Phonon probe of structural variations in FeSe$_{1-x}$Te$_{x}$

In the iron chalcogenide system FeSe$_{1-x}$Te$_{x }$the superconducting compositions are produced by isovalent substitution of Te for Se. The electronic structure and magnetic properties of FeSe$_{1-x}$Te$_{x}$ also show an unusual sensitivity to the crystal structure modification with chalcogen substitution. We present the evolution of Raman active phonons in single crystals of FeSe$_{1-x}$Te$_{x}$ with x = 0.0, 0.5, 0.75, and 1.0 as measured with polarized Raman spectroscopy at room temperature. Lattice dynamics calculations of the studied compositions yield a simple eigenvector for the A$_{1g}$ and B$_{1g}$ modes involving vibrations along the c-axis of only Se(Te) and Fe atoms, respectively. Remarkably, the A$_{1g}$ mode exhibits a single mode behavior thus pointing to the importance of the average structure rather than local one for the properties of iron chalcogenides. The atomic disorder associated with Te for Se substitution produces a Raman band around 150 cm$^{-1}$ that persists also in the composition with x =1 thus reflecting the stoichiometric deviation in the actual structure FeTe$_{0.92}$ and the band is absent in FeSe.