M. Guzik

Application of lanthanide (Eu, Nd) spectroscopy as a structural probe of diluted double phosphates

Abstract The present studies are focused on the application of europium(III) emission as a structural probe of diluted double phosphate and the determination of the number of metal sites, as well as their symmetries. The influence of the M(I) ions on the structure of phosphates is shown in the emission spectra at 77 K. The IR and Raman spectra were measured and used in the assignment of the vibronic components of electronic transitions. The electron–phonon coupling was analyzed in the emission and excitation spectra of europium double phosphate. Moreover, absorption spectra at 293 and 4 K as well as intensity analysis of Nd(III) f–f transition were used in the detection of structural modifications in both types of phosphates—rubidium (1) and sodium (2) salts. The energy level diagram was proposed and compared to the earlier reported data for different types of phosphates.

Eu3+ luminescence from different sites in a scheelite-type cadmium molybdate red phosphor with vacancies

Eu3+-doped scheelite-type cadmium molybdate with the chemical formula Cd1−3xEu2x□xMoO4 (vacancy is denoted by □) was investigated as a red-emitting conversion phosphor for white light emitting diodes (WLEDs) by taking advantage of the Eu3+ spectroscopic probe to analyze in detail the structural properties as a continuation of our previous analysis on both, Cd1−3xNd2x□xMoO4 and Cd1−3xYb2x□xMoO4. A series of microcrystalline samples were synthesized using a high-temperature solid state reaction. X-ray diffraction patterns indicate that the samples show only one phase and crystallize in the tetragonal scheelite-type structure (space group I41/a with point symmetry S4), when the concentration of Eu3+ ions is in the range from 0.05 mol% to 66.67 mol% with respect to Cd2+ ions. Substitution of divalent Cd2+ by trivalent Eu3+ cations leads to the formation of cationic vacancies in the framework due to the charge compensation: 3Cd2+ → 2Eu3+ + □ vacancy. The □ vacancy concentration dependence brings some originality to this research program in optical materials. High-resolution photoluminescence excitation and emission spectra were investigated. Time-resolved laser spectroscopy performed at room temperature and at 77 K allowed the separation of lines corresponding to transitions from the 5D1 and 5D0 levels, respectively, and to define the contributions of the individual emission lines in the steady state spectrum. Low temperature fluorescence spectra obtained by applying laser site-selective excitation confirmed the presence of multisite characteristics of Eu3+ ions in this host lattice, similar to that found for Nd3+ and Yb3+ dopants. Three types of Eu3+ sites have been identified. These phosphors are efficiently excited by UV light, and exhibit a very strong red luminescence corresponding mainly to the electric dipole 5D0 → 7F2 transition at 616 nm, and thus they are considered as promising phosphors for WLEDs. Concentration quenching of Eu3+ luminescence as well as the optimum doping of optically active ions were investigated.

Structural and spectroscopic characterizations of two promising Nd-doped monoclinic or tetragonal laser tungstates

The tetragonal Cd0.25Nd0.50□0.25WO4 compound (□ – statistically distributed vacancies in cation sublattice) and its gadolinium diluted Cd0.25Gd0.50□0.25WO4:Nd3+ analogue as well as the monoclinic CdNd2W2O10 phase and its yttrium diluted CdY2W2O10:Nd3+ analogue were synthesized by the high temperature solid-state reaction. The gadolinium or yttrium diluted samples were doped with different concentrations of the active Nd3+ ion: 0.25, 0.5, 2.5 and 5 mol%. X-ray powder diffraction, IR spectra as well as electron microscopy (SEM and HRTEM) were used to identify their structure and morphology. All micromaterials were obtained as spherical particles with average diameters between 1 and 10 μm. In order to study their spectroscopic properties, high resolution absorption and emission spectra in the visible and IR regions were measured at room temperature, 77 K and 4 K. The kinetics of luminescence from the lowest 4F3/2 energy level as a function of the Nd3+ ion concentration and temperature is also reported. Both the values of the absorption cross-section calculated for 5 mol% of Nd3+ ion, and the very strong emission of the 4F3/2 → 4I11/2 laser channel of Nd3+ recorded under Xe lamp excitation or OPO laser pumping, indicate that the scheelite-type Cd0.25Gd0.50□0.25WO4:Nd3+ phase can be considered as a promising laser material. The radiative lifetimes of IR luminescence are also appropriate for potential applications of this phosphor as a solid-state laser.

