Structure and luminescence analyses of simultaneously synthesised (Lu1−xGdx)2O2S:Tb3+ and (Lu1−xGdx)2O3:Tb3+

Abstract

Herein we describe the synthesis and luminescence of nanosized (Lu_1−y–xGd_x)₂O₂S:Tb_y and (Lu_1−y–xGd_x)₂O₃:Tb_y phosphors with y = 0.1 mol% Tb³⁺ and y = 2 mol% Tb³⁺ and x ranging between 0 and 1. The concentration of Gd³⁺ (x) was varied in steps of 0.1 (molar ratio Gd³⁺). The samples at 0.1 < x < 0.7 contained a mixture of (Lu_1−xGd_x)₂O₃:Tb³⁺ and (Lu_1−xGd_x)₂O₂S:Tb³⁺, while the samples at x = 0 contained only Lu₂O₃:Tb³⁺. At 0.1 < x < 0.7 Lu₂O₂S:Tb³⁺ and Gd₂O₂S:Tb³⁺ did not form a solid solution, but rather crystallised into two slightly different hexagonal structures. This behaviour has been explained in terms of segregation of Lu and Gd between the oxide and oxysulfide phases: the oxide phase is more Lu-rich whereas the second oxysulfide phase is more Gd-rich. The photoluminescence spectra of the phosphors with 0.1 mol% Tb³⁺ showed a modest colour change of the fluorescence light from cyan to green when x was increased from 0 to 1, whereas the samples of the series with 2 mol% Tb³⁺ yielded essentially green light. From this analysis it was concluded that the colour change of (Lu_1−xGd_x)₂O₂S:0.1%Tb³⁺ is caused by increasing energy transfer of the ⁵D₃-level of Tb³⁺ to the charge transfer band of (Lu_1−xGd_x)₂O₂S:Tb³⁺ upon increasing x. Since the samples with 100% Lu consisted of pure cubic Lu₂O₃:Tb³⁺, we had the opportunity to also study the symmetry-related PL of this compound. From this study we concluded that the C₂–C_3i doublet of the Tb^{3+ 5}D₄ → ⁷F₅ transition behaves in the same way as the corresponding doublet in cubic Y₂O₃:Tb³⁺.