IR spectroscopy of rare-earth aluminum borates RAl3(BO3)4 (R = Y, Pr-Yb)

Rare-earth aluminum borates RAl3(BO3)4 (R = Y, Nd-Yb) obtained by spontaneous high-temperature flux crystallization form two polytype modifications described by the space groups R32 (D37) and C2/c (C2h6). They differ in the symmetry versions in mutual arrangement of layers. These borates have been investigated by mid- and far-IR spectroscopy in combination with factor-group analysis of the vibrations of BO33− ions; translational motions of Al3+, R3+ (Y, Nd-Yb), and BO33− ions; and BO33− rotations. Rare-earth aluminum borates are assigned to different space groups according to their IR spectra. Borates with large rare-earth Nd and Pr cations are crystallized into the space group C2/c (C2h6), while borates with smaller (Y, Sm-Yb) ions are crystallized into the space group R32 (D37). The rhombohedral structure of the latter compounds includes monoclinically ordered domains, as is evidenced by the presence of monoclinic-phase bands in the IR spectrum. NdAl3(BO3)4 and SmAl3(BO3)4 can form both monoclinic and rhombohedral polytypes. The IR spectrum of the monoclinic SmAl3(BO3)4 phase contains bands due to the rhombohedral phase, while the IR spectrum of NdAl3(BO3)4 (which is described by the space group R32) contains bands due to the monoclinic polytype. The presence of domains with another arrangement of layers in the polytype structure is a characteristic sign that corresponds to the order-disorder theory; this theory explains the structure of polytypes.