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

Abstract

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 isolated groups of [CO4]­tetrahedra. The structures of lithium and sodium orthocarbonates were found to be isotypic. As well as alkaline earth orthocarbonates, orthocarbonates of alkali metals are structurally similar to orthosilicates of the same metal. Na4CO4-C2/c was found to be dynamically stable at ambient pressure, while all other structures are destabilized on decompression. In addition, the P–T phase diagrams of lithium, sodium, and potassium oxides have been calculated for the first time. All oxides have a high-pressure phase transition from the Fm3̅m (anti-fluorite) structure to the Pnma (anti-cotunnite) structure at 34 GPa for Li2O, at 10 GPa for Na2O, and at 5 GPa for K2O at low temperatures. In the Na2O and K2O systems, high-temperature phase transition to the P6̅2m (barringerite) structure was revealed at temperatures above 1300 and 650 K, respectively.

Novel Calcium sp3 Carbonate CaC2O5-I4̅2d May Be a Carbon Host in Earth’s Lower Mantle

“…Theoretical studies have predicted the formation of sp 3 carbonates at pressures and temperatures much lower than are required for the transformation of conventional carbonates for either oxide- or CO 2 -enriched compounds: i.e., for carbonates which have an oxide:CO 2 ratio other than 1:1. − Some of these predictions have recently been confirmed by the synthesis of the sp 3 carbonates Ca 2 CO 4 - Pnma , Mg 2 CO 4 - P 2 1 / c , Sr 2 CO 4 - Pnma , , and Sr 3 (CO 4 )­O- I 4/ mcm , which can be obtained at p ≈ 20 GPa and where Sr 2 CO 4 - Pnma and Sr 3 (CO 4 )­O- Pnma can be recovered under ambient conditions. These syntheses have all been conducted by reacting mixtures of an alkaline-earth oxide with the corresponding carbonate: e.g., by reacting CaO with CaCO 3 .…”

High-pressure transformations of CaC2O5– a full structural trend from double [CO3] triangles through the isolated group of [CO4] tetrahedra to framework and layered structures

“…A number of new types of carbonates, namely orthocarbonates, in the structure of which carbon atoms are in the sp 3 hybridized state, have been discovered using ab initio crystal structure predictions. [1][2][3][4][5] To date, it has been demonstrated that orthocarbonates of alkaline earth metals could be formed by the reaction of carbonates with the oxides of the corresponding metals. Depending on the ratio of initial reagents, two different phases M 2 CO 4 -Pnma (MCO 3 ÁMO) and M 3 CO 5 -I4/mcm (MCO 3 Á 2MO), where M = Ca, Sr, and Ba, were predicted to be stable beginning from a pressure of 5-20 GPa.…”

High‐Pressure Crystal Structures of Pb2CO4 and PbC2O5 with Tetrahedral [CO4] and Pyrocarbonate [C2O5] atomic groups

“…[9] In addition to the described systems with alkaline earth elements, systems with alkaline elements have been also investigated and stability of 2 : 1 structures above 20 GPa have been shown. [10,11] In the systems with d-elements, sp 3 -hybridized structures of MnC 2 O 5 , [12] FeFeC 3 O 12 , and FeFeC 4 O 13 [13] were synthesized. Systems with p-elements have not yet been investigated, and in the present work, we investigate such a system, namely PbOÀ CO 2 .…”