Sr₃[CO₄]O Antiperovskite with Tetrahedrally Coordinated sp³-Hybridized Carbon and OSr₆ Octahedra

Abstract

We have synthesized the orthocarbonate Sr 3 [CO 4 ]O in a laserheated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO 4 ] 4− groups at ambient conditions. Single-crystal diffraction shows the presence of [CO 4 ] 4− groups, i.e., sp 3hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO 4 ] 4− tetrahedra are located in a cage formed by corner-sharing OSr 6 octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The highpressure phase is tetragonal (I4/mcm). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (Pnma), where the octahedra tilt and deform slightly.

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

“…Among all revealed orthocarbonates, only Na 4 CO 4 - C 2/ c persists dynamic stability at ambient pressure (Figure S4). This shows that Na 4 CO 4 can be synthesized by the reaction of Na 2 CO 3 and Na 2 O using high-pressure experiments and recovered upon cold decompression, as have been shown for Sr 2 CO 4 and Sr 3 CO 5 . ,, …”

“…As a result, it was shown that magnesium orthocarbonate stabilizes in the stoichiometry Mg 2 CO 4 - P 2 1 / c , while calcium orthocarbonate stabilizes in three stoichiometries: Ca 3 CO 5 - Cmcm , Ca 2 CO 4 - Pnma , and CaC 2 O 5 - Pc . ,, Searches for strontium and barium orthocarbonates were carried out in MO–MCO 3 , where M  Sr and Ba. As a result, it was found that strontium orthocarbonate is stabilized in stoichiometries Sr 3 CO 5 - I 4/ mcm and Sr 2 CO 4 - Pnma ,,, and barium orthocarbonate is stabilized in stoichiometries Ba 3 CO 5 - I 4/ mcm and Ba 2 CO 4 - Pnma . , Among all of the aforementioned orthocarbonates, Mg 2 CO 4 - P 2 1 / c and Sr 2 CO 4 - Pnma are dynamically stable at 0 GPa, as seen from ab initio calculations. , More recently, it has been demonstrated by X-ray diffraction that Sr 2 CO 4 and Sr 3 CO 5 can be recovered at ambient conditions. , …”

“…This shows that Na 4 CO 4 can be synthesized by the reaction of Na 2 CO 3 and Na 2 O using high-pressure experiments and recovered upon cold decompression, as have been shown for Sr 2 CO 4 and Sr 3 CO 5 . 13,15,16 Li 4 CO 4 and K 4 CO 4 are dynamically unstable in the lowpressure region. Specifically, for Li 4 CO 4 , imaginary frequencies of phonon dispersion curves present in the pressure range 0− 25 GPa, while for K 4 CO 4 , they appear in the pressure range 0− 10 GPa (Figure S4).…”

“…8,9,12 Searches for strontium and barium orthocarbonates were carried out in MO−MCO 3 , where M  Sr and Ba. As a result, it was found that strontium orthocarbonate is stabilized in stoichiometries Sr 3 CO 5 -I4/ mcm and Sr 2 CO 4 -Pnma 10,11,15,16 and barium orthocarbonate is stabilized in stoichiometries Ba 3 CO 5 -I4/mcm and Ba 2 CO 4 -Pnma. 10,15 Among all of the aforementioned orthocarbonates, Mg 2 CO 4 -P2 1 /c and Sr 2 CO 4 -Pnma are dynamically stable at 0 GPa, as seen from ab initio calculations.…”

“…14,15 More recently, it has been demonstrated by X-ray diffraction that Sr 2 CO 4 and Sr 3 CO 5 can be recovered at ambient conditions. 13,16 Potential stabilization of sp 3 -bonded alkali metal orthocarbonates has been considered in two works. 17,18 Al-Shemali and Boldyrev 17 considered the possibility for the ambient pressure stability of sodium orthocarbonate, Na 4 CO 4 , but did not find a stable phase relative to the decomposition to carbonate and oxide.…”