Spectroscopic characterization of Sm3+ in La3Ga5.5Ta0.5O14 single crystals

Abstract

La3Ga5.5Ta0.5O14 single crystals with nominal Sm3+ concentrations of 0.5 at.% and 3 at.% were grown by the Czochralski method from iridium crucibles. Examination of chemical composition of manufactured crystals revealed that actual Sm3+ concentrations are 0.27 at.% and 1.96 at.%, respectively. Spectroscopic measurement encompassed acquisition of absorption spectra, luminescence spectra and luminescence decay curves at temperatures between 5K and 300K. Analysis of room temperature absorption spectra in the framework of the Judd–Ofelt treatment made it possible to determine radiative transition rates branching ratio values related to the 4G5/2 luminescence of Sm3+ ions. Crystal field splitting of Sm3+ multiplets involved in luminescence phenomena was inferred from absorption and emission spectra recorded at 5K. It was found that absorption and luminescence transitions exhibit strong anisotropy and their intensities for σ-polarized spectra is higher by a factor of two roughly than those for π-polarized spectra. Observed strong inhomogeneous broadening of spectral bands was attributed to certain structural disorder of the host resulting from the substitution of octahedrally coordinated Ga3+ ions by Ta5+ ions. It follows from calculated and experimental values of luminescence branching ratio that about 80% of excitation energy of the 4G5/2 metastable level is converted to visible luminescence distributed into three bands peaking at 550nm, 600nm and 646nm. It was concluded that the La3Ga5.5Ta0.5O14:Sm3+ system is promising for application as an orange–red emitting phosphor and deserves further investigation aimed at assessing its ability to show yellow laser emission.