In thiogallate phosphor, SrGa₂S₄:Eu²⁺, Eu²⁺ substitutes for Sr²⁺ and is surrounded by a square antiprism of eight sulfide ions.

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The lowest-energy 5f7 4f65d excitation is centered at ~480 nm, and the relatively narrow (full width at half maximum = 50 nm at 300 K) 4f5d4f7 emission at 540 nm, which makes this phosphor a potentially useful green emitter in phosphor-converted tricolor LEDs. (a) What is the magnitude of the Stokes shift in eV? (b) The binary rock- salt sulfide SrS:Eu+, with octahedrally coordinated Sr+ sites, emits in the red with a maximum at ~620 nm. What factor(s) is responsible for the longer- wavelength emission in SrS:Eu+?