Agata Lazarowska

Luminescence of Gd2(WO4)3:Ln3+ at ambient and high hydrostatic pressure

This paper presents a spectroscopic characterization of Gd(2)(WO(4))(3):Ln(3+) (Ln=Eu, Pr, Tb and Dy). The luminescence and luminescence kinetics were measured under pressures up to 250 kbar. It was found that pressure quenches the luminescence of Pr(3+) and Tb(3+), whereas the emission of Eu(3+) and Dy(3+) was stable up to 250 kbar. This effect was related to a decrease in the ionization energy of Pr(3+) and Tb(3+) caused by pressure induced increase in energies of the Ln(2+) and Ln(3+) ions with respect to the band edges. Analysis of emission and excitation spectra allowed us to estimate the energies of the ground states of Ln(3+) and Ln(2+) with respect to the valence and conduction band edges of the Gd(2)(WO(4))(3) host. Differences between energies of the ground states of Ln(2+) and Ln(3+) have also been calculated.

Improvement of the Water Resistance of a Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor Synthesized under High Isostatic Pressure through Coating with an Organosilica Layer

A SrLiAl3 N4 :Eu(2+) (SLA) red phosphor prepared through a high-pressure solid-state reaction was coated with an organosilica layer with a thickness of 400-600 nm to improve its water resistance. The observed 4f(6) 5d→4f(7) transition bands are thought to result from the existence of Eu(2+) at two different Sr(2+) sites. Luminescence spectra at 10 K revealed two zero-phonon lines at 15377 (for Eu(Sr1)) and 15780 cm(-1) (for Eu(Sr2)). The phosphor exhibited stable red emission under high pressure up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for the Eu(2+/3+) result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70 % of their initial EQE after aging for 5 days under harsh conditions. White-light-emitting diodes of the SLA red phosphor and a commercial Y3 Al5 O12 :Ce(3+) yellow phosphor on a blue InGaN chip showed high color rendition (CRI=89, R9=69) and a low correlated color temperature of 2406 K.

Pressure effect on the zero-phonon line emission of Mn4+ in K2SiF6

In this work, effects of pressure and temperature on the luminescent properties of the K2SiF6:Mn(4+) system have been presented. At ambient pressure, the luminescent spectrum of Mn(4+) consists of several lines at 610-650 nm attributed to phonon repetitions of the (2)Eg → (4)A2g transition and does not contain the zero phonon line (ZPL). At pressure above 9 kbar, an additional line at about 624 nm occurs, which can be attributed to the ZPL of the (2)Eg → (4)A2g transition in the Mn(4+) ions. This change in the emission spectra is accompanied by shortening of the luminescence decay time. Further increasing pressure up to 220 kbar causes the red shift of all emission lines. Upon releasing pressure, all observed lines are going back to their previous positions. The ZPL remains visible even at ambient pressure. Taking into account XRD and Raman spectra at ambient pressure before and after compression-decompression, we have attributed these changes to pressure-induced local structure change of MnF6 (2-) octahedron.

Pressure-induced phase transition in LiLuF4:Pr3+ investigated by an optical technique

The luminescence and luminescence kinetics of LiLuF(4) doped with 1.5 at.% of Pr(3+) obtained at high hydrostatic pressure changing from ambient to 220 kbar applied in a diamond anvil cell are presented. It has been shown that pressure causes shift of the emission lines toward the red with rates of the order of single cm(-1) kbar(-1). The pressure-induced phase transition from tetragonal to fergusonite structure for pressure above 100 kbar was observed. The crystal field calculations performed showed that this phase transition reduces the point symmetry of the Pr(3+) site from the S(4) to the C(2) point group.

Spectroscopic studies of inclusion complexes of methyl-p-dimethylaminobenzoate and its ortho derivative with α- and β-cyclodextrins

The effects of α- and β-cyclodextrins (CDs) on the both emission modes (LE -locally excited and TICT -twisted intramolecular charge transfer) of the fluorescence spectrum of methyl-p-dimethylaminobenzoate (I) and its o-methoxy (II) derivative in aqueous solution have been investigated using steady-state and time-resolved fluorescence techniques. It is found that the intensity of both fluorescence bands increases with increasing concentration of α- and β-CD. The stoichiometries and equilibrium constants of the fluorophore-cyclodextrin inclusion complexes have been determined by steady-state fluorescence measurements. Performed spectroscopic studies demonstrate that in the case of I in α-CD and β-CD, both 1:1 and 1:2 inclusion complexes are formed, whereas only 1:1 inclusion complex is formed between II and β-CD.

Pressure dependence of the emission in CaF2 : Yb2+

We present a detailed spectroscopic investigation of CaF2 doped with Yb(2+) performed at high hydrostatic pressure which is applied in a diamond anvil cell. At ambient pressure and at temperatures lower than 175 K, the luminescence consists of a single broad band peaked at 18 500 cm(-1), attributed to the recombination of impurity-trapped excitons. Increasing pressure causes the luminescence to be observable at higher temperature. At a pressure of 72 kbar luminescence can be observed up to 275 K. The emission lineshape does not strongly depend on pressure below 85 kbar. However, at 85 kbar it is blue shifted to 21 630 cm(-1). This is attributed to the known phase transition of the CaF2 crystal from cubic to the orthorhombic phase. The absolute energy of the ground and 4f(13)5d states of Yb(2+) as well as the energy of the impurity-trapped exciton with respect to valence and conduction bands have been estimated. The results, are discussed in comparison with the pressure dependences observed for the luminescence of BaF2 : Eu(2+) and CaF2 : Eu(2+). The difference between the spectral properties of Eu(2+) and Yb(2+) is attributable to the fact that the ground and 4f(6)5d states of Eu(2+) are placed deeper in the CaF2 bandgap than the ground and excited 4f(13)5d states of Yb(2+), whereas the energies of the impurity-trapped exciton states for Yb(2+) and Eu(2+) with respect to the conduction band are approximately the same.

