Fig. 5.

Evidence for polar domains and their scalability with crystal size. (A) Bright-field image from a light microscope of a crystal before and after etching in acetone for 120 s. The crystal (2.4 mm) was grown from a 1:1 PbI₂:MAI solution, using ethyl acetate as antisolvent (for comparison, an example for 0.56-mm crystal is shown in SI Appendix, Fig. S9). (ii) Magnification of the yellow rectangle marked in i. (B) The pyroelectric decay from crystal #3 (SI Appendix, Fig. S2B) when measured with amplification of 10⁷ (discharge time, ∼0.02 ms). Crystal #3 was grown from a 1:2 PbI₂-to-MAI solution. Based on the analysis related to Eq. 2, the domain period should be ∼9 μm. (C) Domain periodicity with respect to the crystal thickness in the polar direction. The data are an average derived from microscope images after etching in acetone for ∼120 s. The red dot represents the value obtained from the pyroelectric TSER (crystal #3) from B. The values of γ are obtained by fitting to ω = a·D^γ.