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Stereochemical expression of ns2 electron pairs in metal halide perovskites

Nature Reviews Chemistry volume 5, pages 838–852 (2021)Cite this article

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Abstract

Metal halide perovskites (MHPs) are characterized as strongly anharmonic and dynamic lattices. While there is a consensus on the solvation-like polarization effect in these materials, whether static polarization, that is, ferroelectricity, exists or not in 3D MHPs remains controversial. In this Review, we resolve this controversy by analysing the stereochemical expression (SE) of the ns2 electron pair (NSEP) on group IV metal cations. The SE-NSEP is key to lattice instability, which governs the breaking of inversion symmetry and induces ferroelectricity. The SE-NSEP is diminishingly small in commonly studied 3D lead iodide or bromide perovskites, indicating an absence of ferroelectricity. In contrast, 2D MHPs promote the SE-NSEP and produce unambiguous ferroelectricity or antiferroelectricity. Irrespective of ferroelectricity, the dynamic manifestation of the SE-NSEP provides the missing link to understanding polar fluctuations and efficient dielectric screening in MHPs, thus, contributing to the long carrier lifetimes and diffusion lengths.

Key points

  • The stereochemical expression of the ns2 electron pair (SE-NSEP) on group IV metal cations governs the breaking of inversion symmetry and induces ferroelectricity in metal halide perovskites.

  • The tendency of the SE-NSEP increases with lighter group IV cations, more electronegative halides and larger A-site cations.

  • The SE-NSEP is diminishingly small in commonly studied 3D lead halide perovskites, suggesting the absence of ferroelectricity.

  • Dimensionality reduction promotes the SE-NSEP and produces unambiguous ferroelectricity or antiferroelectricity in 2D lead halide perovskites.

  • The inherent driving force for the SE-NSEP in 3D perovskites results in dynamic symmetry breaking, strong phonon anharmonicity and polar fluctuations, giving rise to efficient dielectric screening of charge carriers.

  • Emerging halide perovskites with the SE-NSEP offer exciting systems to understand the origin of the remarkable photophysical properties in metal halide perovskites.

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Fig. 1: Crystal structures of 3D and 2D halide perovskites.
Fig. 2: Lattice distortions in 3D perovskites.
Fig. 3: Lattice distortion and polarization in a ferroelectric 2D perovskite.
Fig. 4: Lattice distortions in 2D Ruddlesden–Popper halide perovskites.
Fig. 5: Dynamic B-cation off-centre displacement and lattice anharmonicity in halide perovskites.
Fig. 6: Dielectric solvation of charge carriers in 3D halide perovskites.

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Acknowledgements

X.-Y.Z. acknowledges the Vannevar Bush Faculty Fellowship through Office of Naval Research grant no. N00014-18-1-2080 and the US Department of Energy, Office of Energy Sciences, grant DE-SC0010692 for support at various stages of writing of this account. S.J. acknowledges support through the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award DE-FG02-09ER46664.

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Authors and Affiliations

  1. Department of Chemistry, Columbia University, New York, NY, USA

    Yongping Fu & X.-Y. Zhu

  2. Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, China

    Yongping Fu

  3. Department of Chemistry, University of Wisconsin–Madison, Madison, WI, USA

    Song Jin

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Y.F., S.J. and X.-Y.Z. discussed the content of the article and Y.F. wrote the first version of the manuscript. All authors edited the manuscript prior to submission.

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Supplementary information

Glossary

Polaron

A quasiparticle consisting of an electron or a hole dressed by a cloud of local distortion of the lattice. When the distortion is mostly confined within the unit cell, it is a small polaron. If the distortion extends well beyond the unit cell, the result is a large polaron.

Rashba effect

Momentum-dependent splitting of spin bands due to a combined effect of spin–orbit coupling and asymmetry of the crystal potential. In a non-centrosymmetric solid, the static electric field Lorentz transforms into a magnetic field in the reference frame of a moving electron, which then interacts with the electron spin and breaks the spin degeneracy.

Displacive ferroelectrics

Refers to the scenario where ions are displaced from the equilibrium positions to create the spontaneous polarization at temperatures below the Curie temperature.

Second-harmonic generation

A nonlinear optical process in which two photons with the same energy interacting with a nonlinear material are effectively ‘combined’ to form a new photon with twice the energy.

Curie temperatures

The critical temperatures above which a ferroelectric material loses spontaneous polarization. The same concept applies to ferromagnetic materials.

Orthorhombic phase

When the lattice parameters a ≠ b ≠ c and all the angles are 90°.

Stokes shift

Describes the energy difference between the emission peak and the absorption peak.

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Fu, Y., Jin, S. & Zhu, XY. Stereochemical expression of ns2 electron pairs in metal halide perovskites. Nat Rev Chem 5, 838–852 (2021). https://doi.org/10.1038/s41570-021-00335-9

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