Vibrational spectra and factor group analysis of rare-earth chromium borates, RCr3(BO3)4, with R = La–Ho

Anhydrous orthoborates RM3(BO3)4, where R = Y, La–Lu, M = Al, Ga, Cr, Fe, with huntite structure type are considered as multifunctional laser materials. The crystal structure of these borates is either rhombohedral with space group R32 (D37) (Z = 3) or monoclinic with space group C2/c (C2h6) (Z = 4) depending on the growth conditions. Both modifications have very close polytypic structures, and it is difficult to identify them by powder diffraction data. In this context, double borates of rare-earth cations and Cr3+ have been grown from high-temperature solutions and are characterized by Raman and infrared spectroscopy in a crystalline state in combination with factor group analysis of vibrational modes. The assignment for the stretching and bending vibrations of BO33− groups and external modes has been made. Some external modes have been identified by study of mass effect (Al–Cr, La–Ho). Comparison of the Raman spectra of these borates shows redistribution of band intensities of two spectral modifications, related to different symmetry groups. As predicted by factor group analysis, the number of IR-active vibrational modes of stretching and bending vibrations of BO33− units significantly increases in infrared spectra of monoclinic borates in comparison with rhombohedral ones. The dependence of the realized borate space group on the crystal growth conditions and the sort of rare-earth atom was revealed. Both GdCr3(BO3)4 and EuCr3(BO3)4 borates crystallize in space group R32 irrespective of growth conditions. The borates with the large rare-earth elements La–Nd always form the monoclinic structures, irrespective of crystallization temperature. The borates SmCr3(BO3)4, TbCr3(BO3)4 and DyCr3(BO3)4 have been obtained in two modifications in dependence of crystalline borate substance/solvent ratio and related temperature of crystallization.