Orange emission from (Ba_1-XSr_X)₄Al ₂S₇:Eu²⁺ thioaluminate phosphors with visible light excitation

Solid solutions of (Ba_1-XSr_X)₄Al ₂S₇ (X = 0.2 to 0.6) novel thioaluminate compound were synthesized, whose crystal structure was determined. The crystal structure of the (Ba_1-XSr_X)₄Al₂S₇ solid solution is related to that of monoclinic Ba₄Ga ₂S₇ consisting of isolated Ga₂S₇ units. However, the local structures around Ba/Sr in (Ba_1-XSr _X)₄Al₂S₇, such as coordination number and one unusually long bond (d(Ba,Sr)-S 4 Å) in 7- and 8-coordinated (Ba,Sr)-S, are different from those in Ba₄Ga ₂S₇. Eu²⁺-activated (Ba_1-XSr _X)₄Al₂S₇ exhibited orange emission with a peak maximum at 594 nm and a corresponding excitation band at 300 to 550 nm. This is the first report of an orange phosphor in the thioaluminate system. The thermal quenching of luminescence in the (Ba_1-XSr _X)₄Al₂S₇:Eu²⁺ phosphor was comparable to that in a conventional Y₃Al₅O ₁₂:Ce³⁺ phosphor.