Thermochromism versus piezochromism in (PMA)2CuX4 (X = Br, Cl) halide perovskites

Abstract

This study addresses the challenges of poor thermodynamic and structural stability in copper-based perovskites under extreme conditions. By replacing hygroscopic cations with hydrophobic ones like phenylmethylammonium (PMA), the stability of these materials is significantly enhanced. Two-dimensional copper-based perovskites, (PMA)₂CuCl₄ and (PMA)₂CuBr₄, are examined for their large band gap tunability, focusing on thermo- and piezo-chromism. Temperature-dependent transmission and reflectance measurements show that the band gaps narrow by ∼70 nm (from 466 to 536 nm) for (PMA)₂CuCl₄ and by ∼87 nm (from 472 to 559 nm) for (PMA)₂CuBr₄ across the temperature range of 20–320 K. Pressure-dependent transmission reveals that a redshift in the band gap occurs, shifting from 476 to 546 nm (∼70 nm) up to 10.94 GPa for (PMA)₂CuCl₄ and from 482 to 572 nm (∼90 nm) up to 10.86 GPa for (PMA)₂CuBr₄ under ambient conditions. These changes are reversible, confirming the stability under extreme conditions. The observed significant thermo- and piezo-chromic effects make the materials very functional for use in color tuning and temperature and pressure sensor applications. Additionally, the comparison of thermo- and piezo-chromism shows that even if electron–phonon interactions play an important role in both phenomena, the changes in lattice parameters induced by temperature (expansion) and pressure (compression) are fundamentally different, leading to distinct mechanisms of energy gap narrowing.