Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO 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 CaWO, 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 CaWO is far from understanding. In this work, a series of scheelite-based materials (CaEuK□)WO (□ represents the cation vacancy of the Ca 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 Eu at the Ca sites of CaWO is followed by doping of the low-oxidation-state K at a nearly equivalent molar amount. The Eu local symmetry is thus varied from the original point group symmetry to point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu was incorporated in CaWO 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 D → F was first demonstrated for (CaEuK□)WO. 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.