Hexagonal Zn1-xMgxS sheds light on the lattice dynamics of atomic alloys

Cubic pseudo-unary A_1-xB_x high-entropy metallic alloys and pseudo-binary A_1-xB_xC disordered semiconductor alloys set a benchmark to explore how physical properties are impacted by disorder. Through its diversity, the lattice dynamics offers a unique playground to assign the relevant length scales at which operate various kinds of disorders induced by alloying. (i) In high-entropy metallic alloys, the overdamping of the bond-collective (multi-bond→1-mode) acoustic modes at short wavelength originates from force-constant fluctuations. (ii) In semiconductor alloys, the lattice mismatch splits, at any wavelength, the bond-specific (1-bond→1-mode) optical modes in duos distinguishing "same" from "alien" environments, as explained by the percolation model. Zn_1-xMg_xS is ideal to test both univocal assignments. Its force-constant disorder is small, reducing the cause for overdamping of the acoustic modes. Its local strain is inverted, the lighter substituent being the larger one and forming the longer bond. This forecasts a dramatic inversion of the mode-duos. Further, its wurtzite structure enables (iii) to test whether/how the percolation model for the mode-duos transfers under lowering the crystal symmetry from cubic to hexagonal. The triple acoustic-(i)/optical-(ii-iii) test on Zn_1-xMg_xS, combining inelastic neutron scattering with first-principles simulations, is positive. This highlights a few key points behind the lattice dynamics of atomic alloys.