G. Boulon

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 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.

The size effect on the energy transfer in Bi3+–Eu3+ co-doped GdVO4 nanocrystals

Nanocrystals of GdVO4 co-doped with various concentrations of Bi3+ and a constant concentration of Eu3+ ions (5 mol%) were synthesized by Pechinis method and annealed in the range of temperature from 750 °C to 900 °C in order to obtain different sizes of individual crystallites. Upon the excitation of near-UV light, an intense red/orange-red luminescence of Eu3+ was observed. Shifting of the maximum and broadening of the excitation band with increasing Bi3+ concentration were also observed. In particular, the influence of the size effect on the energy transfer in the chain: (O2−–V5+) charge transfer → (Bi3+–V5+) charge transfer → Eu3+ ions was determined and analysed.

An Approach in the Structural and Spectroscopic Analysis of Yb3+-Doped YAG Nano-ceramics by Conjugation of TEM-EDX and Optical Techniques

We show our approach in the structural and spectroscopic analysis of Yb3+-doped YAG nano-ceramics prepared by the low temperature-high pressure sintering technique (LTHP) by conjugation of both TEM-EDX and optical techniques. Pressure sintering dependences of absorption, emission and decays are analyzed and interpreted. The sample pressurized at 8 GPa for sintering is characterized by the highest transparency and confirms the Y3Al5O12 garnet structure of the grains of ∼ 21 nm average size. Yb3+ ion distribution has been analyzed by both TEM-EDX evaluation in grains and grain boundaries and spectroscopy of Yb3+ pairs of small population from the cooperative luminescence phenomenon. EDX analysis at the TEM scale provides unambiguous results on a clear tendency of almost uniform Yb3+ distribution. An important new observation has been made at 4 K and room temperature with the \(^{2}F_{7/2} \rightarrow ^{2}F_{5/2}\) 0-phonon absorption line at 975.7 nm in addition of the 0-phonon line of the YAG structure of grains at 968 nm similar to that of bulky YAG single crystals. We have discussed the origin of this new 0-phonon line relaxing only by non-radiative transitions and conclude that this line might be assigned to Yb3+ distorted sites on the grain surfaces.

Characterisations of New Nd3+-Doped Scheelite-Type Molybdates for Laser Materials

The necessity to develop new laser materials was our motivation to study a new family of molybdates activated by the Nd3+ ions, which crystallizes in the scheelite-type structure (the tetragonal symmetry, the space group I41∕a ). A series of Cd1 − 3 xNd2x_xMoO4 solid solutions with concentration of Nd3+ ions from 0.1 to 66 mol % with respect Cd2+ ions have been successfully synthesized by a high-temperature annealing, using CdMoO4 and Nd2(MoO4)3 as the starting materials. Structural and spectroscopic characterizations of Nd3+in substitution of Cd2+ were carried out. The Nd3+ ions in this matrix most probably occupy two non-equivalent symmetry sites (see Fig. 45.1). The substitution of Cd2+ by trivalent Nd3+ cations leads to the formation of cationic vacancies in a framework (which are denoted in the chemical formula as _ ) and its concentration depends essentially on the composition of initial CdMoO4/Nd2(MoO4)3 mixtures. The analysis of the morphology using SEM reveals high homogeneity of the spherical-shape products with the average grain size of about 10 μm. The optical analysis and the laser performance parameters suggest Cd1 − 3xNd2x_x MoO4 as a potential laser powder material: both the values of the absorption cross-section and the very strong emission of the4F3∕2− > 4I11∕2 laser channel of Nd3+ recorded under Xe lamp excitation or OPO laser pumping, as well as, the radiative lifetimes of IR luminescence are appropriate for potential applications of this phosphor as a solid-state laser. Our earlier studies have shown that the presence of the cationic vacancies in the framework may significantly improve the laser parameters (Guzik et al., J Mater Chem 22:14896, 2012).