Raman spectroscopy and x-ray diffraction of sp3CaCO3 at lower mantle pressures

Abstract

The exceptional ability of carbon to form ${\mathit{sp}}^{2}$ and ${\mathit{sp}}^{3}$ bonding states leads to a great structural and chemical diversity of carbon-bearing phases at nonambient conditions. Here we use laser-heated diamond-anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in $\mathrm{CaC}{\mathrm{O}}_{3}$ at $P>40\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We find that postaragonite $\mathrm{CaC}{\mathrm{O}}_{3}$ transforms to the previously predicted $P{2}_{1}/c\phantom{\rule{4pt}{0ex}}\mathrm{CaC}{\mathrm{O}}_{3}$ with ${\mathit{sp}}^{3}$-hybridized carbon at 105 GPa ($\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ higher than the theoretically predicted crossover pressure). The lowest-enthalpy transition path to $P{2}_{1}/c\phantom{\rule{4pt}{0ex}}\mathrm{CaC}{\mathrm{O}}_{3}$ includes reoccurring ${\mathit{sp}}^{2}$ and ${\mathit{sp}}^{3}\phantom{\rule{4pt}{0ex}}\mathrm{CaC}{\mathrm{O}}_{3}$ intermediate phases and transition states, as revealed by our variable-cell nudged-elastic-band simulation. Raman spectra of $P{2}_{1}/c\phantom{\rule{4pt}{0ex}}\mathrm{CaC}{\mathrm{O}}_{3}$ show an intense band at $1025\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which we assign to the symmetric C-O stretching vibration based on empirical and first-principles calculations. This Raman band has a frequency that is $\ensuremath{\sim}20%$ lower than the symmetric C-O stretching in ${\mathit{sp}}^{2}\phantom{\rule{4pt}{0ex}}\mathrm{CaC}{\mathrm{O}}_{3}$ due to the C-O bond length increase across the ${\mathit{sp}}^{2}\ensuremath{-}{\mathit{sp}}^{3}$ transition and can be used as a fingerprint of tetrahedrally coordinated carbon in other carbonates.