Influence of Al3+ and P5+ ion contents on the valence state of Yb3+ ions and the dispersion effect of Al3+ and P5+ ions on Yb3+ ions in silica glass

Three series of Yb3+-doped silica glasses containing different amounts of Al2O3 and P2O5 were prepared successfully by using a sol–gel method. Absorption, excitation and fluorescence spectra of Yb2+ ions in these silica glasses as well as X-ray photoelectron spectra (XPS) of Yb4d were recorded and analysed systematically. It is found out that the addition of Al3+ or P5+ ions has great influence on the redox state of ytterbium ions. With increasing Al3+ ion contents in these silica glasses, more trivalent Yb3+ ions are reduced to divalent Yb2+. In contrast, the increase of P5+ ion contents greatly promotes divalent Yb2+ ions to be oxidized to trivalent Yb3+. The possible redox mechanisms have been explored and discussed in detail. The influence of Al3+ and P5+ ion contents on the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs was also discussed. Both the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs decrease gradually with increasing Al3+ and P5+ ion contents. The decrease of cooperative luminescence of Yb3+ ion pairs indicates a good dispersion effect of Al3+ and P5+ ions on Yb3+ ions in Yb3+-doped silica glass. The results are useful for optimization of fabrication of the high quality Yb3+-doped silica fiber by the composite design of Yb–Al–P co-doped silica glass.

Structural and spectroscopic characterizations of new Cd1−3xNd2x□xMoO4 scheelite-type molybdates with vacancies as potential optical materials

The necessity to develop new laser materials of tungstates and molybdates is our motivation to study a new family of molybdates activated by Nd3+ ions, which crystallizes in the scheelite-type structure (the tetragonal symmetry, the space group I41/a). A series of Cd1−3xNd2x□xMoO4 stable solid solutions with concentration of Nd3+ ions from 0.1 mol% to 66.67 mol% with respect to Cd2+ ions were successfully synthesized by a high-temperature annealing, using CdMoO4 and Nd2(MoO4)3 as the starting materials. Structural and spectroscopic characterizations of Nd3+ substituting Cd2+ ions were carried out. The substitution of divalent Cd2+ by trivalent Nd3+ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □). The Nd3+ ions in this matrix occupy two non-equivalent symmetry sites depending on the location of these vacancies. Concentration of vacancies depends essentially on the composition of initial CdMoO4–Nd2(MoO4)3 mixtures. The SEM analysis of crystal morphology reveals high homogeneity of the nearly-spherical shaped products with an average grain size of about 1–10 μm. Optical analysis and the laser parameters suggest that Cd1−3xNd2x□xMoO4 can be considered as a potential laser material. Both the values of the absorption cross-section and the strong emission of the 4F3/2 → 4I11/2 laser channel of Nd3+ recorded under LED source excitation or OPO laser pumping, as well as the radiative lifetimes of NIR luminescence, are appropriate for potential applications of this optical material as a solid state laser.

Influence of Stark splitting levels on the lasing performance of Yb 3+ in phosphate and fluorophosphate glasses

Lasing properties have been investigated for Yb(3+) doped glasses with similar emission cross sections (σ(emi)) and lifetime while possessing different Stark levels. Narrow Stark splitting of Yb(3+)-phosphate glass is responsible for severe heat generation, narrow emission band and much smaller σ(emi) at lasing wavelength, making Yb(3+)-phosphate glass unsuccessful to achieve laser output, whereas 1.166W cw laser was obtained in Yb(3+)-fluorophosphate (FP) glass with broader Stark splitting. Analysis on laser system levels reveals that under room temperature, Yb(3+) laser is quasi-3.13-level in phosphate glass and quasi-3.36-level in FP glass. These demonstrations suggest that unless the Stark splitting is enlarged, conventional Yb(3+)-phosphate glass is not a good gain medium for bulk Yb(3+)-laser.

