Raman spectra of lanthanide sesquioxide single crystals with A-type structure

Phonon data are given for La2O3, Pr2O3, and Nd2O3 single crystals. The polarized Raman spectra of Nd2O3 are presented and analysed according to Pauling's structure: Dsub(3d)sup(3)(P3m1), Z=1. A factor group analysis leads to the symmetries 2Asub(1g) + 2Esub(g) for the q = 0 lattice vibrations which give a non zero contribution in the first order Raman scattering. The experimental data are in complete agreement with these previsions. The four Raman active vibrations are shown to be those of Ln2O(II)2 entities within cages of O(I) atoms, O(I) and O(II) lying in Dsub(3d) and Csub(3v) crystal sites, respectively. There are two stretching (Asub(1g) + Esub(g)) and two bending (Asub(1g) + Esub(g)) modes, the assignments of which are made. A tentative comparison with the oxysulphides Ln2O2S, which have also a P3m1 structure, is presented. A linear correlation is found between the Ln-O(II) stretching mode frequency of the three studied sesquioxides and the c/a ratio of their hexagonal A-type structure. (author)