Press-induced phase transitions of KNbO3 studied from the first principle calculation

Using the full-potential linearized augmented plane-wave method (FP-LAPW), we have performed a detailed theoretical investigation on the pressure-induced phase transitions in KNbO3. Results obtained show that the pressure will induce a series of phase transitions from rhombohedral to orthorhombic, to tetragonal and finally to the cubic phase, with transition pressures being 8, 15 and 24 GPa, respectively. By analyzing the internal atomic coordinates, we found with increasing pressure, the oxygen octahedron would distort, and the orthorhombic and tetragonal phases would tend to make transitions to the tetragonal and cubic phase, respectively. We also calculated the optical properties of KNb03, and found that the dielectric function spectra epsilon(omega) of different phases were similar to each other, and the effect of pressure on epsilon(omega) spectra were small. However, the second harmonic generation (SHG) coefficients in different phases were greatly different.