First principles study of the electronic, elastic, vibrational and piezoelectric properties of KAlSiO ₄

Abstract KAlSiO4 is a member of feldspathoid, which has high thermal expansion coefficients and catalytic activity for synthesizing biodiesel. In this work, the energies and phonon spectra of the P31c and P63 phases of KAlSiO4 are calculated and the former is confirmed to be more favorable. Studies indicate that the P31c phase is a material with negative expansion along the c axis while the P63 one can be dynamically stabilized at room temperature. Then the electronic, elastic, vibrational, dielectric and piezoelectric properties of the P31c phase are studied by first principles calculation. Computed band structure indicates that it is a direct band gap insulator. Calculated partial density of states and electron localization function reveal that the Al-O and Si-O bonds are covalent while the crystal is a mixed ionic-covalent compound. Obtained elastic constants imply that P31c KAlSiO4 is mechanically stable but highly anisotropic, as illustrated by the directional bulk and Young’s moduli. Further studies indicate that this crystal has a low hardness and Debye temperature, and is more resistant against volume change than shape deformation. Vibrational modes at the Brillouin zone center are classified by factor theory analysis and displayed by eigenvectors. Infrared and Raman spectra are simulated, compared and analyzed. Moreover, the Born effective charges, dielectric constants and piezoelectric coefficients are also calculated and discussed.