1 research outputs found
Aragonite-II and CaCO<sub>3</sub>‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO<sub>3</sub>
The importance for
the global carbon cycle, the <i>P</i>–<i>T</i> phase diagram of CaCO<sub>3</sub> has
been under extensive investigation since the invention of the high-pressure
techniques. However, this study is far from being completed. In the
present work, we show the existence of two new high-pressure polymorphs
of CaCO<sub>3</sub>. The crystal structure prediction performed here
reveals a new polymorph corresponding to distorted aragonite structure
and named aragonite-II. In situ diamond anvil cell experiments confirm
the presence of aragonite-II at 35 GPa and allow identification of
another high-pressure polymorph at 50 GPa, named CaCO<sub>3</sub>-VII.
CaCO<sub>3</sub>-VII is a structural analogue of CaCO<sub>3</sub>-<i>P</i>2<sub>1</sub>/<i>c</i>-l, predicted theoretically
earlier. The <i>P</i>–<i>T</i> phase diagram
obtained based on a quasi-harmonic approximation shows the stability
field of CaCO<sub>3</sub>-VII and aragonite-II at 30–50 GPa
and 0–1200 K. Synthesized earlier in experiments on cold compression
of calcite, CaCO<sub>3</sub>-VI was found to be metastable in the
whole pressure–temperature range