The effect of halogenated spacer cations on structural symmetry-breaking in 2D halide double perovskites

Herein, we present a strategy to introduce above-room temperature non-centrosymmetry into two-dimensional halide double perovskites (A′4M′M′′X8) using a halogenated A′-site organic linker, 3-chloro/bromo propyl amine. These crystals exhibit anisotropic polarization with three orders of magnitude variation between different crystallographic axes. The non-centrosymmetry is further confirmed by piezo-force microscopy studies and its role in the thermal and optical properties was investigated.