Double scheelite-type oxides of the form A + A ′ 3+ (WO 4 ) 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 ′(WO 4 ) 2 ( A + = Li, Na, K; A ′ 3+ = La, Lu, Bi) series has been resolved. The Na- and La-containing oxides display an average tetragonal I 4 1 / 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 6 s 2 lone pair activity, with symmetry-lowering required to model the bonding requirements of each ( A / A ′)O 8 polyhedron. These requirements are better captured by the partial cation ordering and displacements allowed by a monoclinic I 2 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 LuO 8 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.
Mullens et al. (Tue,) studied this question.