First principles study of pressure induced polymorphic phase transition in KNO₃

We report the structural, elastic, electronic, and vibrational properties of polymorphic phases II and III of KNO₃ based on density functional theory (DFT). Using semi-empirical dispersion correction (DFT-D2) method, we predicted the correct thermodynamic ground state of KNO₃ and the obtained ground state properties of the polymorphs are in good agreement with the experiments. We further used this method to calculate the elastic constants, IR and Raman spectra, vibrational frequencies and their assignment of these polymorphs. The calculated Tran Blaha-modified Becke Johnson (TB-mBJ) electronic structure shows that both the polymorphic phases are direct band gap insulators with mixed ionic and covalent bonding. Also the TB-mBJ band gaps are improved over standard DFT functionals which are comparable with the available experiments