Disorder effects in LaAlO3 : Ho3+ single crystals revealed by optical spectra

High-resolution spectroscopic studies of $\mathrm{LaAl}{\mathrm{O}}_{3}$ 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 $23\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ 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 ${\mathrm{Ho}}^{3+}$ ions which substitute for ${\mathrm{La}}^{3+}$ ions in $\mathrm{LaAl}{\mathrm{O}}_{3}$ at sites with the ${D}_{3}$ 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 $0.3--0.8\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ that substantially exceeds the widths of the doublet hyperfine splitting. The observed line shapes were successfully modeled, assuming the interaction of the ${\mathrm{Ho}}^{3+}$ ions with random deformations of the crystal lattice induced by point defects and ferroelastic domain boundaries.