Double perovskites Double perovskites are an important subclass of perovskite-related materials with the general chemical formula A2BO6, in which two chemically distinct cations occupy the B site of the perovskite lattice. Compared with simple ABO3 perovskites, the introduction of B-site ordering increases crystallographic complexity, leading to symmetry reduction, additional distortion modes, and a wider range of physical properties.
Crystallographic principles and ordering mechanisms Double perovskites can be regarded as ordered superstructures of simple perovskites, where periodic B/ cation ordering enlarges the primitive perovskite unit cell. The B and cations lead to different ordering schemes, which are rock salt, columnar, and layered structures. Rock salt is an alternating, three-dimensional checkerboard of B and B' polyhedra. This structure is the most common from an electrostatic point of view, as the B sites will have different valence states. Columnar arrangement can be viewed as sheets of B-cation polyhedral viewed from the [111] direction. Layered structures are seen as sheets of and B polyhedra.
Common distortion modes and Glazer tilt patterns As in simple perovskites, deviations from ideal ionic size ratios in double perovskites frequently induce octahedral tilting and distortions. These distortion modes can be described using Glazer tilt notation, although the presence of B-site ordering imposes additional symmetry constraints. The coupling between B/ ordering and octahedral tilting leads to a variety of reduced-symmetry structures, with monoclinic and orthorhombic phases commonly observed in oxide double perovskites.
Electronic and magnetic structure trends B-site ordering in double perovskites strongly modifies electronic structure by altering orbital hybridization, bandwidth, and superexchange pathways. In many transition-metal systems, this ordering enables magnetic interactions that are absent or suppressed in chemically disordered perovskites, giving rise to diverse magnetic ground states and tunable electronic behavior. Electronic structure calculations further indicate that compositional tuning within the double perovskite framework provides a versatile route for engineering band gaps and carrier transport characteristics.
Defect chemistry and antisite disorder In real materials, perfect B-site ordering is rarely achieved. A common defect in double perovskites is antisite disorder, in which B and cations exchange lattice positions. Antisite disorder disrupts the periodic potential associated with ideal ordering and can substantially modify magnetic, electronic, and transport properties. The extent of antisite disorder depends sensitively on synthesis conditions, cation similarity, and thermal history, making defect control a central challenge in double perovskite materials.
Representative functional materials The structural flexibility of double perovskites has enabled a broad range of functional materials. Oxide double perovskites have been explored for applications involving magnetism, ferroelectricity, catalysis, and energy conversion, where functional behavior is often closely linked to cation ordering and lattice distortions.
Lower dimensional perovskites Using the metal halide octahedral as a building block, perovskites are subcategorized into 3D, 2D, 1D, or 0D to describe the arrangement of the octahedral units. 3D perovskites form when there is a smaller cation in the A site so octahedra can be corner shared. 2D perovskites form when the A-site cation is larger so octahedra sheets are formed. In 1D perovskites, a chain of octahedra is formed while in 0D perovskites, individual octahedra are separated from each other. Generally, as the dimensions of a crystal are reduced, a material's
band gap and carrier confinement increase, while carrier transport worsens. Both 1D and 0D perovskites lead to quantum confinement They have been widely studied for optoelectronic applications including solar cells, light-emitting diodes, photodetectors, lasers and scintillators because of their strong optical absorption, high carrier mobility, long diffusion lengths and tunable composition-dependent properties. Lead halide perovskites are also frequently described as defect-tolerant relative to many conventional solution-processed semiconductors, although the extent and meaning of this defect tolerance remain active topics of research. Their wider use is limited by concerns including long-term instability under heat, moisture, light and electrical bias, ion migration, and the toxicity of lead-containing compounds.
Lead-free halide perovskites In recent years, considerable attention has focused on lead-free halide perovskites, driven by concerns over the toxicity and environmental instability of Pb-based compounds. Candidate replacements for Pb2+ include Sn2+, Ge2+, Bi3+, Sb3+, and double perovskite combinations. Their low trap densities lead to high efficiencies for solar cells. In addition, the high mobilities of perovskites promote their usage as photodetectors. Combining the properties together, chiral perovskites would have the properties of circular dichroism, circularly polarized photoluminescence, making them suitable for circularly polarized light (CPL) photodetectors and circularly polarized LEDs. The first chiral perovskite was a 1D chiral-perovskite single crystal found in 2003, and the first 2D chiral-perovskite single crystal was found in 2006. The chiroptical study has not been performed until 2017, where Ahn et al explored the circular dichroism performance of (R/S-MBA)PbI4. After that, new kinds of chiral perovskites were found, such as chiral-perovskite nanocrystals, cogels, nanoplatelets, and low-dimensional chiral perovskites. The chiral transfer mechanisms of chiral perovskites include ligand-induced chiral inorganic structure, chiral distortion of the inorganic surface, chiral patterning of the surface ligands, chiral field effect, and chirality through environments. These different mechanisms lead to different designs of chiral perovskite synthesis. The idea of chirality through chiral ligands leads to the methods of direct synthesis using chiral ligands, post-synthetic chiral ligand exchange, chiral-ligand-assisted reprecipitation, and the chiral-ligand-assisted tip-sonication method. On the other hand, chirality can be introduced by the environment, via chiral solvents, strains, self-assembly on chiral templates.
Metal-free perovskites Metal-free perovskites are a class of perovskite-related materials in which the framework contains no metal cations; in the best known examples, the general formula is A(NH4)X3, where A is an organic cation and X is a halide. Interest in this class increased substantially after the report of a family of metal-free three-dimensional perovskite ferroelectrics based on MDABCO–NH4X3 (X = Cl, Br, I), which showed spontaneous polarization values comparable to those of some inorganic ferroelectrics. These materials have been studied as alternatives to conventional metal-containing halide perovskites, particularly for photonic and optoelectronic applications, because they can combine wide band gaps, low optical loss and the absence of toxic heavy metals. Metal-free perovskites have attracted particular interest for nonlinear optics and terahertz photonics: methyl-DABCO ammonium iodide has been reported to show terahertz emission by optical rectification, while metal-free ferroelectric halide perovskites have also been reported to exhibit visible photoluminescence correlated with local ferroelectricity, and related compositions have been investigated as semiconductor-core optical fibers with second-order optical nonlinearity. Although the field is less developed than that of inorganic and hybrid metal halide perovskites, metal-free perovskites illustrate how perovskite-like structural chemistry can be extended to entirely molecular frameworks. == See also ==