Synthesis, physicochemical characterization and computational studies of selected lanthanide-doped luminophores

Wydział Chemii: Zakład Ziem RzadkichBadania dotyczyły materiałów o ogólnym wzorze M3RE2(BO3)4:Ln3+ (gdzie: M = Ca, Sr, Ba; RE = Y, La, Gd oraz Ln3+ = Eu3+, Tb3+, Dy3+). Materiały tego typu były otrzymywane za pomocą metody zol-żelowej Pechini'ego. Otrzymane luminofory mogą być zastosowane w źródłach świtała opartych o LED. Kolejna część pracy była poświęcona badaniom obliczeniowym wpływu domieszkowania na budowę krystaliczną materiału-matrycy, wpływu budowy krystalicznej matrycy na procesy przeniesienia energii oraz na tworzenie się aglomeratów powodowanych jonami domieszki. Wyniki obliczeń pół-empirycznych oraz DFT wykazały, że zwiększenie ilości domieszki Tb3+ w układach CeF3 i Sr3La2(BO3)4 skutkowało zmniejszeniem wymiarów komórki. Nowe podejście do obliczenia szybkości przeniesienia energii pomiędzy jonami Ln3+ w ciele stałym (z uwzględnieniem wielu możliwych odległości Ln-Ln) zostało zastosowane do analizy układu YVO4:Yb3+,Er3+, wykazującego zjawisko upkonversji. Opisywana metoda modelowania może być zastosowana też dla innych układów wykazujących przeniesienie energii. Badania uwzględniały również analizę (za pomocą obliczeń DFT) możliwości tworzenia klasterów jonów domieszki w YVO4:Eu3+. Obliczenie wskazują, że geometria układu odpowiadająca bardziej skupionemu ułożeniu jonów domieszki jest bardziej preferowana termodynamicznie, a więc możliwa jest tendencja do tworzenia aglomeratów jonów domieszki.A series Ln3+-doped photoluminescent materials of general formula M3RE2(BO3)4:Ln3+ (M = Ca, Sr, Ba, RE = Y, La, Gd, Ln3+ = Eu3+, Tb3+, Dy3+) were studied. Such materials were synthesized by the wet-chemistry sol-gel Pechini route. The phosphors can be potentially applied in LED-based light sources. Another part of this work was devoted to the computational studies focused on the effects of dopant addition of the crystal structure of the matrix material, the effects of crystal structure of the matrix on energy transfer processes and dopant ions clustering (agglomeration). According to both both semi-empirical and ab initio DFT quantum-chemical computations, an increase in the Tb3+ dopant content resulted in a gradual decrease in the crystal cell dimensions of such materials as Sr3La2(BO3)4 and CeF3. A novel approach of calculation of energy transfer rates between the Ln3+ ions in solids was applied to YVO4:Yb3+, Tb3+ upconvesion system. The proposed approach resulted in good agreement between the experiment and calculations and can be applied to any other energy transfer processes in any Ln3+-based materials. The inhomogeneity of dopant ions distribution in the structure of YVO4:Eu3+ was analyzed. It was found out using DFT that more dense dopant ion packing, clustering corresponds to a lower energy, and thus can be more preferable from the thermodynamical point of view

Theoretical and Experimental Investigation of the Tb3+-Eu3+ Energy Transfer Mechanisms in Cubic A3Tb0.90Eu0.10({PO}4)3 (A = Sr, Ba) Materials

In this study the optical spectroscopy, the excited state dynamics, and in particular the Tb3+ -> Eu3+ energy transfer, have been investigated in detail both from the theoretical and experimental point of view in eulytite double phosphate hosts A(3)Tb(PO4)(3) (A = Sr, Ba) doped with Eu3+. It has been found that the energy transfer is strongly assisted by fast migration in the donor Tb3+ subset. Moreover, the transfer rates and efficiencies depend significantly on the nature of the divalent elements present in the structure and hence on the distances between Tb3+-Eu3+ nearest neighbors. It is shown that the competition between quadrupole-quadrupole and exchange interaction is crucial in accounting for the transfer rates

Electron Trap Depths in Cubic Lutetium Oxide Doped with Pr and Ti, Zr or HfFrom Ab Initio Multiconfigurational Calculations

