Infrared spectroscopy and the structure of rare-earth chromium borates RCr3(BO3)4 (R = La-Er)

Rare-earth chromium borates RCr₃(BO₃)₄, where R = La-Er, obtained as a result of spontaneous solutionmelt crystallization and belonging to two polytypic modifications (R32 (D₃⁷) and C2/c (C _2h ⁶) space groups) are studied by infrared spectroscopy with the factor group analysis of vibrations. Borates with R = La-Nd are shown to crystallize in the monoclinic C2/c space group. Borates with R = Sm-Er in the 1:1 ratio of batch and solvent form rhombohedral (R32 space group) and monoclinic (C2/c space group) phases at the 2.3:1 ratio, except EuCr₃(BO₃)₄ and GdCr₃(BO₃)₄ that have the rhombohedral structure under all crystallization conditions. Both rhombohedral and monoclinic polytypes may contain oppositely ordered layers. Y-, Tm-, Yb-, Lu-chromium borates are not formed.