Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu–Cl Hybrid PerovskiteJ. Am. Chem. Soc.

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Authors
Adam Jaffe, Yu Lin, Wendy L. Mao, Hemamala I. Karunadasa
Year
2015
DOI
10.1021/ja512396m
Subject
Chemistry (all) / Colloid and Surface Chemistry / Biochemistry / Catalysis

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Article

Pressure-Induced Conductivity and Yellow-to-Black

Piezochromism in a Layered Cu–Cl Hybrid Perovskite

Adam Jaffe, Yu Lin, Wendy L Mao, and Hemamala I. Karunadasa

J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512396m • Publication Date (Web): 12 Jan 2015

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Figure 1. A) X-ray structure of (EDBE)[CuCl4] (1, EDBE = 2,2’(ethylenedioxy)bis(ethylammonium)). Inset: an EDBE dication. Cu: green, Cl: orange, O: red, N: blue, and C: gray. H atoms omitted for clarity. B) A single inorganic sheet viewed along the b axis showing an antiferrodistortive arrangement of the elongated Cl–Cu–Cl axis. C) View of the inorganic sheets along the a or c axes.

Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu–Cl Hybrid Perovskite

Adam Jaffe,†§ Yu Lin,‡ § Wendy L. Mao,‡* and Hemamala I. Karunadasa†* †Department of Chemistry and ‡Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States

ABSTRACT: Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn-Teller distortion of d9 Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper (II)-chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder x-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other CuII–Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in CuII–Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 104 S·cm1 at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0-eV bandgap semiconductor at 39.7 GPa, and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression and ambient pressure powder x-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis.

INTRODUCTION

Compression with gigapascal-scale pressures can yield dramatic structural and electronic changes in solids.1 These include semiconductor-to-metal transitions of low-bandgap organic2 and inorganic3 solids, polymerization of conjugated organic groups,4 spin-polarization reversal,5 and metal-tometal charge transfer.6 In particular, pressure-induced electronic changes of two-dimensional Cu-based solids have been a subject of great interest. This is in part motivated by an attempt to understand the substantial increase in ordering temperature (Tc) observed for high-temperature cuprate superconductors with compression along the copper-oxide planes.7

Owing to their crystallinity and tunability, two-dimensional

Cu–Cl hybrid perovskites8 are well-defined platforms for studying the evolution of structural and electronic properties with applied pressure. These layered materials consist of corner-sharing sheets of CuII–Cl octahedra partitioned by organic cations (Figure 1A), allowing for the compressibility of the inorganic components to be modulated through organic substitution. The 3d9 CuII centers in two-dimensional perovskites show a pronounced Jahn-Teller (JT) distortion owing to the unequal occupancy of degenerate orbitals in an octahedral ligand field, with long and short Cu–Cl distances of ca. 2.9– 3.1 and 2.3 Å, respectively. The elongated axes align in an antiferrodistortive (AFD) arrangement within the plane of the inorganic sheets (Figure 1B). This results in the unpaired electrons residing in orbitals of dx2–y2 character that have minimal overlap with each other (Figure S2). Although a half-filled band should provide a conduction pathway, poor overlap between these orbitals likely leads to the materials’ insulating nature. Indeed, these materials behave as two-dimensional