Exploring the Impact of Structure-Sensitivity Factors on Thermographic Properties of Dy3+-Doped Oxide Crystals

Abstract

Optical absorption spectra and luminescence spectra were recorded as a function of temperature between 295 K and 800 K for single crystal samples of Gd2SiO5:Dy3+, Lu2SiO5:Dy3+, LiNbO3:Dy3+, and Gd3Ga3Al2O12:Dy3+ fabricated by the Czochralski method and of YAl3(BO3)4:Dy3+ fabricated by the top-seeded high temperature solution method. A thermally induced change of fluorescence intensity ratio (FIR) between the 4I15/2→ 6H15/2 and 4F9/2 → 6H15/2 emission bands of Dy3+ was inferred from experimental data. It was found that relative thermal sensitivities SR at 350 K are higher for YAl3(BO3)4:Dy3+ and Lu2SiO5:Dy3+than those for the remaining systems studied. Based on detailed examination of the structural peculiarities of the crystals it was ascertained that the observed difference between thermosensitive features cannot be attributed directly to the dissimilarity of structural factors consisting of the geometry and symmetry of Dy3+ sites, the number of non-equivalent Dy3+ sites, and the host anisotropy. Instead, it was found that a meaningful correlation between relative thermal sensitivity SR and rates of radiative transitions of Dy3+ inferred from the Judd–Ofelt treatment exists. It was concluded that generalization based on the Judd–Ofelt parameters and luminescence branching ratio analysis may be useful during a preliminary assessment of thermosensitive properties of new phosphor materials.

Structure-Dependent Accessibility of Phonon-Coupled Radiative Relaxation Pathways Probed by X-ray-Excited Optical Luminescence

“…Figure B contrasts the energy manifolds for La 3+ and Dy 3+ centers depicting the accessible X-ray absorption pathways and optical emission channels. We observe that, upon X-ray excitation of core La 3+ electrons, the generated electron–hole pairs are thermalized, thereby populating not only the 4 F 9/2 state but also the 4 I 15/2 state of Dy 3+ ; luminescence emission from these states yields bands at 570 and 540 nm, respectively. , The observed emission from thermally populated state 4 I 15/2 suggests the annihilation of lattice phonons, which is a signature of optical cooling …”

“…We observe that, upon X-ray excitation of core La 3+ electrons, the generated electron−hole pairs are thermalized, thereby populating not only the 4 F 9/2 state but also the 4 I 15/2 state of Dy 3+ ; luminescence emission from these states yields bands at 570 and 540 nm, respectively. 14,20 The observed emission from thermally populated state 4 I 15/2 suggests the annihilation of lattice phonons, which is a signature of optical cooling. 21 Panels A and B of Figure 2 show normalized X-ray absorption near-edge structure (XANES) spectra of double and quadruple molybdates with different La/Dy ratios, respectively.…”

“…1200 cm −1 . 14,20,28 The appearance of these emission features for double molybdate suggests that, upon excitation of La 4d core states, high-energy electron−hole pairs are thermalized and populate 4 I 15/2 levels of Dy 3+ centers on the rare-earth sublattice. 22,24 As such, La 3+ acts as a sensitizer and is coupled to the Dy 3+ chromophore through a phonon-mediated crossrelaxation pathway that populates excited 4 I 15/2 levels based on energy transfer across REO 8 polyhedra.…”

“…22,24 As such, La 3+ acts as a sensitizer and is coupled to the Dy 3+ chromophore through a phonon-mediated crossrelaxation pathway that populates excited 4 I 15/2 levels based on energy transfer across REO 8 polyhedra. 13,14,20 Notably, the phonon-assisted cross-relaxation pathway populating thermally excited states is accessible for the double molybdate but not the quadruple molybdate (Figure 2C and Figure S2B of the Supporting Information), delineating structure-dependent sensitization and coupling mechanisms. In other words, energy transfer between the crystal lattice and the color center is strongly dependent upon the connectivity of the polyhedra containing the La 3+ sensitizer and Dy 3+ emitter, which, in turn, is governed by the crystal structure and stoichiometry (Figure 1A).…”

“…It is well-established that 4 F 9/2 and 4 I 15/2 levels of Dy 3+ are thermally coupled and separated by an energy gap of ca. 1200 cm –1 . ,, The appearance of these emission features for double molybdate suggests that, upon excitation of La 4d core states, high-energy electron–hole pairs are thermalized and populate 4 I 15/2 levels of Dy 3+ centers on the rare-earth sublattice. , As such, La 3+ acts as a sensitizer and is coupled to the Dy 3+ chromophore through a phonon-mediated cross-relaxation pathway that populates excited 4 I 15/2 levels based on energy transfer across REO 8 polyhedra. ,, Notably, the phonon-assisted cross-relaxation pathway populating thermally excited states is accessible for the double molybdate but not the quadruple molybdate (Figure C and Figure S2B of the Supporting Information), delineating structure-dependent sensitization and coupling mechanisms. In other words, energy transfer between the crystal lattice and the color center is strongly dependent upon the connectivity of the polyhedra containing the La 3+ sensitizer and Dy 3+ emitter, which, in turn, is governed by the crystal structure and stoichiometry (Figure A).…”