High‐Pressure Synthesis and Crystal Structure of the Structurally New Orthorhombic Rare‐Earth Meta‐Oxoborates γ‐RE(BO2)3 (RE = La ‐ Nd)

The new meta‐oxoborates γ‐RE(BO2)3 (RE = La ‐ Nd) (≡ REB3O6) were obtained as colourless (La, Ce), light green (Pr) and pale violet (Nd) crystalline powders by the reaction of CeO2, Pr6O11, or RE2O3 (RE = La, Nd) and B2O3 in a Walker‐type multianvil apparatus under high‐pressure/high‐temperature conditions of 7.5 GPa and 1000 °C. The single crystal X‐ray structure determination of γ‐Ce(BO2)3 revealed: Pca21, a = 1845.7(2), b = 446.8(2), c = 429.5(2) pm, Z = 4, R1 = 0.0265, wR2 = 0.0451 (all data). For the isotypic compounds γ‐RE(BO2)3 (RE = La, Pr, Nd) the corresponding lattice parameters were obtained from powder data. Structurally, the compounds represent a new type of rare‐earth meta‐oxoborate RE(BO2)3 in addition to the well characterized monoclinic (RE = La ‐ Nd, Sm ‐ Lu; C2/c) and orthorhombic (RE = Tb ‐ Lu; Pnma) phases. Considering the increasing number of polymorphs, we introduced greek letter prefixes to the rare‐earth meta‐oxoborates, labeling the former known monoclinic phases as α‐RE(BO2)3 (space group: C2/c), the orthorhombic phases as β‐RE(BO2)3 (space group: Pnma), and the here presented orthorhombic phases as γ‐RE(BO2)3. Furthermore, we report about temperature‐resolved in‐situ powder‐diffraction measurements, and IR‐spectroscopic investigations into γ‐Ce(BO2)3.