High‐Pressure Synthesis and Crystal Structure of the Structurally New Orthorhombic Rare‐Earth Meta ‐Oxoborates γ‐ RE (BO 2 ) 3 ( RE = La ‐ Nd)

The new meta ‐oxoborates γ‐ RE (BO 2 ) 3 ( RE = La ‐ Nd) (≡ RE B 3 O 6 ) were obtained as colourless (La, Ce), light green (Pr) and pale violet (Nd) crystalline powders by the reaction of CeO 2 , Pr 6 O 11 , or RE 2 O 3 ( RE = La, Nd) and B 2 O 3 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(BO 2 ) 3 revealed: Pca 2 1 , a = 1845.7(2), b = 446.8(2), c = 429.5(2) pm, Z = 4, R 1 = 0.0265, wR 2 = 0.0451 (all data). For the isotypic compounds γ‐ RE (BO 2 ) 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 (BO 2 ) 3 in addition to the well characterized monoclinic ( RE = La ‐ Nd, Sm ‐ Lu; C 2/ 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 (BO 2 ) 3 (space group: C 2/ c ), the orthorhombic phases as β‐ RE (BO 2 ) 3 (space group: Pnma ), and the here presented orthorhombic phases as γ‐ RE (BO 2 ) 3 . Furthermore, we report about temperature‐resolved in‐situ powder‐diffraction measurements, and IR‐spectroscopic investigations into γ‐Ce(BO 2 ) 3 .