Crystal Structures and Photoluminescence across the La₂Si₂O₇–Ho₂Si₂O₇ System

Abstract

It is well-known that when an RE₂Si₂O₇ 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 Ho³⁺-substituted La₂Si₂O₇. Several compositions across the La₂Si₂O₇–Ho₂Si₂O₇ 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)₂Si₂O₇ which extends to the La_0.6Ho_1.4Si₂O₇ composition. Compositions richer in Ho³⁺ show a two-phase domain (G+δ), while δ-(La,Ho)₂Si₂O₇ is the stable phase for Ho³⁺ contents higher than 90% (La_0.2Ho_1.8Si₂O₇). Anomalous diffraction data interestingly indicated that the La³⁺ for Ho³⁺ substitution mechanism in the G-(La,Ho)₂Si₂O₇ polymorph is not homogeneous, but a preferential occupation of Ho³⁺ for the RE2 site is observed. The Ho³⁺-doped G-La₂Si₂O₇ phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho³⁺ content of 10%