Development of Nd3+-doped Monoclinic Dimolybdates La2Mo2O9 as Optical Materials

The work presented is mainly focused on synthesis and study of structural and optical properties of microcrystalline Nd3+-doped monoclinic dilanthanum dimolybdate at both room and cyrogenic temperatures (4K and 77 K). These compounds might be useful for application in the future as optical materials and also as transparent ceramics when the structure is cubic. The Nd3+-doped phases with monoclinic structure (α-form, space group P21, unit cell parameters a = 7:1426, b = 7:1544, c = 7:1618 Å and β = 89:538°) were observed for a concentration of the optically active ions equal to 5%. When the concentration of the Nd3+ ions is higher than 15%, a cubic structure is formed (β-form, space group P213, with the lattice parameter a = 7:155±0:005 Å). A series of Nd3+- doped La2Mo2O9 phases with different concentration of Nd3+ were prepared using conventional solid-state reactions. The formation of phase-pure Nd3+-doped La2Mo2O9 has been monitored by powder X-ray diffraction, DSC, SEM, Raman, and FT-IR absorption techniques. High-resolution absorption and emission spectra, as well as the dynamics of the Nd3+ excited states characterized by decay time measurements were recorded from room temperature to 4 K. At least two slightly different crystallographic sites are available for the Nd3+ ions. First results show that this new Nd3+-doped monoclinic La2Mo2O9 molybdate phosphor is promising for applications of ultra-short pulse lasers. Graphical Abstract Development of Nd3+-doped Monoclinic Dimolybdates La2Mo2O9 as Optical Materials

Spectroscopic studies of lanthanide (Ce, Eu) chlorides in ethane-1,2-diol

Abstract This paper is a consequence of our earlier studies on the structure of solutions of anhydrous and hydrated lanthanide chlorides in different types of alcohols. Such investigations are important mainly in understanding the spectroscopic behavior of silica gels and glasses obtained by alcohol methods and codoped with Ce 3+ and other lanthanide ions. Our work focuses on the spectroscopy of Ce(III) and Eu(III) chlorides in ethylene glycol solutions. This alcohol can play a special role, because it has two OH groups, forming relatively strong hydrogen bonding. Moreover, it can coordinate simultaneously to two metal ions. Absorption, emission and emission excitation spectra were measured at 293 and 77 K and are discussed in terms of CF and Ln(III)–solvent interactions. The role of CT state of Eu(III) in description of the composition of solvates is presented. Low temperature europium spectra were applied as spectroscopic probes of solution structure, the number of species existing in it and their symmetries. Basing on these results, the best conditions for gel and glass formation were found. Adiabatic compressibility data make it possible to discuss the role of lanthanide ions as structure-breakers in solution and evaluation of the number of solvent molecules involved in the solvation of these ions.

Structural and spectroscopic properties of Yb³⁺-doped MgAl₂O₄ nanocrystalline spinel.

Magnesium spinel (MgAl2O4) powders doped with Yb(3+) ions have been synthesized by a sol-gel method and heat-treated in the range of 700-1000 °C for 3 h. XRD patterns indicated that the powders have a cubic structure with high crystallite dispersion. Nanoparticles in the range of 10-30 nm are obtained as a function of the dopant concentration and sintering temperature. The main Yb(3+) zero-phonon line is located at 976 nm. The spectroscopic properties of the Yb(3+) ions are characterized by broad absorption spectroscopy and emission spectroscopy. Even at low temperature, the spectra reveal a strong distorted spinel lattice due to the high inversion rate between Mg(2+) tetrahedral sites and Al(3+) octahedral sites. The substitution of Mg(2+) ions by Yb(3+) ions favors the creation of Yb(3+) ion pairs which are observed in the cooperative luminescence spectra at around 500 nm. The luminescence decays are influenced by both the Yb(3+) content, the energy transfer between ions and by the presence of pairs and aggregates. Detailed analysis of the observed structural and spectroscopic measurements has been described in this manuscript.

Violet-green excitation for NIR luminescence of Yb 3+ ions in Bi 2 O 3 -B 2 O 3 -SiO 2 -Ga 2 O 3 glasses

60Bi(2)O(3)-20B(2)O(3)-10SiO(2)-10Ga(2)O(3) glasses doped with 1-9 mol% Yb(2)O(3) were prepared and investigated mainly on their violet-green excitation for the typical NIR emission of Yb(3+), generally excited in the NIR. Two violet excitation bands at 365 nm and 405 nm are related to Yb(2+) and Bi(3+). 465 nm excitation band and 480 nm absorption band in the blue-green are assigned to Bi(0) metal nanoparticles/grains. Yb-content-dependence of the excitation and absorption means that Bi(0) is the reduced product of Bi(3+), but greatly competed by the redox reaction of Yb(2+) ↔ Yb(3+). It is proved that the violet-green excitations result in the NIR emission of Yb(3+). On the energy transfer, the virtual level of Yb(3+)-Yb(3+) as well as Bi(0) dimers probably plays an important role. An effective and controllable way is suggested to achieve nano-optical applications by Bi(0) metal nanoparticles/grains and Yb(3+).