High pressure phase transitions of paracelsian BaAl(2)Si(2)O(8)

Three new polymorphs of aluminosilicate paracelsian, BaAl(2)Si(2)O(8), have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al(3+) and Si(4+) ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO(4) and SiO(4) tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P2(1)/c) to orthorhombic (Pna2(1)). The structure of paracelsian-III consists of SiO(6) octahedra, AlO(6) octahedra and distorted AlO(4) tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si(4+) and Al(3+) coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna2(1) to Pn. The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O(2−) and Ba(2+) ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.