Optical constants, band gap, and infrared-active phonons of (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (LSAT) from spectroscopic ellipsometry

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO₃)0.3(Sr₂AlTaO₆) 0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies and makes the crystal opaque above 4.8 eV. Using Fourier-transform infrared ellipsometry, the lattice dynamics was studied. Nine pairs of transverse/longitudinal phonons were found and attributed to disorder in the La/Sr sublattice, ordering in the Al/Ta sublattice, and two- phonon absorptio