RAMAN-SPECTRA OF TETRAGONAL LANTHANIDE OXYCHLORIDES OBTAINED FROM POLYCRYSTALLINE AND SINGLE-CRYSTAL SAMPLES

The reported Raman spectra of the tetragonal [lead chlorofluoride-type structure, D4h7-P4/nmm (ITC No. 129)] lanthanide oxychlorides are inconsistent. Nuclear site symmetry analysis predicts six Raman bands. Five peaks are consistently observed, whereas the sixth remains controversial. Also, two different sets of irreducible representations are reported: 2A1g + 3E(g) + B2g (or B1g) + 3A2u + 3E(u) which include the three acoustic modes, A2u{T(z)} and E(u){T(x), T(y)}. The present analysis of the polarized Raman spectra from PrOCl and NdOCl single crystals clearly showed that the correct irreducible representation contains the B2g symmetry. The symmetries of three Raman-active bands are well determined, a fourth band displays two polarizations and a fifth band has no clear polarization and is assigned E(g) by default. The EuOCl Raman spectrum exhibits a weak electronic Raman band which originates in electron-phonon coupling as a result of the closeness between the vibrational and electronic eigenvalues. The low-energy band (LaOCl, 67 cm-1; EuOCl, 70 cm-1; and GdOCl, 65 cm-1) and the high-energy band (525 cm-1 for LaOCl) reported by others for selected lanthanide oxychlorides were not observed in the present work.