Crystal Structure Evolution of Slawsonite SrAl2Si2O8 and Paracelsian BaAl2Si2O8 upon Compression and Decompression

The high-pressure behavior of slawsonite, SrAl2Si2O8, has been studied using in situ single-crystal X-ray diffraction (SCXRD) and Raman spectroscopy up to 31 GPa. Slawsonite undergoes displacive phase transition between 6 and 8 GPa with the formation of slawsonite-II, featuring fivefold coordinated silicon and aluminum. The results have been confirmed by the changes in vibrational modes using Raman spectroscopy. High-pressure evolution of the Raman spectra of isotypic paracelsian, BaAl2Si2O8, was studied upon compression and decompression up to 37.5 GPa. Raman data for paracelsian upon compression are in good agreement with previously obtained SCXRD data, which demonstrated three phase transitions at ∼6, 28, and 32 GPa with the formation of AlO5, SiO5, AlO6, and SiO6 polyhedra. Raman data upon decompression show the possibility to quench the high-pressure modification, containing AlO5 polyhedra. The comparison of the high-pressure behavior of slawsonite with paracelsian reveals that the increasing size of extra framework cation from Sr2+ to Ba2+ reduces the phase transition pressure but does not change the transformation pathway.