Upconversion luminescence of nano-sized Yb and Tm codoped rare earth phosphates, that is, LaPO and YPO, 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), the thermalized states of Tm 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 Tm band ratio (F → H/H → H), 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).
Anti-Stokes luminescence of up-converting nanocrystals SrF:Yb,Er can be used as a high pressure optical sensor alternative to the ruby fluorescence-scale. In nanocrystalline SrF:Yb,Er, high pressure reversibly shortens the emission lifetimes nearly linearly up to 5.29 GPa at least. Its advantage is the use of NIR (≈980 nm) radiation, highly penetrable for many materials. The shortening of up-conversion lifetimes has been attributed mainly to the changes in energy transfer rates, caused by decreased interatomic distances and increased overlap integrals between 4f electrons and the valence shells of ligand ions. The origin of high-pressure effects on the luminescence intensity, band ratio and their spectral position has been explained by the increased interactions and distortions of the crystal-field symmetry around the emitting ions in the compressed structure.
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
A3Tb(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.
Electronic and structural properties
of cubic Lu2O3 with either single oxygen vacancy,
oxygen vacancy–vacancy
pair, or a Frenkel pair (an oxygen vacancy and an interstitial oxygen)
were analyzed using ab initio density functional
theory calculations. A plane-wave ultrasoft pseudopotential approach
with local density approximation functional was used to optimize the
geometries. A full-potential linearized augmented plane-wave method
with meta-generalized gradient approximation was applied to the optimized
geometries to calculate the electronic properties of the systems.
Defect-related bands were observed between the valence band and the
conduction band. Different charge states of the defects were considered.
Both oxygen vacancy and oxygen vacancy–vacancy pairs behave
as moderately deep electron traps (trap depth 1.3–2.7 eV).
Trapped electron localization at the vacancy site(s) was confirmed
using electron density and electron localization function plots. The
bands originating from oxygen vacancy–vacancy pairs in Lu2O3 exhibit some dependence on the distance between
the two entities. The resulting energy differences (considered from
an optical absorption point of view) cover ultraviolet, visible, and
infrared ranges. Thus, it is plausible that oxygen vacancy–vacancy
pairs are responsible for the coloration that is sometimes observed
in Lu2O3 powders and crystals. On the contrary,
the Frenkel pair exhibits no systematic dependence of the defect-related
bands on the distance between its oxygen vacancy and interstitial
oxygen, while the resulting defect-related bands look similar to those
corresponding to the isolated defects. Frenkel pairs are thus considered
a probable mechanism of oxygen vacancy formation and stabilization
of Lu2O3. Additionally, a brief review of the
relevant experimental data is provided in the introduction.
A new strategy for noninvasive temperature probing, applying the temperature-induced configuration crossover between the thermally-coupled 6P7/2 and 5d1 levels of Eu2+ is presented.
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