Pressure-induced changes of the structure and properties of monoclinic α -chalcocite Cu2S

The high-pressure behavior of monoclinic ($P{2}₁/c$) $\ensuremath{\alpha}$-chalcocite, ${\mathrm{Cu}}₂\mathrm{S}$, was investigated at ambient temperature by single-crystal x-ray diffraction, electrical resistance measurements, and optical absorption spectroscopy up to 16 GPa. The experiments were complemented by density-functional-theory-based calculations. Single-crystal x-ray diffraction data show that monoclinic $\ensuremath{\alpha}$-chalcocite undergoes two pressure-induced first-order phase transitions at $\ensuremath{\sim}3.1$ and $\ensuremath{\sim}7.1$ GPa. The crystal structure of the first high-pressure polymorph, HP1, was solved and refined in space group $P{2}₁/c$ with $a=10.312(4)\phantom{\rule{0.16em}{0ex}}\AA{}, b=6.737(3)\phantom{\rule{0.16em}{0ex}}\AA{}, c=7.305(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, and $\ensuremath{\beta}=100.17{(2)}^{\ensuremath{\circ}}$ at 6.2(3) GPa. The crystal structure of the second high-pressure polymorph, HP2, was solved and refined in space group $P{2}₁/c$ with $a=6.731(4)\phantom{\rule{0.16em}{0ex}}\AA{}, b=6.689(2)\phantom{\rule{0.16em}{0ex}}\AA{}, c=6.967(8)\phantom{\rule{0.16em}{0ex}}\AA{}$, and $\ensuremath{\beta}=93.18{(3)}^{\ensuremath{\circ}}$ at 7.9(4) GPa. Electrical resistance measurements upon compression and optical absorption experiments upon decompression show that the structural changes in $\ensuremath{\alpha}$-chalcocite are accompanied by changes of the electrical and optical properties. Upon pressure release, the band gap ${E}_g$ of $\ensuremath{\alpha}$-chalcocite (1.24 eV at ambient conditions) widens across the first structural phase transition, going from 1.24 eV at 2.2 GPa ($\ensuremath{\alpha}$-chalcocite) to 1.35 eV at 2.6 GPa (HP1), and closes significantly across the second phase transition, going from 1.32 eV at 4.4 GPa (HP1) to 0.87 eV at 4.9 GPa (HP2). The electrical resistance shows similar behavior: its highest value is for the first high-pressure polymorph (HP1), and its lowest value is for the second high-pressure polymorph (HP2) of $\ensuremath{\alpha}$-chalcocite. These results are interpreted on the basis of calculated electronic band structures.