Synthesis, structural and luminescence properties of (La1−xLnx)2Ti2O7 (Ln=lanthanides) solid solutions

• Synthesis and structural characterizations of (La1−xLnx)2Ti2O7 solid solutions.
• Determination of optical band-gap of Ln2Ti2O7.
• Luminescence properties of (La1−xErx)2Ti2O7, (La1−xEux)2Ti2O7, (La1−xTbx)2Ti2O7 and (La1−xTmx)2Ti2O7 oxides.
• Thermal dependence and concentration quenching of (La1−xEux)2Ti2O7.
• Measurements of quantum efficiencies and lifetimes.

The stability limit between the layered-perovskite/monoclinic and pyrochlore/cubic structures was investigated for the entire series of (La1−xLnx)2Ti2O7 with Ln = Pr to Lu prepared by solid-state reaction. Complete solid solutions were obtained for Ln = Pr or Nd. However, when the ionic radius of Ln3+ decreases (either, Sm3+ toward Lu3+), only partial solid solutions are achieved. The substitution rate decreases drastically when the ionic radius decreases. The energy bands gap were also determined using UV-visible diffuse reflectance spectroscopy. La2Ti2O7 acts as an excellent host matrix, in particular for the substitution of lanthanide ions, thus leading the creation of new emitting phosphors with a wide variety of colours. The luminescence properties of the (La1−xErx)2Ti2O7, (La1−xEux)2Ti2O7, (La1−xTbx)2Ti2O7 and (La1−xTmx)2Ti2O7 oxides have been especially described in this work. Among these compounds, (La0.6Eu0.4)2Ti2O7 presents the best quantum efficiency (42% under an excitation wavelength set at 393.5 nm) as well as good thermal stability which is suitable for use with near-UV LED chips. Finally, the CIE-1931 Chromaticity coordinates were also calculated for all phosphors.

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