Structure and electrical properties in the K_1/2Bi_1/2TiO₃–K_1/2Bi_1/2ZrO₃ solid solution (KBT–KBZ)

Abstract

The present work is devoted to the study of the Zr4+/Ti4 substitution in the K1/2Bi1/2Ti(1–x)ZrxO3 solid solution (x = 0.0 to x = 1.0), based upon the K1/2Bi1/2TiO3 (KBT) ferroelectric compound. The tetragonal distortion of KBT is suppressed by this substitution and leads to the cubic compound K1/2Bi1/2ZrO3 (KBZ). These results agree with the values of the ionic radii of the Zr4+ and Ti4 ions ($r_{{\rm Ti}^{4 + } } $ = 0.605 Å and $r_{{\rm Zr}^{4 + } } $ = 0.72 Å). Close to KBT (x ≤ 0.05), the symmetry remains tetragonal. For higher values of x, the “a” lattice parameter (cubic indexing) follows Vegard's law, thus confirming the formation of a solid solution. The transformation from tetragonal to cubic proceeds via an intermediate pseudocubic symmetry (0.1 ≤ x < 0.5), for which the X‐ray diffraction peaks present small broadening and asymmetry. For x ≥ 0.5 and up to the KBZ compound (a ≈ 4.158 Å), the samples are cubic but some extra peaks are also observed, indicating the occurrence of a secondary phase. The microstructure shows fine‐grained ceramic samples for the first range, while for the KBZ‐rich range the grains are micrometer‐sized and associated to very small grains of the secondary phase. Piezoelectricity is observed for the tetragonal and pseudocubic range, the substitution quickly reducing the piezoelectric properties. The measurement of the dielectric properties revealed close to KBT a dielectric anomaly probably associated to the tetragonal–cubic phase transition. For the pseudocubic and cubic range, a broad dielectric anomaly is observed around 300 °C, corresponding to a relaxor behavior.

Long- and short-range structure studies of KBT-KBZ solid-solutions using synchrotron radiation

“…Interestingly, the refinement for KBT (x = 0) by Bengagi et al shows the (111) reflection to be significantly undercalculated. 16 A better fit to the peak profile was obtained here by refining for strain factors. The improvement is shown in Fig.…”

“…These systematic trends suggest that Ti/Zr atoms are randomly distributed in the lattice, 13,14 with no significant clustering. The local crystal structure from x = 0.5 to 0.9 ( previously reported as cubic 16 ) does not appear to ever become completely centrosymmetric, with Ti remaining off-centered. This is consistent with our high-resolution SXRD diffraction results, and the observation of weak ferroelectricity up to x = 0.9 by Bengagi et al 16 Fig.…”

“…In the conventional XRD data of Bengagi et al 16 diffraction peaks were generally observed to broaden with increasing Ti content through the pseudocubic region (x = 0.1-0.4). Visually, the same behaviour was observed in our SXRD data for x = 0.1-0.4, with only the pure KBT compound found to possess a clear tetragonal distortion (i.e., split peaks) (Fig.…”

Hydrothermal synthesis of lead-free perovskite (Bi_1/2K_1/2)(Zr<i>_x</i>Ti_1&minus;<i>_x</i>)O₃ powders

“…If a heat-treated sample includes two perovskite phases with different Zr/Ti ratios as reported for the solid-state synthesis, a strong split of the (111) peak in its XRD pattern should be observed. 18) However, none of the (111) peaks for the various compositions are split. Moreover, with increasing Zr content, these peaks obviously shift toward lower angles, as observed in the case of the as-synthesized samples; accordingly, the unit-cell volume monotonically increases, as shown in Fig.…”

“…Bengagi et al attempted to fabricate Zr-substituted BKT [i.e., (Bi 1/2 K 1/2 )-(Zr x Ti 1¹x )O 3 , denoted as BKZT hereafter] ceramics via a conventional solid-state reaction and found that the products included two perovskite phases with different Zr/Ti ratios even after extended thermal annealing as long as 100 h. 18) In another study, Liu et al reported the preparation of single-phase BKZT powders via a solid-state reaction involving a similar long calcination time. 19) However, in their case, a large excess of K 2 CO 3 was added to the samples to compensate for the evaporation of K 2 O during the prolonged calcination.…”