Phonons in the cubic phase of Co3 B7 O13 X (X=Cl, Br, and I) boracites

The phonon structure in the cubic paraelectric phase (F 4̄ 3c) of Co3 B7 O13 X (X=Cl,Br,I) boracites has been studied theoretically by density-functional theory and experimentally by Raman spectroscopy. The calculated Raman frequencies are in good agreement with those determined from the polarized Raman spectra. In addition to the four Raman-allowed phonons of A1 symmetry, a low-frequency broadband of same symmetry is observed in the spectra of Co3 B7 O13 Cl and Co3 B7 O13 Br. The position and width of this band correspond to those of the partial phonon density of states of the halogen vibrations: an indication for disorder and instability of the halogen sublattice. In the case of Co3 B7 O13 I, an even greater number of spectral features reflecting the underlying phonon density of states are observed. An "instability" F2 mode that involves displacements of the X atoms along 111 and of the Co atoms along 100 directions is also identified. It is shown that this mode is closely related to the halogen and metal sublattice instabilities, which trigger the cubic to orthorhombic transition at lower temperature. © 2009 The American Physical Society.