Spontaneous ordering as an intrinsic effect at the mesoscopic scale: A vibrational insight in (Zn,Be)Se by Raman scattering and first-principles calculations

The recent finding of a 1-bond→2-phonon ‘percolation’-type behaviour in several random zincblende alloys, supporting an unsuspected 1-bond→2-mode behaviour in the bond length distribution, renews interest for a discussion of CuPt-type spontaneous ordering (CPSO) as a purely intrinsic effect. We investigate this key issue from both experimental (Raman scattering) and theoretical (first-principles bond length and phonon calculations) sides, focusing on Zn1−xBexSe (0 < x = 0.92). Its large contrast in cation-anion bond lengths should stimulate CPSO to some extent, and is at the origin of a uniquely well-resolved 1-bond→2-phonon behaviour for the Be-Se species. Based on percolation, CPSO finds a previously missing natural explanation, at the mesosocopic scale. Also, the transfer of oscillator strength from the highto the low-frequency Be-Se mode in the Raman spectra potentially emerges as a sensitive probe of CPSO in ZnBeSe. First-principles calculations indicate that the transfer is completed for the value of the long range order parameter of η ∼0.5 in ZnBeSe2 (x ∼0.5), corresponding to a pure 1-bond→1-mode behaviour. With further ordering the alloy is forced to re-adopt a non-rewarding 1-bond→2-mode behaviour, due to the formation of ZnSe/BeSe micro-domains in the crystal. Therefore η ∼0.5 appears as an intrinsic limit to CPSO in ZnBeSe2, and possibly in stoichiometric alloys in general.