Scientists Discover New Form of Crystalline Order for Thermoelectric Devices

Scientists have discovered a completely new form of crystalline order that exhibits two different properties: crystal and polycrystalline. The new material could potentially improve the efficiency of thermoelectric devices.

Since the 1850s, scientists have known that crystalline materials are organized into 14 different basic lattice structures. Now, though, researchers have discovered a completely new form of crystalline order.

The scientists first discovered this unusual arrangement of atoms while studying nanoparticles made from the semiconductor copper-indium sulfide (CIS), which is being actively studied for use in solar cells. The interlaced crystal arrangement has properties that make it ideal for thermoelectric applications that turn heat into electricity.

In crystalline materials, atoms are arranged in periodic arrays of points, a mathematical abstraction called a Bravais lattice. There are 14 different types of Bravais lattices in three dimensions. The same atom or group of atoms sits at each lattice point. The simplest and most symmetric is the "simple cubic" lattice.

Square floor tiles provide a two-dimensional example of this particular lattice. The corners of the tiles create a regular, repeating lattice pattern. A three-dimensional version is the face centered cubic (FCC), which has points both on the corners and at the centers of the faces of a cube.

A number of minerals have an FCC lattice. In contrast, though, CIS is a bit more complicated. You can think of the sulfur atoms occupying one FCC sub-lattice while the copper and indium atoms share a second sub-lattice. Each copper or indium atom is surrounded by four nearest-neighbor sulfur atoms while each sulfur is surrounded by two copper and tow indium nearest-neighbors.

The interlaced crystal structure may be just what is needed to optimize thermoelectric applications for power generation or cooling. Thermoelectric devices need a material that's an excellent electrical conductor and a poor conductor of heat. This material may just accomplish this feat.

"We haven't tested this yet, but we are confident that these materials have high electrical conductivity and low thermal conductivity...just what you need for thermoelectrics," said Sokrates Pantelides, one of the researchers, in a news release. "The field is now wide open for scientists who can fabricate thin films and make thermoelectric measurements."

The findings are published in the journal Nature Communications.