Structure-Dependent Accessibility of Phonon-Coupled Radiative Relaxation Pathways Probed by X-ray-Excited Optical Luminescence

Abstract

Rare-earth scheelites represent a diverse family of compounds with multiple degrees of freedom, which enables the incorporation of a wide range of lanthanide color centers. Precise positioning of quantum objects is attainable by the choice of alkali cations and lattice connectivity of polyanion units. Herein, we report the structure-dependent energy transfer and lattice coupling of optical transitions in La 3+ -and Dy 3+ -containing scheelite-type double and quadruple molybdates NaLa 1−x Dy x (MoO 4 ) 2 and Na 5 La 1−x Dy x (MoO 4 ) 4 . X-ray excitation of La 3+ core states generates excited-state electron−hole pairs, which, upon thermalizing across interconnected REO 8 polyhedra in double molybdates, activate a phononcoupled excited state of Dy 3+ . A pronounced luminescence band is observed corresponding to optical cooling of the lattice upon preferential radiative relaxation from a "hot" state. In contrast, combined X-ray absorption near-edge structure and X-ray-excited optical luminescence studies reveal that such a lattice coupling mechanism is inaccessible in quadruple molybdates with a greater separation of La 3+ −Dy 3+ centers.