  The high pressure structural properties of bismuth oxide Bi2SiO5 have been
investigated up to 28 GPa using in situ powder synchrotron X-ray diffraction
and up to 50 GPa with DFT calculations. The monoclinic structure is found to
persist up to about 20 GPa, where a notable change in the compressibility
occurs. The DFT data imply that this is due to a second-order phase transition
from the ambient condition monoclinic structure with space group Cc to an
orthorhombic polymorph with space group Cmcm. This transition involves the
straightening of the chains formed by corner-connected SiO4 tetrahedra, that
suppresses the ferroelectricity in the high pressure, centrosymmetric phase of
Bi2SiO5. The stereo-chemical activity of the Bi3+ lone electron pair is found
to decrease with increasing pressure, but it can still be identifed in the
calculated electron density difference maps at 50 GPa.
