Control of Crystal Symmetry Breaking with Halogen-Substituted Benzylammonium in Layered Hybrid Metal-Halide Perovskites

Layered
hybrid metal-halide perovskites with non-centrosymmetric
crystal structure are predicted to show spin-selective band splitting
from Rashba effects. Thus, fabrication of metal-halide perovskites
with defined crystal symmetry is desired to control the spin-splitting
in their electronic states. Here, we report the influence of halogen para-substituents on the crystal structure of benzylammonium
lead iodide perovskites (4-XC6H4CH2NH3)2PbI4 (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic
generation, we study structure and symmetry of single-crystal and
thin-film samples. We report that introduction of a halogen atom lowers
the crystal symmetry such that the chlorine- and bromine-substituted
structures are non-centrosymmetric. The differences can be attributed
to the nature of the intermolecular interactions between the organic
molecules. We calculate electronic band structures and find good control
of Rashba splittings. Our results present a facile approach to tailor
hybrid layered metal halide perovskites with potential for spintronic
and nonlinear optical applications.