Tunable Structural Disorder for Emission Band Broadening in Double Scheelite-Type Oxide Solid-State Lasing Materials

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Abstract Double scheelite-type oxides of the form A+A′3+(WO4)2 are promising hosts for rare-earth-doped solid-state lasing materials. Using synchrotron X-ray and neutron powder diffraction with total scattering and pair distribution function analysis, the interplay between long-range symmetry and short-range distortions across the AA′(WO4)2 (A+ = Li, Na, K; A′3+ = La, Lu, Bi) series has been resolved. The Na- and La-containing oxides display an average tetragonal I41/a scheelite-type structure with disorder across the two A-site cations. Pronounced local-scale deviations arise from differences in A-site cation size, polarizability, and the presence of 6s2 lone pair activity, with symmetry-lowering required to model the bonding requirements of each (A/A′)O8 polyhedron. These requirements are better captured by the partial cation ordering and displacements allowed by a monoclinic I2 small-box model. When A+ = Li or K, the Lu-containing analogues order across the A site and display long-range symmetry lowering to nonscheelite monoclinic structures, consistent with the less polarizable LuO8 polyhedra. Reverse Monte Carlo modeling reveals an interplay between cation size and the polarizability of A-site cations, explaining and establishing structural design principles for tuning local environments and optimizing emission bandwidths in scheelite-based solid-state laser hosts.

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