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Lanthanide-doped β-Ca2SiO4 has been regarded as a promising material for use in light-emitting diode (LED) phosphors. Nevertheless, the performance of these luminescent materials activated by lanthanide ions depends strongly on the host matrix. The challenge lies in controlling the phase under ambient conditions in a material that exists in five polymorphic forms. Herein, we report the synthesis of β-Ca2SiO4 containing Ce and Eu ions via the Sol–Gel method. The pure and doped samples were characterized using thermogravimetric (TG) and differential thermal (DT) analysis, X-ray diffraction (XRD) and Rietveld–XRD analysis, scanning electron microscopy (SEM), X-ray absorption near edge structure (XANES), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and defect calculations. XRD analyses show the predominant formation of the β-phase. Morphological analyses by SEM show smaller particle sizes in Ce- and Eu-doped β-Ca2SiO4 than pure samples. This result suggests that Ce3+ is more efficient at stabilizing the β-Ca2SiO4 polymorph. The PL emission of the β-Ca2SiO4:Eu, excited at 393 nm, provides evidence about the existence of two nonequivalent sites, with one of them evolving to a high symmetry site and the other one remaining in a low symmetry. PL results are corroborated by defect calculations in which it is evident that both Eu3+ and Ce3+ ions prefer to be incorporated into the two Ca2 sites compensated by a Ca1 vacancy, including a probable change in the coordination number of the Eu3+ ion. The abrupt distortion of one of the Eu3+ sites is accompanied by the high 5D0 → 7F4 transition. Also, we present XANES and XPS analyses combined with defect calculations for Eu2+ and Ce4+ ions to discuss the likely coexistence of valency states in the host matrix. Lastly, the results offer insights into improving the performance of phosphor-converted white LEDs. [ABSTRACT FROM AUTHOR]

In this work, Gd-filled skutterudite GdxCo4Sb12 was prepared using one step method under high pressure in a piston-cylinder-based press at 3.5 GPa and moderate temperature of 800 °C. A detailed structural characterization was performed using synchrotron X-ray diffraction (SXRD), revealing a filling fraction of x = 0.033(2) and an average bond length of 3.3499(3) Å. The lattice thermal expansion accessed via temperature-dependent SXRD led to a precise determination of a Debye temperature of 322(3) K, from the fitting of the unit-cell volume expansion using the second order Grüneisen approximation. This parameter, when evaluated through the mean square displacements of Co and Sb, displayed a value of 265(2) K, meaning that the application of the harmonic Debye theory underestimates the Debye temperature in skutterudites. Regarding the Gd atom, its intrinsic disorder value was ~5× and ~25× higher than those of the Co and Sb, respectively, denoting that Gd has a strong rattling behavior with an Einstein temperature of θ E = 67(2) K. As a result, an ultra-low thermal conductivity of 0.89 W/m·K at 773 K was obtained, leading to a thermoelectric efficiency zT of 0.5 at 673 K. [ABSTRACT FROM AUTHOR]