The Difference One Extra Selenium Makes

To develop materials that can turn heat into electricity, scientists at the Center for Revolutionary Materials for Solid State Energy Conversions (RMSSEC) used powerful theories and computers to explain why seemingly similar compounds have completely different abilities to conduct heat. Previous experimental results show that the copper-antimony-selenium material, shorthanded Cu₃SbSe₃, is a good candidate, while its counterpart, Cu₃SbSe₄, is not. Addressing this abnormal phenomenon, team members specialize in complex calculations − density functional theory (DFT) calculations − and demonstrate that how the atoms move in their crystal structure is strikingly different.

Thermoelectrics Are Power Engines: Thermoelectric devices convert waste heat into electricity or convert electrical power to thermal energy for cooling or heating, and are increasingly seen as having the potential for reducing greenhouse gas emissions. Scientists in the center also believe it will provide cleaner forms of energy and become a substitute for fossil fuels. At the heart of a thermoelectric device are two types of semiconductors arranged in series. The efficiency of the device can be improved either by increasing the electrical conductivity or by decreasing thermal conductivity. However, in classical semiconductors, the electrical and thermal conductivity are linked such that increasing one also increases the other. Thermal conductivity is the property of a material’s ability to transmit heat.

Using Theory to Learn Why the Materials Are Different: The chemical and stoichiometric composition of Cu₃SbSe₃ and Cu₃SbSe₄ are quite similar, but the crystal structures, used to carry out DFT calculations, are different: orthorhombic and zinc-blende-based tetragonal, respectively, with different lattice parameters. The difference in crystal structures creates different chemical environments for each atom, and as a result the thermal conductivity is affected. The DFT results showed that the structure of Cu₃SbSe₃ is more complicated than that of Cu₃SbSe₄, which would suggest a lower thermal conductivity.

To calculate the thermal conductivity of Cu-Sb-Se compounds, the lattice vibrations, also known as phonons, inside the lattice must be understood, because the heat is carried out by phonons. By applying DFT calculations to the two Cu-Sb-Se compounds, researchers observed opposite phonon behaviors. The reason is that the chemical state of Sb in Cu₃SbSe₃ and Cu₃SbSe₄ is different, +3 and +5, respectively, which means that there is a lone pair of electrons around the Sb site in Cu₃SbSe₃. The electrostatic repulsion between the lone pair of electrons significantly weakens the atomic vibrations in Cu₃SbSe₃, lowering its thermal conductivity, relative to Cu₃SbSe₄.

With the help of DFT calculations, scientists now have a proven model and better insights to design new types of thermoelectric semiconductors with intrinsically low thermal conductivity.