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–xCl_x (A = K, Rb, Cs) and Eu²⁺-doped strontium borohydride chloride Sr(BH₄)_2–xCl_x have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH₄)_2–xCl_x (λ_em= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH₄)_3–xCl_x (A = K, Rb, Cs) (λ_em = 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–xCl_x (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.