Unravelling the complexity of co-existing phases in piezoelectrics

Piezoelectric materials are critical for the operation of a vast array of contemporary electronics and devices including sensors, actuators, transducers, capacitors, non-volatile memory devices and they show great promise for energy harvesting.Their performance is dependent on cation displacements that are allowed in non-centrosymmetric structures.Unfortunately, the use of piezoelectric materials is temperature constrained by thermally activated non-centrosymmetric to centrosymmetric phase transitions.This is especially true for the industry standard piezoelectric, PbTi 1-x Zr x O 3 (PZT), which is a solid solution between PbTiO 3 and PbZrO 3 .The highest performance in the PZT solid solution falls at the so-called morphotropic phase boundary (MPB) between rhombohedral (R3m) and tetragonal (P4mm) phases.This region contains a low-symmetry monoclinic structure.Around the MPB, the rhombohedral structure is better described in space group R3c.The complexity of the MPB can be further elaborated by considering the tilting of the corner-sharing octahedra.In the rhombohedral R3c crystal structure, the octahedra have out-of-phase tilting about the pseudocubic [111] direction, described by the Glazer notation a a a , and the cations are displaced along the same direction.In the tetragonal P4mm structure there is no tilting (a 0 a 0 a 0 ) and the polarization is along [001].Like the rhombohedral R3c structure, the two intermediate monoclinic structures only contain out-of-phase tilts, in Cm the tilting is modified from a a a to be more like a b b , whereas in Cc it is more like a a c .The polarization in Cm is rotated toward the monoclinic plane (e.g. between [001] and [110]), whereas in Cc it is noncollinear, generally between [001] and [111].Thus, the superlattice reflections diagnostic of the tilting all involve softening of a phonon mode at the cubic R point.Couple to this the pathological tendency for perovskites to show pseudosymmetry resulting in extensive overlap of reflections, the challenge of identifying the phases present in the MPB is apparent.But wait, there is more to consider.Anisotropic peak broadening emerges across the MPB associated with the phase transitions.Two of the most pressing goals in the search for new and/or improved piezoelectrics is to reduce, and ultimately eliminate, the amount of Pb and to stabilize the noncentrosymmetric to higher temperatures.The complex ferroelectric ceramic (1 x)BiFe 2/8 Ti 3/8 Mg 3/8 O 3 -xPbTiO 3 (BFTM-PT) with x = 0.25-0.373does both but the question is how?Richtik, Manjo n-Sanz and Dolgos (2025) address this in current issue of Acta Crystallographica Section B. Bi-based perovskites are an obvious target to replace PZT as Bi 3+ is isoelectronic with Pb 2+ , both containing a stereochemical active 6s 2 lone pair of electrons.Despite numerous studies, PZT remains the benchmark.Establishing accurate and precise structures in BFTM-PT ceramics is further complicated considering the relative X-ray and neutron scattering lengths.The presence of the very heavy lead and bismuth cations limits the power of X-ray methods to determine the positions of the oxygen atoms that describe the octahedral tilting, and the negative neutron scattering length of Ti (b = 3.438 fm), often quoted as a strength of neutron scattering, results in very low effective scattering lengths (b = 0.64-1.46fm) when Ti occupies the same crystallographic site as Fe (9.45 fm) and Mg (5.375 fm).For reference, vanadium, which is often considered to be 'neutron silent', has a scattering length of 0.38 fm.Thus, the polar displacement of the B-site cation may be poorly described.The solution adopted by Richtik et al. was to refine the structures within their studied BFTM-PT ceramics using combined X-ray and neutron diffraction data, measured at the Canadian Light Source and Spallation Neutron Source, respectively.The structures are refined using TOPAS.The synchrotron X-ray diffraction (SXRD) data provide sensi-

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