Pressure induced structural phase transition and piezochromism in photovoltaic sillen compounds PbBiO2X (X = Cl, Br & I)

Hydrostatic pressure is an effective and clean method that can give rise to the emergence of novel crystal structures and physical properties. Thus, hydrostatic pressure can be used for designing new functional materials. Sillen materials or PbBiO2X (X = Cl, Br & I) are a class of materials which have gained considerable interest in the field of ferroelectrics, photocatalysis, etc. In this work, we have used first principles methods to predict the crystal structure and phase of sillen materials at different pressure points from 0 to 50 GPa. It was then followed by the study of their structural, electronic and optical properties upon compression. Upon compression, the band gaps decreased within a particular phase and showed kinks on phase transitions. In the compressed states, the band gaps are in the suitable range to be used in the visible range of the spectrum for photovoltaic properties. Piezochromism is also reported upon compression as can been seen from the changes in the optical absorption spectra. This work should remove any confusion with regards to the responsible phase at different pressures, and the possible existence of new phases with the respective band gaps should pave the way for future experimental works on photocatalysis and other energy applications.