Titanium Atom That Exists in Two Places at Once in Crystal to Blame for Unusual Phenomenon

Date: 11 December 2020
Source: ORNL

BaTiS3 This high-resolution scanning electron microscope (SEM) image of BaTiS3 crystals is overlaid with illustrations showing the orientation of individual atoms in the crystal. Credit: Caltech / USC / ORNL

The crystalline solid BaTiS₃ (barium titanium sulfide) is terrible at conducting heat, and it turns out that a wayward titanium atom that exists in two places at the same time is to blame.

The discovery, made by researchers from Caltech, USC, and the Department of Energy’s Oak Ridge National Laboratory (ORNL), was published on November 27 in the journal Nature Communications. It provides a fundamental atomic-level insight into an unusual thermal property that has been observed in several materials. The work is of particular interest to researchers who are exploring the potential use of crystalline solids with poor thermal conductivity in thermoelectric applications, in which heat is directly converted into electric energy and vice versa.

“We have found that quantum mechanical effects can play a huge role in setting the thermal transport properties of materials even under familiar conditions like room temperature,” says Austin Minnich, professor of mechanical engineering and applied physics at Caltech and co-corresponding author of the Nature Communications paper.

Crystals are usually good at conducting heat. By definition, their atomic structure is highly organized, which allows atomic vibrations – heat – to flow through them as a wave. Glasses, on the other hand, are terrible at conducting heat. Their internal structure is disordered and random, which means that vibrations instead hop from atom to atom as they pass through.

BaTiS₃ belongs to a class of materials called Perovskite-related chalcogenides. Jayakanth Ravichandran, an assistant professor in USC Viterbi’s Mork Family Department of Chemical Engineering and Materials Science, and his team have been investigating them for their optical properties and recently started studying their thermoelectric applications.

“We had a hunch that BaTiS₃ will have low thermal conductivity, but the value was unexpectedly low. Our study shows a new mechanism to achieve low thermal conductivity, so the next question is whether the electrons in the system flow seamlessly unlike heat to achieve good thermoelectric properties,” says Ravichandran.