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Based on the structural framework concept that atoms are capable of maintaining a nearly tetrahedral coordination environment through the formation of vacancies, clathrate chemistry is an innovative and emerging science that has found a way to generate colloidal crystals with unique control over the symmetry and lattice parameter of the structure. The term clathrate is used to describe a structure that consists of polyhedral cages that contain large pores in the center of the structure1. Within these pores, host-guests, such as noble gases or small organic molecules, are capable of occurring within these naturally occurring molecules. Originally discovered by chemist Humphrey Davy in 1810, clathrates typically occur as a hydrate that is present in a variety of host molecules such as methane, which is the most abundant natural form of clathrate.
An abundance of research in the field of clathrate chemistry is currently taking place, and its presence in a variety of areas ranging from climate instability to planetary concentrations are remarkable. For example, the recent synthesis of an H2 hydrate not only presented with the smallest guest occupancy possible but also established a newly proposed theory in the stability of clathrate hydrates as a whole2. While this research is impressive, it has generally maintained a consistency in the area of clathrate hydrates, whereas recent work by The University of California Davis’ Kirill Kovnir is more interested in clathrates built out of atoms such as copper (Cu), zinc (Zn), barium (Ba) and phosphorus (P) that are present at room temperature3.
What is so unique about the crystal structure of this newly discovered Ba8M24P28+σ (M=Cu/Zn) clathrate is that it has the potential to be composed of five, six, or more bonds, whereas all clathrates that have been studied since its induction into science have all been based on a tetrahedral structure, where each atom in the cage is bonded with four other atoms4. Almost like a mistake, chemists in the lab of Kovnir were initially interested in probing the stability of the clathrate structure when they began adding bonds to it. While Kovnir initially expected the addition of atoms containing more electrons, such as zinc, to break the clathrate structure, the team of researchers instead discovered that an entirely new and more stable structure was capable of being formed2.
As a result of a greater amount of bonds present surrounding the cage of the clathrate, the cage is relatively larger as compared to previously studied clathrates. This increase in the size of the cage allows for atoms to be trapped and rattled around the inside of the clathrate, therefore preventing heat conduction from occurring4. The intrinsically low thermal conductivity capability of these developed clathrates, therefore, allows for these molecules to potentially serve as viable thermoelectric materials. The presence of Ba atoms within the large cages of these molecules is additionally advantageous, as these atoms are highly anisotropic4. The displacement of the Ba atoms within the pentagonal dodecahedra cage supposes a flattened-disc shape along the plane as well.
Well known for their impressive structures and flexibility, clathrates are known for adopting a wide range of properties such as the previously mentioned low thermal conductivity and potential property tenability. The ability of the Ba8M24P28+σ clathrates to resist heat production has caused an interest in applying these materials towards harvesting waste heat and then converting this energy into electricity. This process of harvesting energy typically involves the capturing of small amounts of heat, light, sound, vibration, or movement that would otherwise be lost to be converted to practical electricity5. The loss of energy is present in any type of industrial process and technology that is used every day, therefore a mechanism that could be applied in an effort to convert all of this lost heat into electricity could have been profitable in the future.
Sources and Further Reading
- Lin, Haixin, Sangmin Lee, Lin Sun, Matthew Spellings, Michael Engel, Sharon C. Glotzer, and Chad A. Mirkin. "Chemists Create Colloidal Clathrate Crystals." C&EN Global Enterprise 95.10 (2017): 10-11. Web.
- Patchkovskii, S., and J. S. Tse. "Thermodynamic Stability of Hydrogen Clathrates." Proceedings of the National Academy of Sciences 100.25 (2003): 14645-4650. Web.
- "Compounds Could Be Basis for Devices That Turn Waste Heat into Electricity." Phys.org. 6 Mar. 2017. Web. https://phys.org/news/2017-03-compounds-basis-devices-electricity.html.
- Juli-Anna Dolyniuk et al, Breaking the Tetra-Coordinated Framework Rule: New Clathrate BaP(=Cu/Zn), Angewandte Chemie International Edition (2017).
- "Energy Harvesting." Institute of Physics.
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