Spectroscopic and crystal field studies ofNd3+inGdCa4O(BO3)3andYCa4O(BO3)3

Abstract

The calcium rare-earth oxoborate crystals $R{\mathrm{Ca}}_{4}\mathrm{O}({\mathrm{BO}}_{3}{)}_{3}\ensuremath{-}R\mathrm{COB}$ with ${R}^{3+}$ as ${\mathrm{Gd}}^{3+}$ or ${\mathrm{Y}}^{3+}$ represent promising laser and nonlinear materials for the development of compact near IR and visible laser sources. New results on ${\mathrm{Nd}}^{3+}$ spectral characteristics in GdCOB and YCOB crystals in connection to the crystal structure are presented. Low-temperature absorption and selectively excited emission spectra of ${\mathrm{Nd}}^{3+}$ in $R\mathrm{COB}$ crystals, grown by the Czochralski method in iridium crucible, present one prevailing center corresponding to ${\mathrm{Nd}}^{3+}$ ions in the ${R}^{3+}$ site of ${C}_{s}$ symmetry and at least three minority centers. Crystal field modeling gives a set of free ion and crystal field parameters that describe well the experimentally obtained energy level schemes for the main centers. A comparison between the ${\mathrm{Nd}}^{3+}$ crystal field splittings in $R\mathrm{COB}$ and those of the ${C}_{2}$ site in C-type ${\mathrm{Y}\mathrm{}}_{2}{\mathrm{O}}_{3}$ in terms of rare-earth environments is made. The selectively excited emission, lifetimes, and structural data were used to elucidate the nature of the minority centers. Two of the them were associated with ${\mathrm{Nd}}^{3+}$ in ${R}^{3+}$ sites slightly perturbed by charged intrinsic lattice defects of nonstoichiometric or inversion ${\mathrm{Gd}}^{3+}({\mathrm{Y}}^{3+})\ensuremath{\leftrightarrow}{\mathrm{Ca}}^{2+}$ type, while the third ${\mathrm{Nd}}^{3+}$ center is assigned to ${\mathrm{Nd}}^{3+}$ in a ${\mathrm{Ca}}^{2+}$ site. Other features of the spectra such as vibronics or homogeneous linewidths are also discussed. The Gaussian line shape and main contributions to inhomogeneous broadening are analyzed. An additional source of broadening for ${\mathrm{Nd}}^{3+}$ in YCOB is revealed.