Alkali Metal (Li, Na, and K) Orthocarbonates: Stabilization of sp³-Bonded Carbon at Pressures above 20 GPa

In the present work, based on the density functional theory and crystal structure prediction algorithms, we found novel structures of alkali metal orthocarbonates Li4CO4-C2/c, Na4CO4-C2/c, and K4CO4-P21/c. The pressure of thermodynamic stabilization of the obtained orthocarbonates with respect to decomposition into the corresponding carbonate and oxide ranges from 21 GPa (for K4CO4) to 25 GPa (for Li4CO4). All obtained structures contain...