  A mixed II-VI semiconductor Zn[1-x]Be[x]Se possesses non-trivial vibration
properties, because its two constituent compounds, ZnSe and BeSe, show very
different degree of covalency and hence high elastic contrast. An anomalous
Be-Se vibration line has been observed mostly at intermediate Be content in the
Raman spectra of thin (Zn,Be)Se films. In order to explain microscopic origins
and the detailed composition of these lines, a first-principles calculation of
vibration frequencies in a mixed crystal has been done, with frozen-phonon
technique and supercell setup within the density functional theory, by the
Siesta method, which uses norm-conserving pseudopotentials and strictly
localized numerical basis functions. The calculations confirmed an earlier
assumption that the anomalous Be-Se line appears due to the formation of
continuous chains of a more rigid Be-rich pseudo-continuous phase formed within
the more soft Zn-rich host region on crossing the Be-Se bond percolation
threshold (~0.19). Different local deformation in percolated and non-percolated
regions affect interatomic elastic interactions and split corresponding
vibration lines. Besides confirming the percolation model qualitatively, the
calculation provides details about vibration patterns in different phonon
modes.
