Far-infrared reflectivity and Raman spectra of Ba5Nb4O15

We report low-temperature, far-infrared reflectivity, and Raman-scattering measurements for layered Ba5Nb4O15. We find that this material is characterized by a strong anharmonic lattice where the symmetric stretching vibration of the empty octahedra, a singular feature of this layer compound, splits into two narrow Raman-active bands. We assign them to the same phonon in a slightly different environment, and suggest a small local departure of the reported centrosymmetric D-3d(3)-P3ml space group, We think that the infrared and mainly the Raman band profiles indicate that the lattice of Ba5Nb4O15 is close to collapsing into a lower symmetry structure.