Recently the attention turned towards Be-chalcogenides as they exhibit a large amount of covalent bonding, which is unique among the highly ionic II-VI materials. This results in a reduced lattice parameter for Be-compounds, by ~7-9% in comparison with the Zn-counterparts, and a remarkably high shear modulus C*s, i.e. in the III-V standard. Therefore Zn1-xBex(Se,Te) need to be seen as the first interclass-like ternaries in view of the mechanical properties.
We expect that the sharp contrast between the stiffness of the cation-anion bonds results in a large-scale 'mechanical disorder' when x goes above the critical values associated with the first formation of pseudo-continuous chains of the Be-VI and Zn-VI bonds. These are defined as the bond percolation thresholds, and are estimated at xBe-VI=0.19 and xZn-VI=0.81 in zinc-blende systems. Raman scattering is well-suited to investigate such percolation effects as it addresses directly the force constant of the bond, which is extremely sensitive to the mechanical properties of the host matrix.
We have shown in earlier work that the ZnBe(Se,Te) alloys exhibit the same atypical Raman multi-mode behavior. The picture which emerges is that the two pseudo-infinite ZnVI- and BeVI-like interlaced treelike chains in the percolation regime delimit unbounded volumes with different mechanical properties: a hard-like Be-rich region, and a relatively soft Zn-rich region. Out of the percolation regime the picture of a dispersion-in-continuum prevails. At x<xBe-Se this corresponds to Be-rich hard bounded clusters embedded within a soft ZnVI-like host matrix; the situation is reversed at the other end (x>xZn-Se).
In this work the picture is enriched by introducing a straightforward percolation-based model to achieve full lineshape analysis of the atypical longitudinal (LO) and transverse optical (TO) Raman multi-mode at any composition. No adjustable parameter is needed.