Aragonite-II and CaCO₃-VII: New High-Pressure, High-Temperature Polymorphs of CaCO₃

The importance for the global carbon cycle, the P–T phase diagram of CaCO3 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 CaCO3. 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 CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P21/c-l, predicted theoretically earlier. The P–T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure–temperature range.