic samples as well as the inability of the poling electric field to align all the polar domains. ${P}_{S}$ increases monotonically with decreasing temperature, reaching a value of $\ensuremath{\sim}44\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ at 12 K.