Characterization of the simple cubic multivariate exponential families

AbstractThe Letac–Mora class of real cubic natural exponential families has been characterized by a property of 2-orthogonality of an associated sequence of polynomials (see [G. Letac, M. Mora, Natural real exponential families with cubic variance functions, Ann. Statist. 18 (1990) 1–37; A. Hassairi, M. Zarai, Characterization of the cubic exponential families by orthogonality of polynomials, Ann. Probab. 32 (2004) 2463–2476]). The present paper introduces a notion of transorthogonality for a sequence of polynomial on Rd to extend the characterization to the multivariate version of the Letac–Mora class of real natural exponential families

On the projections of the Dirichlet probability distribution on symmetric cones

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Tunable luminescence and energy transfer properties in YPO4:Tb3+, Eu3+/Tb3+phosphors

Green emitting YPO4:Tb3+and orange-red emitting YPO4:Tb3+, Eu3+phosphors were prepared by conventional solid-state reaction and characterized in order to develop materials for applications in white light-emitting diodes (w-LED). First, the doping level of singly doped YPO4:Tb3+was systematically increased in order to obtain the highest green-to-blue emission intensity ratio through the quenching of the5D3emission, by maintaining an acceptable degree of total luminescence. The best emission performance, in terms of color purity and intensity, was obtained with a 5 mol% concentration. Starting from this result, we have synthesized and measured the emission properties of the co-doped phosphors and investigated the Tb3+to Eu3+energy transfer (ET) mechanism. The possibility of tuning the color of the luminescence by properly adjusting Eu3+concentration was finally discussed

Tuning white upconversion emission in GdPO 4 :Er/Yb/Tm phosphors

International audienceNanoscale phosphors of Yb3+/Er3+/Tm3+-doped GdPO4 with different morphologies were synthesized using the solvothermal method with annealing at 750 °C and 1000 °C. A high concentration of Yb3+(35%mol) ions was successfully doped in the powders. Crystal structures were characterized through powder XRD measurement and Fourier transform infrared spectroscopy. SEM images have shown that the morphologies of the samples can be controlled by adjusting the pH value during their synthesis. Upconversion luminescence was investigated under 980 nm diode laser excitation. The upconversion processes were assessed on the basis of the power dependence of upconversion emission intensity. The light colour was investigated with respect to the 1931 CIE diagram. The samples show good emission tunability by monitoring the annealing temperature and the diode laser pump intensity. By properly adjusting the excitation power, white upconversion luminescence can be attained. Yb3+/Er3+/Tm3+-doped GdPO4 nanophosphors appear, which have potential applications in the field of upconversion-based white lighting devices

Synthesis, characterization and optical spectroscopy of GdPO4:Er3+

A series of Er3+-doped GdPO4 phosphors was synthesized using a conventional solid-state reaction. The monazite structure (space group P121/n1) of the obtained materials was confirmed using X-ray diffraction and Fourier transform infrared spectroscopy. Their optical spectra (excitation, emission, absorption, decay curves) were measured at room temperature in the visible and near-infrared (NIR) regions. The UV–visible–NIR optical absorption spectrum of GdPO4:7% Er3+ was analyzed based on Judd–Ofelt (J–O) theory and the J–O intensity parameter (Ω2, Ω4, Ω6) was calculated. J–O intensity parameters were used to evaluate spontaneous emission properties such as branching ratios, transition probabilities, and radiative lifetime. The calculated quantum efficiency of the 1.5 μm emission (4I13/2−4I15/2) was calculated to be 89%. This result proved that GdPO4:Er3+ is suitable for use in optical amplifiers and is a potential host for laser applications. The most interesting transitions, located at about 540 nm, and 1.0 and 1.5 μm were investigated as a function of doping level and of temperature, to assess the conditions needed for the highest emission performance and to explore the range of application, in particular in the fields of lighting, thermal sensing, and of phosphors for bio-imaging