Interplay of thermal diffuse scattering and correlated compositional disorder in KCl1-xBrx

Single-crystal X-ray diffuse scattering measurements are reported of the compositional series KCl1-xBrx a model system for the broader family of disordered rocksalts. Using a combination of Monte Carlo simulations and lattice dynamical calculations, we show that the observed diffuse scattering is well described in terms of (i) non-statistical anion distributions, (ii) local lattice relaxations accompanying Cl/Br substitution, and (iii) the contribution from low-energy phonons. It is found that a tendency for compositional domain formation broadens the thermal diffuse scattering by splitting and softening the acoustic phonon branches. This effect, which is strongest for intermediate compositions, is seen in both experiment and calculation alike. These results establish a link between local compositional order and unconventional lattice dynamics in this system, and reinforce emerging design principles of exploiting compositional fluctuations to tailor physical properties, such as thermal conductivity, that depend on phonon broadening.

Automated summary

This study reports single-crystal X-ray diffuse scattering measurements of a model system for disordered rocksalts, KCl1-xBrx. By combining Monte Carlo simulations and lattice dynamical calculations, the researchers show that the observed diffuse scattering can be explained by non-statistical anion distributions, local lattice relaxations, and low-energy phonons. The study reveals a link between local compositional order and unconventional lattice dynamics in this system, which has implications for tailoring physical properties through compositional fluctuations.