Elastic properties and electronic structure of tetragonal KDP crystal under polishing pressures from first principles

The elastic properties and electronic structure of tetragonal Potassium dihydrogen phosphate (KDP) under polishing pressures were investigated using the plane-wave pseudopotential method based on density functional theory. The results show that the calculated lattice constant, elastic constants and bandgap agreed well with the results of the experiments and the other calculations at ambient pressure. The elastic constants and the elastic moduli of KDP increase with increasing pressure, but Vickers hardness of KDP decreases. KDP crystal changes from brittleness to ductility beyond the pressure of 3 GPa. The anisotropy of KDP increases and the ratio of E-[100]/E-[110] increases with the increase of pressure. When the pressure reaches 4.5 GPa, the tetragonal KDP will undergo structural phase transition. As pressure increases, the bandgap between and O-2p and P-3s states increases. The interatomic distances were shortened under external pressure, and the interaction between K+ and the neighboring H2PO4- was enhanced, which leads to the increase of elastic mechanical properties.