Solid solutions of (Ba 1-X Sr X ) 4 Al 2 S 7 (X = 0.2 to 0.6) novel thioaluminate compound were synthesized, whose crystal structure was determined. The crystal structure of the (Ba 1-X Sr X ) 4 Al 2 S 7 solid solution is related to that of monoclinic Ba 4 Ga 2 S 7 consisting of isolated Ga 2 S 7 units. However, the local structures around Ba/Sr in (Ba 1-X Sr X ) 4 Al 2 S 7 , 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 Ba 4 Ga 2 S 7 . Eu 2+ -activated (Ba 1-X Sr X ) 4 Al 2 S 7 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-X Sr X ) 4 Al 2 S 7 :Eu 2+ phosphor was comparable to that in a conventional Y 3 Al 5 O 12 :Ce 3+ phosphor.In recent years, the technology and development for energy conservation and low environmental impact have become critical challenges to realize a sustainable society. In contrast to conventional incandescent or fluorescent lights, white light emitting diodes (LEDs) are recognized as a next-generation light source with advantages such as low power consumption, long life, and no use of toxic mercury, and are therefore applied in lighting and back-lighting for displays and signs. 1 White LEDs consist of a blue or near-ultraviolet LED chip and a phosphor, and produce artificial white light from the mixed colors of luminescence from the LED chip and the phosphor. 1 Therefore, the spectral characteristics of the white LED are strongly dependent on the emission characteristics of the phosphor. To realize improved efficiency and various color temperatures of white LEDs, the development of highly efficient phosphors that can be excited by blue to near-ultraviolet lights is required.Phosphors that exploit the 4f-5d transition of Eu 2+ have been widely studied as phosphors of blue light excitation. The 5d orbital level, which is the outermost shell of Eu 2+ , is affected by electrostatic repulsion due to the surrounding negative ions and crystal field. Therefore, the emission color of a Eu 2+ -activated phosphor changes significantly depending on the host materials. For this reason, host materials for the Eu 2+ -activated phosphor, such as oxides, 2,3 phosphates, 4 sulfides, 5,6 oxynitrides, 7 and nitrides, 8 have been reported with luminescence colors of the phosphors changing from blue to red. Paying attention to the influence of electrostatic repulsion between Eu 2+ and the surrounding anions, since the Eu 2+ -activated sulfide phosphor has larger covalency than the corresponding oxide, the electrostatic repulsion with the anions around Eu 2+ decreases, as a result of which the excitation 5d level of Eu 2+ shifts to lower energy. 9 For this reason, many Eu 2+ -activated sulfide phosphors are available, in which excitation from near-ultraviolet to blue light is possible. Therefore, Eu 2+ -activated sulfide phosphors are ...