Site-selective excitation and polarized absorption and emission spectra of trivalent thulium and erbium in strontium fluorapatite

Abstract

Polarized fluorescence spectra produced by site-selective excitation, and conventional polarized absorption spectra were obtained for Tm3+ and Er3+ ions individually incorporated into single crystals of strontium fluorapatite, Sr5(PO4)3F, also known as SFAP. Substitution of the trivalent rare earth ion for divalent strontium was achieved by passive charge compensation during Czochralski growth of the fluorapatite crystals. Spectra were obtained between 1780 and 345 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure [P63/m(C6h2)]. The polarized fluorescence spectra due to transitions from multiplet manifolds of Tm3+(4f12), including D21, G41, and H43 to manifolds H63 (the ground-state manifold), F43, H53, H43, and F33 were analyzed for the details of the crystal-field splitting of the manifolds. Fluorescence lifetimes were measured for Tm3+ transitions from D21, G41, and H43 at room temperature and from G41 at 16 K. Results of the analysis indicate that the majority of Tm3+ ions occupy sites having Cs symmetry. A point-charge lattice-sum calculation was made in which the crystal-field components, Anm, were determined assuming that trivalent thulium replaces divalent strontium in the metal site having Cs symmetry. Results support the conclusion that the nearest-neighbor fluoride (F−) is replaced by divalent oxygen (O2−), thus preserving overall charge neutrality and local symmetry. Crystal-field splitting calculations predict energy levels in agreement with results obtained from an analysis of the experimental data. By varying the crystal-field parameters, Bnm, we obtained a rms difference of 7 cm−1 between 43 calculated and experimental Stark levels for Tm3+(4f12) in Tm:SFAP. Absorption and fluorescence spectra are also reported for Er3+ ions in Er:SFAP. Measurement of the temporal decay of the room temperature fluorescence from the I11/24 and I13/24 manifolds yielded fluorescence lifetimes of 230±20 μs and 8.9±0.1 ms, respectively. The experimental Stark levels obtained from an analysis of the spectroscopic data were compared with a crystal-field splitting calculation. The initial set of Bnm parameters for Er3+(4f11) was established from the three-parameter theory and the final set of Bnm parameters obtained for Tm3+(4f12) in Tm:SFAP. The best overall agreement between calculated and experimental Stark levels is 8 cm−1 for 48 Stark levels, representing 12 observed multiplet manifolds of Er3+(4f11) in Er:SFAP.