High pressure phase transitions of paracelsian BaAl2Si2O8

Liudmila A Gorelova,Anna S Pakhomova,Anatoly V Kasatkin,Leonid S Dubrovinsky,Sergey V Krivovichev

doi:10.1038/s41598-019-49112-1

Abstract

Three new polymorphs of aluminosilicate paracelsian, BaAl2Si2O8, 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 Al3+ and Si4+ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO4 and SiO4 tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P21/c) to orthorhombic (Pna21). The structure of paracelsian-III consists of SiO6 octahedra, AlO6 octahedra and distorted AlO4 tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si4+ and Al3+ 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 Pna21 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 O2− and Ba2+ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.

Highlights

Local coordination of atoms in crystalline compounds is an important characteristic that determines their physical properties and behaviour under changing thermodynamic conditions
In our recent high-pressure studies on feldspar-related minerals such as danburite, CaB2Si2O815, and hurlbutite, CaBe2P2O832, we have discovered polymorphs of these materials, featuring TO5 trigonal bipyramids and TO6 octahedra (T = Si, Be, P)
Crystalline materials with the BaAl2Si2O8 composition attract considerable attention due to their various technological applications. They are widely used in glass and ceramic industries, including the production of low temperature co-fired ceramic (LTCC) materials[38,39], due to the low dielectric constants and good microwave dielectric properties[40,41,42,43,44]

Summary

Introduction

Local coordination of atoms in crystalline compounds is an important characteristic that determines their physical properties and behaviour under changing thermodynamic conditions. In our recent high-pressure studies on feldspar-related minerals such as danburite, CaB2Si2O815, and hurlbutite, CaBe2P2O832, we have discovered polymorphs of these materials, featuring TO5 trigonal bipyramids and TO6 octahedra (T = Si, Be, P). Observations of these exotic structural units has motivated us to investigate high-pressure behavior of closely related mineral paracelsian, BaAl2Si2O833, in order to illuminate correlations between transformation pathways and chemical and topological properties. From the mineralogical point of view, compounds with the BaAl2Si2O8 stoichiometry belong to the feldspar group, members of which are important rock-forming minerals found in all types of rocks and constituting over 50% of the Earth’s crust They are essential constituents of most igneous rocks, and found in association with metamorphic and sedimentary rocks. Paracelsian, which is the subject of the current study, is associated with exhalative hydrothermal processes, low-and medium-grade metamorphism, and have been found in sedimentary and meta-sedimentary rocks

Results

Discussion

Conclusion