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

Abstract

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.

Changing the Usual Interpretation of the Structure and Ground State of Cu2+-Layered Perovskites

“…The present results thus prove that the calculated pressure dependence for several layered copper compounds cannot be explained under the usual assumption of a JTE [9][10][11][12][13][14][15][16][30][31][32][33][34][35] . For clarifying the actual origin of the local structure in these layered systems and the nature of the electronic ground state an insight on (CH3NH3)2CdCl4:Cu 2+ , where Cu 2+ enters as impurity 40 , sheds light on this relevant matter.…”

“…In this work we have derived from first-principles calculations the pressure dependence of Cu 2+ -X -(X = Cl, F) distances in copper layered compounds. As up to now there are not reliable experimental data on the sensitivity to pressure of RZ, RS and RL distances the information provided in this work can be valuable for a further insight into properties displayed by these layered systems and in particular the large piezochromic effect observed in RCuCl4 compounds [14][15][16] .…”

“…In recent years, a great deal of investigation has been focused on layered RCuX4 compounds, where R is an organic group and X a halogen. Indeed, very promising results have been obtained with this kind of compounds in the realm of lightemitting diodes 8 , solar cells 8,9 , multiferroic materials [10][11][12] , cathode materials for Li + -ion batteries 13 or systems displaying huge piezochromism and thermochromism [14][15][16] . Fig.…”

“…This situation is thus seemingly similar to that found for Cu 2+ -doped cubic lattices, like KZnF3 [20][21][22][23] or NaCl [25][26][27] , where the ligand octahedron is elongated and the local symmetry tetragonal as a result of a static Jahn-Teller effect (JTE) 28,29 . For this simple reason, it has systematically been accepted [9][10][11][12][13][14][15][16]30,31 that the JTE is behind the local distortion around the metal cation in RCuCl4 compounds. A similar situation holds for purely inorganic materials displaying a layer structure, such as K2CuF4 [32][33][34] , Rb2CuCl4 35 , Cs2AgF4 36 or Na3MnF6 37,38 .…”

High Compression‐Induced Conductivity in a Layered Cu–Br Perovskite

“…The highest‐energy filled state has 75 % Br p orbital character and 22 % Cu d character whereas the CBM has, within our DFT‐HSE calculations, 63 % Cu d orbital character and 35 % Br p contributions. In the analogous Cu–Cl perovskite (Figure S14), the relative contributions of Cl and Cu states in the CBM are 30 % and 69 % respectively, and the highest filled state has 27 % Cu d character and 72 % Cl p character, consistent with prior calculations . The slightly greater halide contribution in the predominantly d‐orbital bands of the Cu–Br perovskites compared to the Cu–Cl perovskites corroborates the hypothesis that the Br compound possesses greater covalency.…”