Suppression of Jahn–Teller Distortions and Origin of Piezochromism and Thermochromism in Cu–Cl Hybrid Perovskite

While
pressure-induced changes in the electronic, magnetic, and
optical properties of Cu–Cl hybrid perovskites have been studied
intensively, the correlation between these properties and pressure-induced
structural changes is still vaguely understood. Here, by first-principles
calculations on a model system (EDBE)­[CuCl4] (EDBE = 2,2′-(ethylenedioxy)­bis­(ethylammonium))
(a Cu–Cl hybrid perovskite), we correlate the evolution of
a series of exciting physical properties with pressure while resolving
some of the long-standing debates on the fundamental electronic nature
of this important class of material. The material shows two structural
phase transitions and an anisotropy in compressibility with increasing
pressure. After a critical pressure of 17 GPa, the structure becomes
highly symmetric, thereby suppressing the Jahn–Teller distortions.
At zero pressure, mapping the optical transitions with the Laporte
selection rules, lower and higher energy excitations are found to
be of Mott–Hubbard (MH) and charge transfer (CT) type, respectively,
signifying the material to be a Mott insulator. The material shows
a red shift in the charge transfer band edge with increasing pressure
and temperature, demonstrating the piezochromism and the thermochromism,
respectively. Piezochromism originates from the changes in mixing
of Cl–Cu p–d states, while thermochromism is due to
broadening of conduction band states, thereby showing different electronic
and structural evolution with pressure and temperature. Furthermore,
the magnetic ordering in the material was found to be stable up to
higher pressures, making pressure a tool to tune the electronic property
without perturbing the magnetic property.