We propose a picture based on simple percolative concepts for the basic understanding of vibrational properties in the new attractive class of ternary semiconductor alloys made of materials with highly contrasted bond stiffness. In Znl-xBexSe this accounts for the activation of a strong extra BeSe-like optical mode for x between the percolation thresholds of the Be-Se and Zn-Se bonds. Latter mode is attributed to Be-Se bonds within the quasi-continuous Be-rich hard-like cluster which forms above the percolation threshold. [References: 13]
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