A new monoclinic variation of Mg 2 C 3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg 2 C 3 ), the new monoclinic C2/m structure (β-Mg 2 C 3 ) contains linear C 3 4− chains that are isoelectronic with CO 2 . Unlike α-Mg 2 C 3 , which contains alternating layers of C 3 4− chains oriented in opposite directions, all C 3 4− chains within β-Mg 2 C 3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β-Mg 2 C 3 yields C 3 H 4 , as detected by mass spectrometry, while Raman and NMR measurements show clear CC stretching near 1200 cm −1 and 13 C resonances confirming the presence of the rare allylenide anion.
New experimental data are reported on high-pressure polymorphism of CaCO 3 . The CaCO 3 -III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm 3 of powder sample. The interpretation of XRPD indicates that CaCO 3 -III and CaCO 3 -IIIb structures are present simultaneously and are in similar proportions. The lack of any unindexed peaks demonstrates that these two polymorphs are the only phases in this experiment, indicating that CaCO 3 -III and CaCO 3 -IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO 3 -III and CaCO 3 -IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.
The high-pressure and high-temperature formation and stability of recently discovered magnesium carbide Mg 2 C were studied by in situ X-ray diffraction up to 20 GPa and 1550 K. The insights into the thermodynamics of Mg 2 C under extreme conditions, and its metastability at 0.1 MPa and 300 K, were provided by ab initio calculations of total energies and phonon density of states as a function of pressure. We illustrate how the compound found occasionally in high-pressure experiments could be systematically predicted and discovered in a time-saving way. Similar theoretical approaches can be useful for prediction of synthesis conditions and recovery of new solids.
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