Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO₄ by Hydrothermal Conditions

Doping chemistry has become one of the most effective means of tuning materials’ properties for diverse applications. In particular for scheelite-type CaWO₄, high-oxidation-state doping is extremely important, since one may expand the scheelite family and further create prospective candidates for novel applications and/or useful spectral signatures for nuclear forensics. However, the chemistry associated with high-valence doping in scheelite-type CaWO₄ is far from understanding. In this work, a series of scheelite-based materials (Ca_{1–x–y–z}Eu_xK_y□_z)­WO₄ (□ represents the cation vacancy of the Ca²⁺ site) were synthesized by hydrothermal conditions and solid-state methods and comparatively studied. For the bulk prepared by the solid-state method, occupation of high-oxidation-state Eu³⁺ at the Ca²⁺ sites of CaWO₄ is followed by doping of the low-oxidation-state K⁺ at a nearly equivalent molar amount. The Eu³⁺ local symmetry is thus varied from the original S₄ point group symmetry to C_2v point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu³⁺ was incorporated in CaWO₄ at two distinct sites, and its amount is higher than that of the low-oxidation-state K⁺ even though KOH was used as a mineralizer, creating a certain amount of cation vacancies. Consequently, an apparent split emission of ⁵D₀ → ⁷F₀ was first demonstrated for (Ca_{1–x–y–z}Eu_xK_y□_z)­WO₄. The doping chemistry of high oxidation states uncovered in this work not only provides an explanation for the commonly observed spectral changes in rare-earth-ion-modified scheelite structures, but also points out an advanced direction that can guide the design and synthesis of novel functional oxides by solution chemistry routes.