Electronic structure, optical properties, and pressure behavior of the CdB$_{4}$O$_{7}$ and HgB$_{4}$O$_{7}$ compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB$_{4}$O$_{7}$ and HgB$_{4}$O$_{7}$ tetraborate compounds in three structural modifications with the $Pbca$, $Cmcm$, and $Pmn 2_1$ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the $Pbca$ and $Pmn 2_1$ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.