Fig. 1
Taxonomy of high pressure Raman spectra of zincblende A1−xBxC semiconductor alloys based on the percolation model. Schematic TO-overview of the percolation model (PM) that distinguishes between bond vibrations (subscript in the used TO notation) in homo and hetero environments (superscript), originally worked out for the highly-contrasted zb-Zn1−xBex-chalcogenides (central panel). The PM-scheme notably features a change in slope in the “TO-frequency vs. \(x\)” variations at the B-C (\({x}_{B-C,perco}\)~0.19) and A-C (\({x}_{A-C,perco}\)~0.81) bond percolation thresholds (emphasized by dotted arrows), in principle. (a) The PM covers the historical MREI and cluster behaviors corresponding to unimodal—(thick lines) and quadrimodal-TO (colored lines) behaviors per bond, respectively, with three MREI variants depending on whether the two modes per alloy are well-separated (2-mode type), close (1-mode type) or cross each other (intermediary type). The MREI model is blind to the local environment, by construction (see text). The cluster model distinguishes between vibrations of, e.g., B–C bonds depending on the C-centered tetrahedron-cluster they belong to, with one to four B atoms at the vertices (abbreviated C:1B to C:4B). Each MREI branch accordingly divides in four cluster sub-branches (top-left panel). (b) Under pressure the distinct PM-doublet due to the short/stiff bond (usually) vibrating at high frequency, e.g., B–C, either diverges or converges depending on the relative hardening rates of the homo and hetero environments, governed by the volume dependence of the bond ionicity (\({f}_{i}\)), as indicated. The convergence follows distinct paths depending on whether B–C is dispersed in chain (\(x\)≤\({x}_{A-C}\)=0.81, marking the A–C bond percolation threshold) or matrix-like (\(x\)≥\({x}_{A-C}\))—leading either to the achievement of a “phonon exceptional point” at the resonance (interrupting the convergence process, ⊡), or to a proper inversion post resonance, ⊠—as sketched out.
