Pressure-Induced Phase Transformation of Botallackite α-Cu2(OH)3Cl with a Two-Dimensional Layered Structure Synthesized via a Hydrothermal Strategy

Botallackite (α-Cu2(OH)3Cl) exhibits exotic unconventional magnetic transitions and magnetic ordering structures and thus attracts considerable research enthusiasm. In this work, free-standing α-Cu2(OH)3Cl nanoflakes with high purity and crystallinity are synthesized via a template-free hydrothermal strategy. Powder X-ray diffraction studies confirm that α-Cu2(OH)3Cl crystallizes in a brucite-like monoclinic lattice characteristic of a hydrogen-bonded two-dimensional layered structure. The quantitative elemental analysis by using EDX spectra indicates the perfect stoichiometry of the prepared sample. The morphological features observed via SEM and TEM techniques reveal that the prepared sample is composed of single-crystalline nanosheets with a thickness ranging from 20 to 30 nm. The vibrational modes and their frequencies observed in FTIR absorption and Raman scattering measurements may be assigned to the bonding nature of α-Cu2(OH)3Cl. The band gap of the obtained α-Cu2(OH)3Cl nanoflakes is estimated to be 2.82 eV through UV–vis absorption spectra recorded under ambient conditions. The compression behavior of α-Cu2(OH)3Cl nanoflakes is investigated by in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction of up to 48.4 GPa. A first-order reversible structural phase transformation is observed to begin at 13.8 GPa and is complete at 24.4 GPa, with coexistence of the two phases in the intermediate pressure range. A disordered rutile-type CdOHF-related structure is postulated for the novel high-pressure phase, which persists to the highest pressure in this study. The pressure dependences of the unit cell volumes of the two phases are fitted to the third-order Birch–Murnaghan equation of state to acquire more information about the mechanical properties of Cu2(OH)3Cl. The variation of the electronic energy band gap with pressure is measured by using the in situ high-pressure UV–vis absorption spectra. A sharp decrease in Eg in the pressure range of 11.1–13.0 GPa confirms the onset of the structural phase transformation.