We propose a universal approach to model intervalence charge transfer (IVCT) and metal-to-metal charge transfer (MMCT) transitions between ions in solids. The approach relies on already well-known and reliable ab initio RASSCF/CASPT2/RASSI-SO calculations for a series of emission center coordination geometries (restricted active space self-consistent field, complete active space second-order perturbation theory, and restricted active space state interaction with spin-orbit coupling). Embedding with ab initio model potentials (AIMPs) is used to represent the crystal lattice. We propose a way to construct the geometries via interpolation of the coordinates obtained using solid-state density functional theory (DFT) calculations for the structures where the activator metal is at specific oxidation (charge) states of interest. The approach thus takes the best of two worlds: the precision of the embedded cluster calculations (including localized excited states) and the geometries from DFT, where the effects of ionic radii mismatch (and eventual nearby defects) can be modeled explicitly. The method is applied to the Pr activator and Ti, Zr, Hf codopants in cubic Lu2O3, in which the said ions are used to obtain energy storage and thermoluminescence properties. Electron trap charging and discharging mechanisms (not involving a conduction band) are discussed in the context of the IVCT and MMCT role in them. Trap depths and trap quenching pathways are analyzed

Multifunctional Optical Sensors for Nanomanometry and Nanothermometry: High-Pressure and High-Temperature Upconversion Luminescence of Lanthanide-Doped PhosphatesLaPO₄/YPO₄:Yb³⁺–Tm³⁺

Upconversion luminescence of nano-sized Yb³⁺ and Tm³⁺ codoped rare earth phosphates, that is, LaPO₄ and YPO₄, has been investigated under high-pressure (HP, up to ∼25 GPa) and high-temperature (293–773 K) conditions. The pressure-dependent luminescence properties of the nanocrystals, that is, energy red shift of the band centroids, changes of the band ratios, shortening of upconversion lifetimes, and so forth, make the studied nanomaterials suitable for optical pressure sensing in nanomanometry. Furthermore, thanks to the large energy difference (∼1800 cm^–1), the thermalized states of Tm³⁺ ions are spectrally well-separated, providing high-temperature resolution, required in optical nanothermometry. The temperature of the system containing such active nanomaterials can be determined on the basis of the thermally induced changes of the Tm³⁺ band ratio (³F_2,3 → ³H₆/³H₄ → ³H₆), observed in the emission spectra. The advantage of such upconverting optical sensors is the use of near-infrared light, which is highly penetrable for many materials. The investigated nanomanometers/nanothermometers have been successfully applied, as a proof-of-concept of a novel bimodal optical gauge, for the determination of the temperature of the heated system (473 K), which was simultaneously compressed under HP (1.5 and 5 GPa)

Effects of Dopant Addition on Lattice and Luminescence Intensity Parameters of Eu(III)-Doped Lanthanum Orthovanadate

A series of La_1–<i>x</i>Eu_<i>x</i>VO₄ samples with a different Eu³⁺ content was synthesized via a hydrothermal route. An increase in the dopant content resulted in a decrease in lattice constants of the materials. Plane-wave DFT calculations with PBE functional in CASTEP confirmed this trend. Next, CASTEP calculations were used to obtain force constants of Eu–O bond stretching, using a novel approach which involved displacement of the Eu³⁺ ion. The force constants were then used to calculate charge donation factors <i>g</i> for each ligand atom. The chemical bond parameters and the geometries from DFT calculations were used to obtain theoretical Judd–Ofelt intensity parameters Ω_λ. The effects of geometry changes caused by the dopant addition were analyzed in terms of Ω_λ. The effects of distortions in interatomic angles of the Eu³⁺ coordination geometry on the Ω_λ were analyzed. Effects of distortions of atomic positions in the crystal lattice on the Ω_λ and photoluminescence intensities of Eu³⁺ 4f–4f transitions were discussed. It was shown that the ideal database geometry of LaVO₄ corresponds to the highly symmetric coordination geometry of Eu³⁺ and very low Ω₂. On the contrary, experimental intensities of the ⁵D₀ → ⁷F₂ transition and the corresponding Ω₂ parameters were high. Consequently, distortions of crystal structure that reduce the symmetry play an important role in the luminescence of the LaVO₄:Eu³⁺ materials and probably other Eu³⁺-doped phosphors based on zircon-type rare earth orthovanadates