Pressure-Induced
Conductivity and Yellow-to-Black
Piezochromism in a Layered Cu–Cl Hybrid Perovskite
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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
d<sup>9</sup> 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 Cu<sup>II</sup>–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 Cu<sup>II</sup>–Cl perovskites. The conductivity increases by 5 orders
of magnitude between 7 and 50 GPa, with a maximum measured conductivity
of 2.9 × 10<sup>–4</sup> S·cm<sup>–1</sup> at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature
conductivity measurements indicate that the perovskite behaves as
a 1.0 eV band-gap 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