Symmetry dependent evolution of the Tb3+ photoluminescence in Ba6(RE1−xTbx)9B79O138 (RE = La and Y, 0 ≤ x ≤ 1) and Ce3+-to-Tb3+ sensitization effect: A re-visit to REBaB9O16

Abstract

Extensive studies on photoluminescence (PL) were devoted to the well-known “REBaB9O16” family, while the structure remained unsolved until the recent work on the structure determination of its new analogue Ba6Bi9B79O138. Accordingly, a re-visit to “REBaB9O16” with the new formula was rationally performed by the syntheses and Tb3+-PL study on Ba6La9B79O138 (BLBO) and Ba6Y9B79O138 (BYBO). Powder XRD patterns of Ba6(RE1−xTbx)9B79O138 (RE = La and Y, 0 ≤ x ≤ 1) show a linear shrinkage or expansion of the unit cell in BLBO: xTb3+ and BYBO: xTb3+, respectively. The ratio of Tb3+ cyan/green (C/G) emission was proposed to evaluate the evolution principle of the RE-site symmetry in both series of phosphors. The C/G value remains unchanged in BLBO: xTb3+, while it is sensitive to the Tb3+-to-Y3+ substitution in BYBO: xTb3+. For example, C/G keeps almost constant (∼0.30) when 0.10 ≤ x ≤ 0.5; a dramatic increase of C/G (from 0.33 to 0.67) occurs in the range of 0.50 ≤ x ≤ 0.90, accompanied with a RE-site symmetry transition from centrosymmetric to noncentrosymmetric; the value of C/G remains unchanged thereafter. The Ce3+-to-Tb3+ sensitization effect is applied in Ba6(RE0.3−yCeyTb0.7)9B79O138 (RE = La or Y, 0 ≤ y ≤ 0.3), and the samples with y = 0.2 possess the strongest green emission.