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.

中文翻译：

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通过水热条件了解白钨矿 CaWO4 中高氧化态的掺杂化学

掺杂化学已成为为各种应用调整材料性能的最有效手段之一。特别是对于白钨矿型 CaWO ₄，高氧化态掺杂极其重要，因为人们可以扩展白钨矿家族并进一步为核法医学的新应用和/或有用的光谱特征创造潜在的候选物。然而，与白钨矿型CaWO _{4 中}的高价掺杂相关的化学还远未得到理解。在这项工作中，一系列白钨基材料 (Ca _{1– x – y – z} Eu _x K _y □ _z )WO ₄ (□ 代表 Ca²⁺位点）通过水热条件和固态方法合成并进行了比较研究。对于通过固态方法制备的块体，在CaWO ₄的 Ca ²⁺位点占据高氧化态 Eu ³⁺之后，以几乎等量的摩尔量掺杂低氧化态 K ⁺。因此，Eu ³⁺局部对称性从原始的S ₄点群对称性变为C _{2 v}点群对称性。令人惊讶的是，与批量情况不同，对于通过水热条件制备的纳米级对应物，高氧化态 Eu ³⁺在两个不同的位点掺入 CaWO ₄中，即使使用 KOH 作为矿化剂，其量也高于低氧化态 K ^{+ 的}量，从而产生一定量的阳离子空位。因此，(Ca _{1– x – y – z} Eu _x K _y □ _z )WO ₄首次证明了⁵ D ₀ → ⁷ F ₀的明显分裂发射. 这项工作中发现的高氧化态掺杂化学不仅为稀土离子修饰白钨矿结构中常见的光谱变化提供了解释，而且指出了可以指导新型功能的设计和合成的先进方向。通过溶液化学路线的氧化物。