Atomic-level structural correlations across the morphotropic phase boundary of a ferroelectric solid solution: xBiMg1/2Ti1/2O3-(1 − x)PbTiO3

Abstract

Revelation of unequivocal structural information at the atomic level for complex systems is uniquely important for deeper and generic understanding of the structure property connections and a key challenge in materials science. Here we report an experimental study of the local structure by applying total elastic scattering and Raman scattering analyses to an important non-relaxor ferroelectric solid solution exhibiting the so-called composition-induced morphotropic phase boundary (MPB), where concomitant enhancement of physical properties have been detected. The powerful combination of static and dynamic structural probes enabled us to derive direct correspondence between the atomic-level structural correlations and reported properties. The atomic pair distribution functions obtained from the neutron total scattering experiments were analysed through big-box atom-modelling implementing reverse Monte Carlo method, from which distributions of magnitudes and directions of off-centred cationic displacements were extracted. We found that an enhanced randomness of the displacement-directions for all ferroelectrically active cations combined with a strong dynamical coupling between the A- and B-site cations of the perovskite structure, can explain the abrupt amplification of piezoelectric response of the system near MPB. Altogether this provides a more fundamental basis in inferring structure-property connections in similar systems including important implications in designing novel and bespoke materials.

Favorable Concurrence of Static and Dynamic Phenomena at the Morphotropic Phase Boundary of xBiNi0.5Zr0.5O3<

“…a multi-component ferroic system, which are often very subtle to be categorized distinctly in a quantitative or qualitative manner. [6][7][8] Although there have been many recent experimental attempts to build more convincing structural models mimicking the atomic-level structural correlations in complex systems, [9][10][11][12][13][14][15][16][17] the challenge in formulating robust atomistic model still exists.…”

Distinct temperature behavior of the local structure of (1‐x)PbTiO3‐xBiNi0.5Ti0.5O3 at the morphotropic phase boundary

“…By analogy with other complex perovskite‐type ferroelectrics, the Raman peaks in the spectral ranges 20 to 100 cm −1 and 100 to 200 cm −1 are attributed to modes dominated by vibrations of off‐centered A‐site cations and to BO 3 ‐translations against the A‐site cation vibrations (A‐BO 3 mode), respectively . The Raman scattering in the range 180 to 300 cm −1 arises mainly from vibrations of off‐centered B‐site cations, while that near 350 cm −1 comprises tilting of the BO 6 octahedra in the case of x ≠0 . The spectral range 450 to 780 cm −1 is dominated by internal BO 6 modes, composed mainly of oxygen vibrations.…”

“…The uniqueness of the MPB composition as compared with x > x MPB is however not apparent, if considering only the temperature behavior of the A‐site‐cation vibrations. Figure shows the temperature dependence of the ETO2 and (B 1 +E) modes, involving predominantly vibrations of the B‐site cations as well as of the quasi‐E2(LO+TO) mode, consisting of B‐site cations and oxygen vibrations as well as of BO 6 tilting that develops with increasing x . These trends also demonstrate that the cosubstitution enhances the order‐disorder character of phase transition and smears out the coupling processes: (a) the minimum in ω ETO 2 ( T ), which occurs precisely at T C for x =0, disappears already for x =0.20; (b) for x >0.20, the ETO2 mode cannot be distinguished from the (B 1 +E) mode at any temperature, while the quasi‐E2(LO+TO) mode merges into the (B 1 +E) mode near T C and the average wavenumber is approximately the same in the entire temperature range.…”

“…The x ‐dependencies of the mode wavenumbers indicate multistep composition‐induced transformations at room temperature: (a) the initial incorporation of BNiT ( x =0.1) increases the anisotropy of the A‐site local potentials by introducing a new lower‐energy state (Figure C); (b) the anisotropy in the local potentials in the subsystem of off‐centered B‐site cations gradually decreases upon BNiT doping and diminishes at x =0.50< x MPB (Figure D), where the A‐cation‐localized ETO1 mode shows a pronounced minimum in wavenumber (Figure C) indicating coupling processes within the A‐cation subsystem; and (c) at the MPB composition, the A‐BO 3 mode that corresponds to the quasi‐TO1(A 1 +E) excitation in PT‐based ceramics softens and broadens (Figure B), revealing coupling between the adjacent local dipoles at the A and B sites, that is, between the sublattices of off‐centered A‐site and B‐site cations, a phenomenon that appears to be universal for perovskite‐type ferroelectric solid solutions with MPB . All together, this indicates that at room temperature, the composition‐induced local‐scale rearrangements within the A‐cation subsystem as well as within the B‐cation subsystem occur slightly below the MPB, which probably facilitates the change of the ferroelectric long‐range order at the MPB.…”

“…[21,22] Nowadays, it is widely accepted that a single or multiple phases of low symmetry (monoclinic, triclinic, orthorhombic) exist at the MPB, acting as a structural bridge between the tetragonal and rhombohedral/pseudorhombohedral phases in a solid solution and allowing for easy rotation of the unit-cell polarization. [6,23] Recent studies have, however, emphasized the importance of the mesoscopic-scale structure for the composition-driven property enhancement at room temperature, revealing that at the MPB, the dispersion of local atomic surroundings of the A-and B-site cations is enlarged, [24,25] which facilitates the flattening of the energetic barrier between the different types of ferroelectric long-range order and the optimal domain reorientation during the poling process. Moreover, very recent studies of the high-temperature state of pure PT indicate multistep coupling processes of local ferroic distortions occurring already above T C [9] and imply that the MPB in PT-based solid solutions is a chemically stabilized temperature-independent ergodic state of locally synchronized dipoles.…”