&Bullet; physics 14, p40

Introducing correlated disorder into a crystalline material could provide a way to tune the phonon properties and thermal conductivity of the material.

Crystals are one of the easiest solids to theoretically describe. Their large-scale structures consist of a pattern that is repeated in space and can be predicted from a few parameters. Daniel Chaney from the University of Bristol, UK, and colleagues have now created a crystal whose structural order is locally modified by a second, smaller pattern [1] . They show that this “correlated disorder” changes the propagation of phonons in a material, suggesting a method of controlling its thermal properties.

Two crystals with similar structures can be grown on top of each other so that they interlock like pieces of a puzzle. If a layer then tries to change its crystal structure, the interlocking puzzle can thwart the change. The team used this effect to create a crystallographic “conflict” in a 300 nm thick film of a uranium-molybdenum (UMo) alloy. They grew a UMo layer on top of a niobium underlayer

$8th00{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C.}$

, a temperature at which both materials have cubic lattices. On cooling, UMo normally decomposes into orthorhombic uranium and cubic molybdenum. Instead, the niobium UMo gearing retained the alloy’s mixed state, forcing it to retain its original cubic structure.

X-ray scattering experiments showed that this conflict distorts UMo’s cubic lattice, with atoms in alternating planes shifted in opposite directions. This pattern remained coherent over ranges of 5-30 Å, the exact correlation length being dependent on factors such as the molybdenum content of the alloy.

The team found that phonons in the disturbed lattice had a significantly shorter lifespan than in a simulated, undisturbed lattice. In UMo, the heat transport is dominated by electrons, not phonons. But, according to the researchers, the correlated disorder could be used to tune the thermal conductivity of materials in which phonons play a larger role.

–Marric Stephens

Marric Stephens is the corresponding editor for physics based in Bristol, UK.

## References

1. D. Chaney et al., “Tunable Correlated Disturbance in Alloys”, Phys. Rev. Mater.5, 035004 (2021).

## areas of expertise

Condensed Matter PhysicsMaterial Science

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