Crystal Structure of BaCa(CO₃)₂ Alstonite Carbonate and Its Phase Stability upon Compression

Abstract

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO−BaO−CO 2 system is more complex than previously thought. We characterized the BaCa(CO 3 ) 2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V 0 = 1608(2) Å 3 , B 0 = 60(3) GPa, B′ 0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO 3 ) 2 polymorphs was investigated.

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate

“…The compressibility of barytocalcite lies in between those of the two end-member carbonates: 67(2) GPa for CaCO 3 calcite (B′ 0 = 4) 24 , 66.5(7) GPa (B′ 0 = 5.0(1)) for CaCO 3 aragonite 25 and 48(1) GPa for BaCO 3 witherite 26 , and it is comparable to the 62.7(6) GPa of SrCO 3 strontianite 26 , 27 . Barytocalcite’s bulk modulus is slightly smaller than that of other BaCa(CO 3 ) 2 polymorph, alstonite, which is reported to have an experimental value of B 0 = 60(3) GPa 13 . Theoretical compressibility results on alstonite using the same methodology (B 0 = 62.8(2) GPa) confirm this smaller B 0 value for barytocalcite.…”

“…This system is also characterized by the formation of, at least, five different polymorphs of the barium calcium BaCa(CO 3 ) 2 double carbonate 10 – 14 . These phases, namely barytocalcite (space group (SG): P2 1 /m) 10 , paralstonite (SG: P321) 11 , two variants of alstonite (SGs: P31m 12 and P321 13 ) and a synthetic monoclinic phase (SG: C2) 14 , have been identified at ambient conditions. According to recent theoretical DFT investigations, barytocalcite is the thermodynamically stable phase but the differences in enthalpies with the alstonite and paralstonite variants are very small (< 0.08 eV/formula unit), which suggests that either of these phases could be found in nature, as it occurs in fact 13 .…”

“…These phases, namely barytocalcite (space group (SG): P2 1 /m) 10 , paralstonite (SG: P321) 11 , two variants of alstonite (SGs: P31m 12 and P321 13 ) and a synthetic monoclinic phase (SG: C2) 14 , have been identified at ambient conditions. According to recent theoretical DFT investigations, barytocalcite is the thermodynamically stable phase but the differences in enthalpies with the alstonite and paralstonite variants are very small (< 0.08 eV/formula unit), which suggests that either of these phases could be found in nature, as it occurs in fact 13 . Regarding the high-temperature structural behavior of barytocalcite, a phase transition to a disordered cation calcite form above 520 ºC has been reported 15 .…”

“…Thus, barytocalcite has the Ca and Ba atoms in 7- and 11-fold coordination by oxygen atoms, respectively 10 . The coordination of the cations in paralstonite and alstonite variants differ from barytocalcite, as Ba is coordinated by ten oxygen atoms and the Ca by eight oxygen atoms 11 – 13 . The synthetic phase 14 and the HT disordered calcite phase 15 present each Ba/Ca atom octahedrally coordinated.…”

Short Photoluminescence Lifetimes Linked to Crystallite Dimensions, Connectivity, and Perovskite Crystal Phases

“…The three different morphologies show maximum emission centered at different (transition) temperatures for the same laser excitation, where this temperature is mainly determined by the speed of the process. 56 − 63 Interestingly, the configuration that exhibits shorter lifetimes than the other configurations for the PL intensity, as it is the case of the 103 nm-IS, is the first one to undergo the phase transitions (by decreasing the temperature). This can be understood if we consider that the phase transformation will be more homogeneous in the case of 103 nm-IS because of the strain relaxation isolated islands.…”