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

A synthetic \(\hbox {CaCO}_{3}\)–\(\hbox {SrCO}_{3}\) solid solution with composition \(\hbox {Ca}_{0.82}\hbox {Sr}_{0.18}\) \(\hbox {CO}_{3}\) 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 \(\sim\)9 GPa and Ar up to \(\sim\)22 GPa. At ambient conditions, \(\hbox {Ca}_{0.82}\hbox {Sr}_{0.18}\) \(\hbox {CO}_{3}\) crystallizes in a monoclinic structure, isostructural to \(\hbox {CaCO}_{3}\)-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, \(\beta = 108.064(1)^\circ\) and \(V=248.60(1)\) Å\(^3\) (where the number in parenthesis is 1\(\sigma\) 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, \(\beta = 103.856(6)^\circ\) and \(V = 462.39(5)\) Å\(^3\)), different from any pure \(\hbox {CaCO}_{3}\) polymorph. At 12 GPa, the sample transformed to a triclinic structure, Sr-calcite-IIIb (Sr-CC-IIIb), with space group \(P{\bar{1}}\) and \(Z=4\) ( \(a=6.059(5)\) Å, \(b=6.280(2)\) Å, \(c=6.331(2)\) Å, \(\alpha =95.20(3)^\circ\), \(\beta =108.89(5)^\circ\), \(\gamma =110.52(5)^\circ\) and \(V=207.7(2)\) Å\(^3\)), isostructural to end-member \(\hbox {CaCO}_{3}\)-IIIb. Finally, at 17 GPa, a transition is observed to Sr-calcite-VI (Sr-CC-VI), with space group \(P{\bar{1}}\) and \(Z=2\) (\(a=3.444(3)\) Å, \(b=4.985(4)\) Å, \(c=5.761(5)\) Å, \(\alpha =77.05(7)^\circ\), \(\beta =84.92(7)^\circ\), \(\gamma =89.00(7)^\circ\) and \(V=96.0(1)\) Å\(^3\)), isostructural to end-member \(\hbox {CaCO}_{3}\)-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 \(\hbox {CaCO}_{3}\)–\(\hbox {SrCO}_{3}\) solid solution. The phase evolution of \(\hbox {Ca}_{0.82}\hbox {Sr}_{0.18}\hbox {CO}_3\) and the crystallization of a new phase, Sr-CC-IIIc, different from the high-pressure polymorphs of end-member \(\hbox {CaCO}_{3}\), point to the importance of extending the study of carbonates to more complex systems than pure end-member compositions.