Perovskite B-Site Compositional Control of [110]p Polar Displacement Coupling in an Ambient-Pressure-Stable Bismuth-based Ferroelectric**

Piezoelectrics are key functional materials in actuator and sensor applications. Current technology exploits lead-based materials displaying a morphotropic phase boundary (MPB) between two ferroelectric solid solutions of different symmetries and polarization directions. This is best exemplified by the PbZr1 xTixO3 (PZT) perovskite system where the distortions driving the piezo response are produced by the stereo-active electron lone pair of the Pb cation on the A site of the ABO3 perovskite structure. [2] Due to the environmental impact of lead, there is a considerable focus on the synthesis of lead-free electroceramics. It is not however clear if the complex crystal chemistry underlying the structural phase boundaries at the MPB in lead-based systems can be simply translated into lead-free analogues. In particular the balance between distortions driven by the A and B site cations and the role of octahedral tilting and tolerance factor considerations are expected to be quite different in non-lead systems, as Pb is significantly larger than the more highly charged Bi often considered as an alternative polarizationgenerating cation. However, the smaller size of bismuth ions generally requires high-pressure synthesis conditions to form a perovskite. Only a small group of known perovskites exist with A-site bismuth cations that are stable at ambient pressure, based on BiFeO3, [4] Bi2Mn4/3Ni2/3O6, [5] and Bi(Fe2/8Ti3/8Mg3/8)O3 (BFTM). [6] Bi2Mn4/3Ni2/3O6 is antiferrodisplacive, while both BiFeO3 and BFTM adopt the R’ R3c structure where the [111]p displacements of untilted R (space group R3m in the PZT case) are coupled with octahedral rotation about the same axis (the prime symbol denotes the presence of tilts). In order to access MPBs, polar structures with distinct polarization directions away from [111]p in these Bi-based families are required. We investigated the pseudoternary phase field BFTM– LaFeO3 (LFO)–La2MgTiO6 (LMT) where the introduction of LFO (tolerance factor t= 0.95) would alter the average structural asymmetry on the A site by combining the aspherical Bi and spherical La cations, and the addition of Mg and Ti from LMT (t= 0.95) on the B site would reduce the dielectric loss caused by the increase in Fe content. LFO adopts the Pmnb structure of GdFeO3 with an ab b tilt system and 0.18 antiferrodistortive La displacements along the [110]p direction. LMT (P21/a) has the sameA-site displacement pattern and tilt system plus rock salt ordered B-site cations. Phases in the BFTM–LMT–LFO (BLFTM) field (Figure 1a) were prepared according to the protocol set out in Section 1 of the Supporting Information. Four distinct classes of powder diffraction patterns are observed. BFTM (t= 0.96) is the only material with the rhombohedral R3c space group, and is surrounded by a two-phase region. At low LFO content and along the BFTM–LMT line, an orthorhombic perovskite phase and an Aurivillius phase coexist (XRD data Figure S1, Supporting Information). However, single phases emerge at higher substitution levels. These phases adopt the nonpolar Pmnb structure at high LFO/LMT content, but a new orthorhombic phase emerges between the Pmnb and multiphase regions, as shown by the evolution of the lattice parameters along the (1 x)BFTM–xLFO line in Figure 1b. Powder second harmonic generation (SHG) measurements show that the non-Pmnb BLFTM samples are non-centric (x= 0.28, 0.37, 0.40, 0.42), whereas the Pmnb materials (x= 0.5, 0.67) are centric as expected. Thin films can be grown by pulsed laser deposition (Section 5, Supporting Information). The structural characterization of the new non-centric BLFTM phase will focus on the Bi0.72La0.28(Fe0.46Ti0.27Mg0.27)O3 (0.72BFTM–0.28LFO) composition. Of the candidate space groups which gave similar modelindependent Lebail fits in the observed 2ap ffiffiffi