Chiral Hybrid Perovskites Insights

• Chiral hybrid perovskites are materials wherein enantiopure organic cations induce noncentrosymmetric metal halide frameworks, resulting in distinct chiroptical and spin phenomena.
• Their properties arise from asymmetric hydrogen-bond networks and group-theoretical coupling, which enable improper ferroelectricity and tunable spin-polarized transport.
• Chiral phonons and exciton dynamics in these systems facilitate enhanced nonlinear optical effects and efficient energy transport, promising advanced photonic and spintronic applications.

Chiral hybrid perovskites are a class of organic–inorganic materials where broken inversion and mirror symmetry, typically induced by the packing of chiral organic cations within metal halide frameworks, yields a noncentrosymmetric lattice with emergent chiroptical, spin, and energy transport phenomena. These compounds exemplify a highly tunable architecture—structural chirality, vibrational angular momentum, improper ferroelectric phases, and intricate coupling between electronic and phononic degrees of freedom—positioning them at the intersection of condensed matter physics, materials chemistry, and advanced optoelectronic applications.

1. Structural Origin and Classification of Chirality

Chirality in hybrid perovskites emerges primarily when enantiomerically pure organic cations are incorporated into layered or three-dimensional metal halide matrices. The organic cations (e.g., methylbenzylammonium (MBA), phenylethylammonium (PEA), or (S)-1-Methyl-2-pyrrolidinemethanol) reside between or within inorganic layers, imparting a handedness to the entire crystal via asymmetric hydrogen bonding networks and collective distortions of the metal halide octahedral framework (Zuri et al., 2022, Pols et al., 31 Jul 2025).

A canonical descriptor of such structural chirality is the vector chirality,

$$\chi = \frac{1}{N} \sum_{i=1}^{N} \hat{\mathbf{u}}_i \times \hat{\mathbf{u}}_{i+1}$$

where the $\hat{\mathbf{u}}_i$ are normalized bond vectors connecting adjacent atoms; the projection along the layer normal quantifies the net handedness. The degree of chirality is further influenced by temperature, as molecular dynamics simulations reveal that while the organic sublattice retains its chiral arrangement to high temperatures, the inorganic framework’s chirality diminishes due to the thermal weakening and reorientation of hydrogen bonds (Pols et al., 28 May 2024).

In two-dimensional (2D) perovskites, these descriptors can differentiate organic-induced chirality from the transfer of chirality to the inorganic sublattice, an essential mechanism for pronounced chiroptical activity.

2. Group-Theoretical Coupling Schemes and Improper Ferroelectricity

Chiral hybrid perovskites support a broad array of symmetry-breaking mechanisms beyond typical polar distortions. Group-theoretical analysis identifies trilinear coupling invariants in the free energy—$\beta A B P$—where two nonpolar distortions ($A$ and $B$, transforming as irreps with opposite inversion parity at the same k-point) can induce a secondary polar order ($P$) (Boström et al., 2017). For example, combining molecular dipolar order (e.g., at the $A$-site) and octahedral tilt or shift (e.g., $R_5^-$ mode), activates improper ferroelectricity and may result in net polar, and frequently chiral, space groups.

Molecular substitution on the $A$-site or $X$-site (e.g., the adoption of organic cations with multipolar moments or anionic formate/azide ligands) enriches the spectrum of distortions available for symmetry breaking. As a result, the crystal design strategies for acentric and chiral perovskites are informed by deliberate engineering of cation and anion chemistry to target specific irreducible representations, enabling rational synthesis of functional materials for nonlinear optics, sensors, and magnetoelectric coupling.

3. Vibrational Chirality: Chiral Phonons and Angular Momentum

Chiral phonons—lattice vibrations with circular polarization—are a direct consequence of the underlying structural chirality. In chiral 2D perovskites such as (R-MBA)$_2$PbI$_4$, low-energy optical phonons (notably a 2.5 meV mode) exhibit angular momentum and are experimentally distinguished from racemic samples via femtosecond transient absorption spectroscopy (Ramesh et al., 14 Jul 2025). The phonon spin (or angular momentum) is quantified by

$$s^{\alpha}_{\mathbf{q},\sigma} = \sum_{i=1}^{N} e^*_{i,\mathbf{q},\sigma} S^{\alpha} e_{i,\mathbf{q},\sigma}$$

with $e_{i,\mathbf{q},\sigma}$ the phonon polarization vectors and $S^{\alpha}$ spin-1 matrices.

Under a temperature gradient, the occupation asymmetry leads to a net phonon angular momentum,

$$J^{(\mathrm{ph},\alpha)} = -\frac{\hbar \tau}{V} \sum_{\mathbf{q},\sigma}\sum_{\beta=x,y,z} s^{\alpha}_{\mathbf{q},\sigma} v^{\beta}_{\mathbf{q},\sigma} \left( \frac{\partial f_0(\omega_{\mathbf{q},\sigma})}{\partial T} \right) \left( \frac{\partial T}{\partial x^{\beta}} \right)$$

enabling heat and spin currents—a mechanism underpinning the spin Seebeck effect and chirality-induced spin selectivity (CISS) (Pols et al., 26 Nov 2024, Pols et al., 31 Jul 2025). Comparative studies demonstrate that resonance modes in Sn-based perovskites exhibit substantially enhanced phonon chirality relative to Pb-based analogues, although contributions to net angular momentum may mutually compensate, limiting macroscopic chiral heat currents (Pols et al., 31 Jul 2025).