Aluminate Red Phosphor in Light-Emitting Diodes: Theoretical Calculations, Charge Varieties, and High-Pressure Luminescence Analysis

Searching for a non-rare-earth-based oxide red-emitting phosphor is crucial for phosphor-converted light-emitting diodes (LEDs). In this study, we optimized a blue and UV-light excited Sr4Al14O25:Mn phosphor exhibiting red emission peaked at ∼653 nm, which was successfully synthesized by solid-state reaction. The crystal structure, micromorphology, and luminescent properties of Sr4Al14O25:Mn phosphors were characterized by X-ray Rietveld refinement, high-resolution transmission electron microscopy, and photoluminescence spectra. The band gap and electronic structure of Sr4Al14O25 were analyzed by density functional theory calculations using the hybrid exchange-correlation functional. The crystal field environment effect of Al sites from introducing activator Mn ions was investigated with the aid of Raman 27Al nuclear magnetic resonance spectra and electron spin resonance. The pressure dependent luminescent properties and decay time of this compound were presented. The tricolor display spectrum by combining blue InGaN chips, commercial β-SiAlON:Eu2+ green phosphor, and Sr4Al14O25:Mn red phosphor were evaluated for commercial applications: using the present Sr4Al14O25:Mn red phosphor converted LED as a backlighting source.

Structural phase transitions and photoluminescence properties of oxonitridosilicate phosphors under high hydrostatic pressure.

Spectroscopic properties of a series of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) compounds has been studied under high hydrostatic pressure applied in a diamond anvil cell up to 200 kbar. At ambient pressure the crystal structures of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) are related to the ratio of strontium to barium and three different phases exists: orthorhombic Pbcn(0.78 ≤ x ≤ 0.98), triclinic P1 (0 < x ≤ 0.65) and triclinic P1 (0.65 < x < 0.78). It was found that Eu2+ luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu2+ sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (Sr0.98-xBaxEu0.02)Si2O2N2 into orthorhombic structure. Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (Sr0.98-xBaxEu0.02)Si2O2N2 series. Additionally, very different pressure shifts of the Eu2+ luminescence in different phases of (Sr0.98-xBaxEu0.02)Si2O2N2:Eu from −40 cm−1/kbar to 0 cm−1/kbar have been observed. This effect is explained by different interaction of the Eu2+ 5d electron with the second coordination sphere around the impurity cations.

Temperature evolution of the luminescence decay of Sr0.33Ba0.67Nb2O6 : Pr3+.

This article presents a spectroscopic investigation of Sr(0.33)Ba(0.67)(NbO2)3, doped with 1 mol% of Pr(3+). Photoluminescence and luminescence kinetics were measured at different temperatures at ambient (ferroelectric phase) and 76 kbar pressures (paraelectric phase). The photoluminescence spectrum is dominated by (1)D2 → (3)H4 transition of Pr(3+) in both phases. At ambient pressure when the system is excited with UV radiation, the intensity of dominant (1)D2 → (3)H4 emission evidently increases in the 200-293 K temperature range. This effect is attributed to enhancement of the excitation of the (1)D2 state through the praseodymium trapped exciton state, which at higher temperatures does not populate the higher lying (3)P0 state. Additionally, under UV radiation the material exhibits afterglow luminescence activated by temperature that can also have an impact on the increase of the (1)D2 emission. We propose that the afterglow luminescence is related to the existence of electron traps. At a pressure of 76 kbar the depth of the electron traps decreases in comparison to the ones observed at ambient pressure. However, the phase transition does not change the number of electron traps.

Thermal stabilization and energy transfer in narrow-band red-emitting Sr[(Mg2Al2)1−y(Li2Si2)yN4]:Eu2+ phosphors

Cuboid-coordinated nitridomagnesoaluminate Sr[Mg2Al2N4]:Eu2+ and a solid solution of Sr1−x[(Mg2Al2)1−y(Al2Si2)yN4]:Eux2+ were prepared using all-nitride precursors by gas pressure sintering. X-ray diffraction (XRD) data of the phosphors were validated by Rietveld refinement of synchrotron X-ray powder XRD data with the space group I4/m indexed to a tetragonal crystal system. The 7Li solid-state magic-angle spinning nuclear magnetic resonance (ss-MAS-NMR) data prove the successful incorporation of the Li+–Si4+ couple. The blue-light excitable property with the excitation band peaking at 460 nm gave rise to emission at 620–630 nm at x = 0.004, with a full-width-at-half-maximum of ∼77 nm. The inhomogeneous broadening of the Eu2+ luminescence in the system due to the effective energy transfer from one activator to another was observed which concurrently resulted in spectral peak shifts from ∼615 nm to ∼680 nm as a function of the amount of Eu2+. The Li+–Si4+-tuned solid solution increased the emission intensity without significantly shifting the emission wavelength. Such a phenomenon was accompanied by an improvement in thermal stability from Δ = 965 cm−1 for y = 0, to Δ = 1365 cm−1 for y = 0.1 wherein Δ corresponds to the activation energy from the 5d states to the conduction band. The simple gas pressure sintering strategy using all-nitride starting materials resulted in the desired blue light-excitable narrowband red emitting thermally stabilized phosphor.