Mehmet A. Akbas

Chemical order in PMN-related relaxors: structure, stability, modification, and impact on properties

High temperature thermal treatments were used to modify the cation order in several tantalate and niobate members of the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxors. The observation of complete 1:1 structural order in several compositions, and the refined cation occupancies of well-ordered samples conflict with the predictions of the “space charge” model, and support the “random site” description for the B-site order. In this charge balanced model one of the positions in the ordered structure is solely occupied by Ta (Nb), while the other contains a random distribution of Mg and the remaining Ta (Nb) cations. The stability of the order and magnitude of the domain growth is strongly influenced by solid solution additives. For Pb(Mg1/3Ta2/3)O3 (PMT), ordering-enhancing Zr, Sc, or La substituents increase the cation order–disorder transition temperature (∼1375°C in pure PMT) and promote extensive domain coarsening. For pure PMN a low temperature (<1000°C) order–disorder transition prevents any structure modification, and domain growth could only be realized with additives (Tb, Sc, or La) that stabilize the order to temperatures where the samples are “kinetically active”. The retention of relaxor behavior in all the fully 1:1 ordered, large-domain PMT and PMN-based ceramics suggests that the disorder on the random site is critical in frustrating ferroelectric order. By systematically controlling the concentration of ferroelectrically active cations on this position in fully ordered (1−x)Pb(Mg1/3Ta2/3)O3–(x)Pb(Sc1/2Ta1/2)O3 solid solutions, a crossover from relaxor to normal ferroelectric behavior was induced at x=0.5.

Cation ordering transformations in Ba(Mg13Ta23)O3-BaZrO3 perovskite solid solutions

The effect of the substitution of BaZrO3 on the cation ordering in Ba(Mg13Ta23)O3 was studied using TEM and X-ray and neutron diffraction. Almost no solubility of Zr was found in the 1:2 ordered, trigonal structure of the Ba(Mg13Ta23)O3 end-member (P 3ml), and a transformation to a 1:1 ordered, cubic (Fm3m) phase with a = 2aper occurs for substitution levels between ~10 and 25 mol% BaZrO3. The structure of this Ba(β′12β″sol12)O3-type phase consists of two distinct octahedral sites, β′ and β″. The occupancies of the two cation positions, refined using the Rietveld method, were found to be consistent with a “random site” model in which β″ is occupied by Ta, and β′ by a random distribution of the remaining cations. The homogeneity range of the 1:1 solid solutions predicted by this model, Ba{[Mg(2 − y)3Ta(1 − 2y)3 Zry]12[Ta]12}O3 (0.0 < y ≤ 0.5), is in excellent agreement with that observed experimentally.

Processing and characterization of lead magnesium tantalate ceramics

Using a processing route that employed closed platinum crucibles, single phase ceramics of Pb(Mg{sub 1/3}Ta{sub 2/3})O{sub 3} (PMT) relaxor ferroelectrics were prepared with densities greater than 95{percent} of their theoretical value. The improvements in the sintering characteristics of this system that result from this route were reflected by the dielectric properties, {epsilon}{sub max}{sup {prime}}=6300 at 182 K, which are similar to those reported for single crystal PMT. Contrast originating from nanosized polar clusters was evident in dark-field TEM images collected from the PMT ceramics at room temperature and showed little change upon cooling through the permittivity maximum. The electron diffraction patterns contained weak superlattice reflections at (h{plus_minus}1/2, k{plus_minus}1/2, l{plus_minus}1/2) that originate from a 1:1 ordering of the B-site cations. High resolution imaging indicated that the lengthscale of the chemical ordering in PMT is essentially identical to niobate relaxors such as PMN, with the 1{endash}2 nm ordered domains being surrounded by a disordered matrix. {copyright} {ital 1997 Materials Research Society.}

Structure, stability, and dielectric properties of chemically ordered relaxors in the Pb(Mg1/3Ta2/3)O3–PbZrO3 (PMT–PZ) system

Abstract The nano-scale phase separated domain structures of Pb(Mg 1/3 Ta 2/3 )O 3 (PMT) relaxors are shown to be metastable and responsive to high temperature heat treatments. For pure PMT the 2–3-nm ordered domains in an as-sintered sample coarsen to approximately four times their original size (∼8 nm) after thermal annealing at 1325°C for 48 h. The alterations in the domain coarsening and degree of cation order are greatly enhanced in solid solutions of PMT with PbZrO 3 (PZ). For 5–15 mol% substitutions fully 1:1 ordered microstructures comprised of ∼40-nm domains were stabilized after the same type of annealing. Quenching experiments revealed that the enhancements in the degree of order were related to a thermodynamic stabilization of the 1:1 order by the Zr cations. Models based on the random site description gave the most consistent interpretation of the cation ordering. The retention of a relaxor ferroelectric response in all the 1:1 ordered large domain PMT-PZ ceramics, indicates that randomness on one of the positions of the ordered structure, and not the chemical domain size, is the critical factor in inhibiting normal ferroelectric coupling.

Growth of the chemically ordered domains in PMN-type relaxor ferroelectrics

Abstract The “space charge” models for the structure of the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxors are based, in part, on the apparent absence of any change in the degree of chemical order with ...

Cation ordering and dielectric properties of PMN-PSN relaxors

Extended thermal annealing treatments were used to modify the B-site cation order in the (1−x)PMN−(x)PSN perovskite system. Extensive 1:1 ordering could be induced in compositions with x⩾0.1. The substitution of PSN into PMN produces a large increase in the thermal stability of the 1:1 ordered phase, with the maximum disordering temperature of ∼1360 °C being observed for x=0.5. The order-disorder transition temperature for pure PMN was calculated to be 913 °C. The changes in stability could be rationalized using the random site model for the cation order. The well ordered, large chemical domain ceramics exhibited relaxor behavior up to x∼0.6, for higher values normal ferroelectric behavior was observed. Alterations in the size of the chemical domain size had no significant effect on the properties of the lower x compositions, but induced a transition to relaxor behavior for x>∼0.6.

Chemical Order and Dielectric Properties of Lead Scandium Tungstate Relaxors

The tungstate relaxor ferroelectric perovskite, Pb(Sc2/3W1/3)O3 (PSW), adopts a 1:1 B‐site ordered “random site” structure with one cation position occupied by Sc3+ and the other by a 2:1 distribution of W6+ and Sc3+. The ordering is destabilized in solid solutions of PSW with PbZrO3 (PSW‐PZ), but stabilized by PbTiO3 in the (1−x)PSW‐(x)PT system. For PSW‐PT complete structural order is retained up to x = 0.25 (Pb(Sc)1/2(W1/2Ti1/2)1/2O3). The different response of PSW to the two substitutions can be explained by the changes in the ion size/charge mismatch on the random site and the size difference of the two ordered positions. The changes in order are accompanied by differences in the dielectric response of the two systems. For PSW‐PZ the temperature of the permittivity maximum (Te,max) increases linearly with x; however, in PSW‐PT Te,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of PSW‐2...