Equally charged with orderly conduct

Double perovskite oxides are valued for their versatile cation arrangements, which impart notable physical properties such as ferrimagnetism and large magnetoresistance. B′ and B″ cation arrangements in such A₂B′B″O₆ materials span from entirely disordered to impeccably ordered. Cation ordering usually occurs when the oxidation states and radii of the ions are substantially different. A difference in oxidation state of two or more is generally required to achieve B′/B″ cation-ordered double perovskites. There are approximately 650 known ordered double perovskites, but all cases to date featuring isovalent cations (A³⁺₂B′³⁺B″³⁺O₆) are disordered. Now, writing in Angewandte Chemie, Athinarayanan Sundaresan and co-workers describe unusual ordering of Fe³⁺ and Al³⁺ in the double perovskite Bi₂FeAlO₆.

The bismuth-based perovskite oxide BiAlO₃ is ferroelectric at room temperature, whereas its iron counterpart — BiFeO₃ — is multiferroic with weakly coupled magnetism and ferroelectricity. A polar phase in which half of the B sites in BiFeO₃ are replaced with non-magnetic Al³⁺ ions may exhibit a spontaneous magnetization that in turn may enhance magnetoelectric coupling. Since both BiAlO₃ and BiFeO₃ have similar crystalline phases, it is reasonable to expect that a BiFe_1−xAlO₃ solid solution is accessible. However, previous exploration of this composition space showed that the BiFe_1−xAlO₃ solid solution was limited to x = 0.1 in bulk and x = 0.4 in thin film. Attempts to stabilize a BiFe_0.5Al_0.5O₃ phase at ambient pressure were unsuccessful.

Credit: Image adapted with permission from De, C. et al. (2018), Wiley-VCH.

A previous similar experience with the polar oxide AlFeO₃ inspired the group to persevere. Al₂O₃ and Fe₂O₃ also have similar crystalline phases, but again an isostructural solid solution could not be formed. Instead, a 1:1 mixture unexpectedly crystallizes as AlFeO₃ in a different space group. The isovalent cations in AlFeO₃ are largely ordered, albeit with some partial anti-site disorder in which Al³⁺ and Fe³⁺ exchange sites. Although unprecedented, a 1:1 mixture of BiAlO₃ and BiFeO₃ may also yield ordered Al³⁺ and Fe³⁺ ions in a polar structure. Polar materials, or those with permanent dipoles, are required for ferroelectricity and other interesting behaviours.

Considering that high pressure stabilizes BiAlO₃ as well as perovskite oxides with ordering of multiple cations (for example, double double perovskites), the group turned to a technique described earlier by collaborator and co-author Attfield. Polycrystalline samples of BiFe_1−xAl_xO₃ with x = 0.2–0.5 were prepared by pressing metal oxides at 6 GPa (~59,200 atm) and 1,000 °C. Rietveld refinement of the expected polar rhombohedral R3c model, with disordered Fe³⁺ and Al³⁺ ions, fitted X-ray powder diffraction data for several Fe³⁺:Al³⁺ ratios, but not for BiFe_0.5Al_0.5O₃. Instead, rock salt ordering of the Fe³⁺ and Al³⁺ cations on B′/B″ sites occurs in the BiFe_0.5Al_0.5O₃, or Bi₂FeAlO₆ double perovskite, in the R3 space group, which is supported by time-of-flight neutron powder diffraction.

The cation ordering itself is highly surprising as ordering of two 3⁺ cations at perovskite B-sites is unprecedented

By replacing half of the Fe³⁺ ions in BiFeO₃ with non-magnetic Al³⁺, the magnetic transition temperature from paramagnetic to an antiferromagnetic phase is decreased from 640 K to 280 K. The dielectric constant exhibits conventional temperature dependence with a magnitude similar to that of BiFeO₃, and lower conductivity that is dominated by a d-electron hopping mechanism. Piezoelectric measurements confirm that Bi₂FeAlO₆ exhibits a polar structure and ferroelectric properties at room temperature. The size difference between the Al³⁺ and Fe³⁺ cations as well as the nature of Al–O and Fe–O bonding may partly account for the cation ordering. However, there must be another factor at play because cation ordering is not observed in another similar compound, LaAl_0.5Fe_0.5O₃. Distortions around the Bi³⁺ cations may accommodate the difference in isovalent cation radii, facilitating order. Although it is not well understood, the electron lone pair on Bi³⁺ might be key in achieving ordering. “The cation ordering itself is highly surprising as ordering of two 3⁺ cations at perovskite B-sites is unprecedented,” explains Sundaresan. “With appropriate selection of B′/B″ cations, many polar materials of the type Bi₂B′B″O₆ may be accessible by the same high pressure methods.”