The union of structural and spectroscopic modeling can accelerate the discovery and improvement of phosphor materials if guided by an appropriate principle. Herein, we describe the concept of “chemical unit cosubstitution” as one such potential design scheme. We corroborate this strategy experimentally and computationally by applying it to the Ca2(Al1–xMgx)(Al1–xSi1+x)O7:Eu2+ solid solution phosphor. The cosubstitution is shown to be restricted to tetrahedral sites, which enables the tuning of luminescent properties. The emission peaks shift from 513 to 538 nm with a decreasing Stokes shift, which has been simulated by a crystal-field model. The correlation between the 5d crystal-field splitting of Eu2+ ions and the local geometry structure of the substituted sites is also revealed. Moreover, an energy decrease of the electron–phonon coupling effect is explained on the basis of the configurational coordinate model.
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