Vibrational properties of bulk LaAlO₃ from Fourier-transform infrared ellipsometry

We used Fourier-transform infrared spectroscopic ellipsometry to determine the dielectric function of twinned single-crystalline bulk lanthanum aluminate at 300 K in the region of lattice vibrations from 250 to 1000 cm⁻¹. We fit the experimental data using a classical sum of Lorentz oscillators as well as a factorized model. We were able to determine the parameters of five infrared-active optical phonons within our spectral range. Transverse phonons appear as peaks in the imaginary part of the dielectric function which are clearly visible without fitting. By transforming the data to obtain the loss function, we are able to observe the longitudinal phonons as peaks in the imaginary part. The polar nature of LaAlO₃ causes a strong splitting between the transverse optical (TO) and longitudinal optical (LO) phonon energies. We report energies, amplitudes and broadenings of five TO/LO phonon pairs and compare the two models used to describe the data. - Highlights: • Infrared spectra of bulk LaAlO₃ taken using FTIR ellipsometry • Peaks analyzed using a factorized phonon model including broadenings • Transverse and longitudinal optical phonon parameters reported