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A synthetic CaCO 3 – SrCO 3 solid solution with composition Ca 0.82 Sr 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 0.82 Sr 0.18 CO 3 crystallizes in a monoclinic structure, isostructural to CaCO 3 -II, Sr-calcite-II (Sr-CC-II), with space group P 2 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 P 2 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 P 1 ¯ 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 P 1 ¯ 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 0.82 Sr 0.18 CO 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. [ABSTRACT FROM AUTHOR]