Host‐Sensitized Photoluminescence and Coordination Environment Evolution in Ba 6 (Bi 1– x Tb x ) 9 B 79 O 138 (0 ≤ x ≤ 1)

Borates are well‐known candidates for optical materials and, here, we are particularly interested in Ba 6 Bi 9 B 79 O 138 (BBBO), which has an interesting layered‐type structure and Bi 3+ ‐containing characteristics that has prompt us to explore its host‐sensitized photoluminescence (PL). Solid solutions of BBBO: x Tb 3+ (0 ≤ x ≤ 1) were prepared and its host‐sensitized photoluminescence was confirmed, which is a result of the efficient Bi 3+ ‐to‐Tb 3+ energy transfer. Furthermore, an evolution of the coordination environments of the Bi 3+ /Tb 3+ sites was observed when increasing x . This evolution was found to correlate to the variation of the PL spectra. In particular, the value of the ratio of cyan/green [defined as I ( 5 D 4 → 7 F 6 )/ I ( 5 D 4 → 7 F 5 ) of Tb 3+ ] was about 0.28 when x < 0.4, and it increased to 0.71 at x = 0.9 and thereafter remained almost constant. Ba 6 (RE 0.3– y Bi y Tb 0.7 ) 9 B 79 O 138 (RE = La or Y, 0 ≤ y ≤ 0.3) were also prepared for better understanding of the host‐sensitization effect and in both series, Ba 6 (RE 0.1 Bi 0.2 Tb 0.7 ) 9 B 79 O 138 show the highest emission.