Refined structural studies on the fluorite-related polymorphs of sol-gel undoped and Eu³⁺-doped yttrium tantalates

Compounds with the general formula RE₃MO₇ (RE = rare earth ions; M = Ta, Nb, Sb, Ru, Ir, Os, Re, etc.), crystallize as a fluorite-related structure, forming polymorphs with different space groups. The space group strongly depends on the RE³⁺ and M⁵⁺ ionic radii and processing conditions. Structural characterization is well-established for the lanthanide series, but literature studies have divergent views about how to attribute yttrium tantalate (Y₃TaO₇) space groups-some authors have described the Y₃TaO₇ structure as orthorhombic and belonging to space group C222₁ or Ccmm, whereas others have assigned a cubic Fm3̄m structure to it. Here, we have characterized the structure of undoped and Eu³⁺-doped Y₃TaO₇ (0.1 to 50 mol% of Eu³⁺) samples synthesized by the sol-gel method that crystallized as a cubic disordered fluorite-type structure, space group Fm3̄m. Their powder X-ray diffraction measurements, Rietveld analyzes and Raman spectra were used as a conclusive technique of the structural properties. We have also investigated whether a secondary phase (M'-YTaO₄) emerged in the samples and compared the phase composition of each sample to their Raman spectra. Low-temperature photoluminescence measurements (∼15 K) using Eu³⁺ as a structural probe helped us analyze the inhomogeneous broadening observed in the emission spectra. These measurements can be used as an important tool to attribute the crystalline phases of rare earth tantalates and niobates.