Electronic, Optical, and Lattice Dynamical Properties of Tetracalcium Trialuminate (Ca₄Al₆O).

The electronic, optical, and lattice dynamical properties of tetracalcium trialuminate (Ca₄Al₆O) with a special sodalite cage structure were calculated based on the density functional theory. Theoretical results show that Ca₄Al₆O is ductile and weakly anisotropic. The calculated Young's modulus and Poisson ratio are 34.18 GPa and 0.32, respectively. Ca₄Al₆O is an indirect-gap semiconductor with a band gap of 5.41 eV. The top of the valence band derives from O 2 states, and the bottom of conduction band consists of Ca 3 states. Transitions from O 2, 2 states to empty Ca 4, 3 and Al 3, 3 states constitute the major peaks of the imaginary part of the dielectric function. Ca₄Al₆O is a good UV absorber for photoelectric devices due to the high absorption coefficient and low reflectivity. The lattice vibration analysis reveals that O atoms contribute to the high-frequency portions of the phonon spectra, while Ca and Al atoms make important contributions to the middle- and low-frequency portions. At the center of the first Brillouin zone, lattice vibrations include the Raman active modes (E, A₁), infrared active mode (T₂), and silentmodes (T₁, A₂). Typical atomic displacement patterns were also investigated to understand the vibration modes more intuitively.