Infrared spectroscopy and the structure of polytypic modifications of RM3(BO3)4 borates (R - Nd, Gd, Y; M - Al, Ga, Cr, Fe)

By means of IR spectroscopy with the use of factor group analysis for vibrations of the B-O bond, rare-earth borates with a general formula of RM3(BO3)(4) (R is Nd, Gd, and Y; M is Al, Ga, Cr, and Fe) and related polytypic structures are assigned to space groups R32 or 2/c. Compounds with both almost homogeneous structures and with inclusions of the monoclinic polytype are revealed among rhombohedral borates. The latter is most typical of phases with a small octahedral Al cation. It is shown that in the monoclinic modification of NdAl and NdCr borates fragments of the rhombohedral phase are always present. Alternation of differently ordered structural fragments is determined by temperature conditions of crystallization.