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' 4 M'M''X 8 ) 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.