Crystalline material conducts heat worse than glass and water

A research team from Aarhus University, Denmark, has measured and explained the exceptionally low thermal conductivity of the crystalline material AgGaGe₃Se₈. Despite its ordered structure, the material behaves like a glass in terms of heat transport – making it one of the least heat-conductive crystalline solids known to date.

At room temperature, AgGaGe₃Se₈ exhibits a thermal conductivity of just 0.2 watts per meter-kelvin – which is three times lower than water and five times lower than typical silica glass. The material is composed of silver (Ag), gallium (Ga), germanium (Ge), and selenium (Se), and has previously been studied for its optical properties.

Now, for the first time, researchers from iMAT – the Aarhus University Centre for Integrated Materials Research – have measured its thermal transport properties and identified the structural origin of its unusually low thermal conductivity.

The key lies in the behaviour of the silver atoms. Instead of being fixed in place within the crystal lattice, the silver atoms are loosely bound and move erratically. This internal disorder disrupts the propagation of phonons – the vibrations that normally carry heat through solids – and causes thermal transport to break down in a way typically seen in amorphous materials like glass.

Remarkably, this glass-like behaviour persists over a broad temperature range, from 2 to 700 kelvin (−271 °C to 400 °C), which is highly unusual for a crystalline material.

Materials with very low thermal conductivity are of interest in a wide range of applications, such as thermoelectric modules that convert waste heat into electricity, or as thermal barriers in microelectronics and high-temperature environments. However, AgGaGe₃Se₈ is not immediately applicable in its current form, as it does not conduct electricity well and contains germanium – a relatively rare and expensive element.

The study is instead a contribution to fundamental materials science. By exploring how specific structural features influence thermal transport, researchers gain valuable insight into how to design materials with tailored heat conduction properties – an important aspect in the development of future technologies in energy, computing, and aerospace.

The findings are based on a combination of thermal measurements and advanced synchrotron X-ray scattering data collected at the Spring-8 facility in Japan.