Study Discovers Why a Perfect Crystal is not Good at Conducting Heat

Barium titanium sulfide (BaTiS3), a crystalline solid, is a poor conductor of heat. It has now been found that the occurrence of a wayward titanium atom simultaneously in two positions is responsible for this phenomenon.

Study Discovers Why a Perfect Crystal is not Good at Conducting Heat
This high-resolution scanning electron microscope (SEM) image of BaTiS3 crystals is overlaid with illustrations showing the orientation of individual atoms in the crystal. Despite the atomic perfection of the crystal, it is unexpectedly poor at transporting thermal energy. Image Credit: Caltech/USC/ORNL.

The new finding by scientists from Caltech, USC, and the Department of Energy’s Oak Ridge National Laboratory (ORNL) was reported in the Nature Communications journal on November 27th, 2020. The study offers a basic, atomic-level understanding of an unusual thermal property observed in various materials.

The research is of specific interest to scientists who have been investigating the prospective use of crystalline solids with bad thermal conductivity in thermoelectric applications, where heat is directly transformed 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.

Austin Minnich, Study Co-Corresponding Author and Professor of Mechanical Engineering and Applied Physics, Caltech

In general, crystals are good conductors of heat. Theoretically, their atomic structure is extremely organized, enabling atomic vibrations (i.e. heat) to flow through them in the form of a wave. By contrast, glasses are poor conductors of heat. They have a random and disordered internal structure, meaning that vibrations instead jump from one atom to another as they pass through.

BaTiS3 falls into a family of materials known as perovskite-related chalcogenides. Jayakanth Ravichandran, an assistant professor in USC Viterbi’s Mork Family Department of Chemical Engineering and Materials Science, and his colleagues have been analyzing these materials for their optical properties and recently started exploring 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.

Jayakanth Ravichandran, Assistant Professor, Mork Family Department of Chemical Engineering and Materials Science, Viterbi School of Engineering, University of Southern California

The researchers found that BaTiS3, and many other crystalline solids, exhibited “glass-like” thermal conductivity. Apart from being comparable to the thermal conductivity of disordered glasses, the thermal conductivity of BaTiS3 actually turns worse with a decrease in temperature, which is in contrast to most materials. At cryogenic temperatures, its thermal conductivity is the worst ever observed in any fully dense (nonporous) solid.

The titanium atom in each BaTiS3 crystal was found to occur in a so-called double-well potential—in other words, the atom wants to be in two spatial locations within the atomic structure. The existence of the titanium atom simultaneously in two places gives rise to a so-called “two-level system.”

In this case, the titanium atom exhibits two states—a ground state and an excited state. The titanium atom absorbs passing atomic vibrations and goes from the ground to the excited state, following which it quickly decays back to the ground state. The energy absorbed is released in a random direction in the form of a vibration.

Due to the cumulative effect of this absorption and emission of vibrations, energy is scattered instead of being transferred cleanly. An analogy is shining a light via a frosted glass, where the titanium atoms are the frost; the titanium deflects off the incoming waves, where only a portion passes through the material.

For a long time, researchers have known the occurrence of two-level systems, but this is the first-ever direct observation of one that was adequate to affect thermal conduction in a single crystal material over a higher temperature range, measured here from 50 to 500 K.

The team visualized the effect by bombarding neutrons onto BaTiS3 crystals as part of a process called inelastic scattering. The Spallation Neutron Source at ORNL was used for this process. The neutrons passing through the crystals either lose or gain energy. This shows that in certain cases, energy is absorbed from a two-level system, and in others, it is imparted to them.

It took real detective work to solve this mystery about the structure and dynamics of the titanium atoms. At first it seemed that the atoms were just positionally disordered, but the shallowness of the potential well meant that they couldn’t stay in their positions for very long.

Michael Manley, Study Co-Corresponding Author and Senior Researcher, ORNL

That was exactly when Raphael Hermann, researcher at ORNL, recommended performing quantum calculations for the double well. “That atoms can tunnel is well known, of course, but we did not expect to see it at such a high frequency with such a large atom in a crystal. But the quantum mechanics is clear: if the barrier between the wells is small enough, then such high-frequency tunneling is indeed possible and should result in strong phonon scattering and thus glass-like thermal conductivity,” added Manley.

The traditional method for making crystalline solids with low thermal conductivity involves forming numerous defects in those solids, which has an adverse effect on other properties like electrical conductivity.

Thus, a technique to make low-thermal-conductivity crystalline materials without any adverse on the optical and electrical properties is much preferable for thermoelectric applications. Since very few crystalline solids show the same terrible thermal conductivity, the next step for the researchers is to investigate whether this phenomenon is responsible in those materials as well.

This study was financially supported by the Defense Advanced Research Projects Agency, the U.S. Department of Energy, the Office of Naval Research, the National Science Foundation, the Army Research Office, the Air Force Office of Scientific Research, and the Link Foundation.

Journal Reference:

Sun, B., et al. (2020) High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals. Nature Communications.


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