Crystal structure and high-pressure phase behavior of a CaCO3–SrCO3 solid solution

A synthetic CaCO_3 – SrCO_3 solid solution with composition Ca_0.82Sr_0.18 CO_3 was investigated by single-crystal X-ray diffraction in the pressure range between 0 and 22 GPa using different pressure-transmitting media. The samples were compressed in DACs using Ne up to ∼ 9 GPa and Ar up to ∼ 22 GPa. At ambient conditions, Ca_0.82Sr_0.18 CO_3 crystallizes in a monoclinic structure, isostructural to CaCO_3 -II, Sr-calcite-II (Sr-CC-II), with space group P2_1/c , 4 formula units per unit cell, Z, a = 6.4237(7) Å, b = 5.0176(1) Å, c = 8.1129(1) Å, β = 108.064(1)^∘ and V=248.60(1) Å ^3 (where the number in parenthesis is 1 σ uncertainties on the last digit). At 1.72(5) GPa, a structural phase transition is observed to a new monoclinic structure, Sr-calcite-IIIc (Sr-CC-IIIc), with space group P2_1/m and Z=8 ( a = 6.2683(2) Å, b = 9.9220(5) Å, c = 7.6574(6) Å, β = 103.856(6)^∘ and V = 462.39(5) Å ^3 ), different from any pure CaCO_3 polymorph. At 12 GPa, the sample transformed to a triclinic structure, Sr-calcite-IIIb (Sr-CC-IIIb), with space group P1̅ and Z=4 ( a=6.059(5) Å, b=6.280(2) Å, c=6.331(2) Å, α =95.20(3)^∘ , β =108.89(5)^∘ , γ =110.52(5)^∘ and V=207.7(2) Å ^3 ), isostructural to end-member CaCO_3 -IIIb. Finally, at 17 GPa, a transition is observed to Sr-calcite-VI (Sr-CC-VI), with space group P1̅ and Z=2 ( a=3.444(3) Å, b=4.985(4) Å, c=5.761(5) Å, α =77.05(7)^∘ , β =84.92(7)^∘ , γ =89.00(7)^∘ and V=96.0(1) Å ^3 ), isostructural to end-member CaCO_3 -VI, which is preserved up to the maximum investigated pressure of 22 GPa. The results of this study show the effect of Sr/Ca cationic substitution on the high-pressure behavior and physical properties of a CaCO_3 – SrCO_3 solid solution. The phase evolution of Ca_0.82Sr_0.18CO_3 and the crystallization of a new phase, Sr-CC-IIIc, different from the high-pressure polymorphs of end-member CaCO_3 , point to the importance of extending the study of carbonates to more complex systems than pure end-member compositions.