Disorder effects in LaAl O3: Ho3+ single crystals revealed by optical spectra

Abstract

High-resolution spectroscopic studies of LaAlO3 single crystal doped with holmium ions are reported. Polarized and unpolarized absorption and luminescence spectra were measured in the broad spectral range from 2000 to 23000cm-1 at temperatures of 4.5-5 K. Additional measurements were fulfilled using site-selective laser spectroscopy. Energies and symmetry properties of the corresponding wave functions of crystal-field levels of Ho3+ ions which substitute for La3+ ions in LaAlO3 at sites with the D3 symmetry were determined with high accuracy, and, on this basis, crystal-field calculations were performed. A thorough analysis of spectral line profiles with the fine doublet structure corresponding to singlet-doublet transitions in the trigonal crystal field was made, which showed the existence of the random deformation splitting phenomenon. The value of deformation splitting lies in the region 0.3-0.8cm-1 that substantially exceeds the widths of the doublet hyperfine splitting. The observed line shapes were successfully modeled, assuming the interaction of the Ho3+ ions with random deformations of the crystal lattice induced by point defects and ferroelastic domain boundaries.