Crystal Structures and Photoluminescence across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>–Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> System

Abstract

It is well-known that when an RE<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho<sup>3+</sup>-substituted La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>. Several compositions across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>–Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> system were synthesized using the sol–gel method followed by calcination at 1600 °C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> which extends to the La<sub>0.6</sub>Ho<sub>1.4</sub>Si<sub>2</sub>O<sub>7</sub> composition. Compositions richer in Ho<sup>3+</sup> show a two-phase domain (G+δ), while δ-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> is the stable phase for Ho<sup>3+</sup> contents higher than 90% (La<sub>0.2</sub>Ho<sub>1.8</sub>Si<sub>2</sub>O<sub>7</sub>). Anomalous diffraction data interestingly indicated that the La<sup>3+</sup> for Ho<sup>3+</sup> substitution mechanism in the G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> polymorph is not homogeneous, but a preferential occupation of Ho<sup>3+</sup> for the RE2 site is observed. The Ho<sup>3+</sup>-doped G-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho<sup>3+</sup> content of 10%

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