RAMAN-ACTIVE PHONONS OF RFE3(BO3)(4), R=LA OR ND, SINGLE-CRYSTALS

Oriented single crystals of RFe3(BO3)(4), with R = La or Nd, have been studied by Raman spectroscopy. Spectra with the relevant polarization configurations have been recorded in order to obtain the symmetry of the observed phonons. The factor group analysis and the correlation wi th the free (BO3)(3-) ion are done in order to identify most of the ph onons associated with the two different types of (BO3)(3-) ion present in the crystal. The number and symmetries of the optical Raman active modes are 7A(1) + 19E, among which 4A(1) + 8E can be assigned as most ly due to (BO3)(3-) vibrations. 7A(1) + 18E modes were observed.The hi ghest energy peaks have been assigned to the regular planar (BO3)(3-) and to the three irregular (BO3)(3-) groups. The two lowest energy pea ks of A(1) symmetry (around 180 and 300 cm(-1)) are very probably rela ted to the BO3 rotatory mode and to Fe displacements. R ions do not pa rticipate in A(1) symmetry modes. The E mode around 90 cm(-1) (the low est frequency mode) is probably due to the R ions which have the longe st bonds and are the heaviest ions.