One Ion, Many Facets: Efficient, Structurally and Thermally Sensitive Luminescence of Eu²⁺ in Binary and Ternary Strontium Borohydride Chlorides

The Eu2+-doped mixed alkaline metal strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) and Eu2+-doped strontium borohydride chloride Sr(BH4)2-xClx have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH4)2-xClx (λem= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) (λem = 490 nm) is already observable after short milling times. Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all ASr(BH4)3-xClx (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally induced emission band broadening can serve as a sensitive response signal for temperature sensing, and this unique behavior has, to the best of our knowledge, not been reported in any Eu2+ doped phosphor so far. Additionally, bright luminescence, high quantum efficiencies, and very low thermal quenching of these Eu2+-doped borohydrides show that such host materials could serve in solid state lighting applications.