4. Chiroptical Effects and Nonlinear Optical Response

The noncentrosymmetric lattice of chiral hybrid perovskites enables second-order nonlinear optical effects, including second harmonic generation (SHG) and the linear electro-optic (LEO) effect. The susceptibility tensor for SHG,

$$\chi^{(2)}_{abc}(-2\omega;\omega,\omega) = \chi^{(2)}_{\mathrm{inter}} + \chi^{(2)}_{\mathrm{intra}} + \chi^{(2)}_{\mathrm{mod}}$$

is enhanced when the band gap is small and the crystal is strongly distorted, with particularly high SHG coefficients found in CH$_3$NH$_3$SnI$_3$ (up to 500 pm/V), rivaling that of GaAs (Song et al., 2020). The band gap and SHG response are highly sensitive to the choice and arrangement of both cations and anions.

Circular dichroism (CD) and circularly polarized photoluminescence (CPL) can be tuned via external stimuli: high pressure induces pronounced lattice distortion and interlayer coupling, raising the degree of CPL from near-zero to 10% at 8.5 GPa (Dai, 2023). The tight-binding approach, parameterized by DFT and extended to include electric quadrupole and magnetic dipole transitions, reveals that CD is strongly correlated with the structural helicity of the metal halide layers—specifically out-of-plane halide displacement—rather than the magnitude of spin–orbit coupling (Apergi et al., 2023). This points toward design rules prioritizing framework distortion over cation identity for targeted chiroptical applications.

5. Spin Selective Transport, Spintronics, and Multifunctionality

Chiral hybrid perovskites provide a nonmagnetic route to spin-polarized transport via CISS. The chiral distortion breaks inversion and mirror symmetry, linking spin and momentum and enabling highly spin-polarized currents without magnetic elements or external fields (Liu et al., 2 Jul 2025). In 2D PbBr$_4$ frameworks, orbital-based symmetry analysis and effective Hamiltonians trace the emergence of valley-spin locking and unconventional spin splitting. The Edelstein effect enables bulk spin polarization upon application of an electric field, with the spin density $S_a = \chi_{ab}E_b$ arising from the interplay of SOC and asymmetric scattering between spin-polarized valleys.

Chiral perovskites are further proposed as tunable altermagnets, capable of spin splitting absent net magnetization—a functionality controlled by both chemical composition and crystal symmetry. The optically active CISS and Edelstein effect establish chiral perovskites as promising candidates for spin filters, spin-LEDs, or even spin-lasers operating at room temperature.

6. Exciton Transport and Chirality Transfer

Energy transport in chiral hybrid perovskites is dominated by ultrafast, density-dependent exciton diffusion; in (MBA)$_2$PbI$_4$, diffusion coefficients as high as 2 cm$^2$/s and transport distances exceeding 100 nm are observed at room temperature (Terres et al., 12 Aug 2024). The presence of pure enantiomers correlates with lower disorder (Urbach energies ≈ 40–41 meV), yielding more efficient exciton diffusion compared to racemic mixtures (≈48.6 meV). Exciton localization follows initial fast propagation, with implications for single-photon emission and quantum optical devices.

Chirality transfer is also realized through electronic coupling to molecular dopants: doping a chiral perovskite matrix with F4TCNQ introduces charge-transfer states with strong visible-range CD, as the dopant inherits the host’s chiral dipole alignment (Chen et al., 15 Jan 2025). This mechanism broadens the spectral range for chiral optoelectronic applications and enables selective detection of CPL in photodetectors, with enhanced electrical conductivity via additional charge transport pathways.

7. Multiphase Behavior, Anharmonicity, and Surface Effects

Many chiral perovskites exhibit complex ground-state landscapes due to structural anharmonicity. In (PEA)$_2$PbI$_4$, low-temperature X-ray diffraction identifies four intrinsically coexisting chiral configurations, stemming from bi-oriented organic cation arrangements and perpendicular inorganic lattice twists (Zuri et al., 2022). The population of these phases is uneven and can be tuned by surface effects, leading to spatially varying grain boundaries and domain interfaces. Such intrinsic disorder impacts charge transport, exciton dynamics, and may give rise to local Rashba-type spin splitting, affecting device performance.

In bulk perovskites such as CsSnBr$_3$, harmonic phonon instabilities induce transitions into polar, chiral phases (P2$_1$ symmetry), exhibiting noncollinear electric dipoles, bulk spontaneous polarization ($\approx$4.5 $\mu$C/cm$^2$), strong SHG, and pronounced negative thermal expansion ($\alpha_V = -9 \times 10^{-5}$ K$^{-1}$), with large spin–orbit splitting compatible with Rashba-type effects (Fabini et al., 15 Jan 2024).

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Chiral hybrid perovskites represent a versatile and multifunctional family of materials defined by the interplay of organic and inorganic lattice symmetry breaking, phononic and electronic chirality, spin selectivity, and strong nonlinear optical responses. Their emergent properties, tunable via chemical substitution, structural design, and external stimuli, supply a fertile ground for developing next-generation spintronic, photonic, and quantum devices. Contemporary research continues to explore the limits and optimization of chirality transfer mechanisms, multiphase ground-state control, and the coupling of spin, charge, and phonon degrees of freedom intrinsic to this material class.