Synthesis, crystal structure and structure–property relations of strontium orthocarbonate, Sr₂CO₄

Abstract

Carbonates containing CO4 groups as building blocks have recently been discovered. A new orthocarbonate, Sr2CO4 is synthesized at 92 GPa and at a temperature of 2500 K. Its crystal structure was determined by in situ synchrotron single-crystal X-ray diffraction, selecting a grain from a polycrystalline sample. Strontium orthocarbonate crystallizes in the orthorhombic crystal system (space group Pnma) with CO4, SrO9 and SrO11 polyhedra as the main building blocks. It is isostructural to Ca2CO4. DFT calculations reproduce the experimental findings very well and have, therefore, been used to predict the equation of state, Raman and IR spectra, and to assist in the discussion of bonding in this compound.

Fe3+ -hosting carbon phases in the deep Earth

“…Calculations in which a low spin state was imposed gave too small lattice parameters, which differed markedly from the experimental values. Similar to recent results on other carbonates in which the carbon is fourfold coordinated by oxygen [24,25], the DFT calculations show that all C-O bonds are covalent, with bond populations ranging from 0.8-0.6 e − /Å 3 . The carbon atoms are therefore tetrahedrally coordinated by oxygen and thus sp 3 hybridized.…”

“…These investigations allowed us to derive an understanding of pressure-induced structural phase transitions and changes in the structure-property relations and the electronic state, depending on thermodynamic conditions and composition [7][8][9][10][11][12][13][14][15]. Only recently has it been shown that carbonates in which carbon is tetrahedrally coordinated by oxygen atoms, i.e., sp 3 -hybridized carbon, are stable at high pressures [16][17][18][19][20][21][22][23][24][25]. Specifically, at conditions prevalent in the deeper mantle with pressures >50 GPa, siderite (FeCO 3 ) transforms via self-oxidation-reduction reactions into new phases containing sp 3 carbon [19][20][21][22].…”

Orthocarbonates of Ca, Sr, and Ba—The Appearance of sp3-Hybridized Carbon at a Low Pressure of 5 GPa and Dynamic Stability at Ambient Pressure

“…The angles between C–O bonds correspond to that of an ideal tetrahedron with deviations of 4.5–5° for Sr 2 CO 4 - Pnma and Ba 2 CO 4 - Pnma , 0.2° for Sr 3 CO 5 - I 4/ mcm , and 1–3° for Ba 3 CO 5 - I 4/ mcm at 50 GPa. The Mulliken population analysis performed by Laniel and co-authors has confirmed the covalent nature of all four C–O bonds within the [CO 4 ] tetrahedron of Sr 2 CO 4 - Pnma .…”

“…Our results on the stability of the orthocarbonates of the other alkaline-earth metals, in particular Sr 2 CO 4 - Pnma , have been presented in the form of a conference presentation and the corresponding book of abstracts . A recent experimental synthesis has confirmed the existence of the Sr 2 CO 4 - Pnma phase …”

“…13 A recent experimental synthesis has confirmed the existence of the Sr 2 CO 4 -Pnma phase. 14 Thus, the discovered crystal structures of orthocarbonates decrease the lower-pressure stability limit for the appearance of sp 3 -hybridized carbon within [CO 4 ] tetrahedra nearly 5 times that for simple carbonates. However, as orthocarbonates are in a newly discovered class of compounds, the lower-pressure limit of their stability is unclear.…”