Tellurium - A Stellar Material

February 29, 2012

Many materials scientists, myself included, have worked with tellurium, a rare chemical element with atomic number 52. My application was thermopower energy-harvesting. Bismuth telluride (Bi₂Te₃) and its close cousins are the star materials for this application, but tellurium is useful in so many ways.

Mercury cadmium telluride and lead telluride are excellent infrared detectors. Cadmium telluride is a useful photovoltaic material. You may even have tellurium in your home, since its oxide is used as a phase-change material in rewritable optical discs. Tellurium glasses, such as Ge₂Sb₂Te₅, can be used as the active material in phase change memory, a technology I mentioned in two previous articles (Nano Memory, September 30, 2010 and Phase Change Conductance, June 21, 2011).

Tellurium and its neighbors in the Periodic Table. When younger, I was confused as to why antimony should be called Sb. This derives from its Latin name, "stibium." (Via Wikimedia Commons).

Well, this "star" material has now been confirmed as a "stellar material." A team of astrophysicists with members from many US institutions has discovered neutral tellurium in three so-called metal-poor stars. This discovery was made using the Hubble Space Telescope, managed by the Space Telescope Science Institute. Details are published in a recent issue of The Astrophysical Journal Letters.[1-4]

One self-referential aspect of materials science is that materials scientists are themselves made from materials. As is well known to all with even a passing interest in astronomy, we are all made from "star stuff." The universe stared out with mostly hydrogen, some helium, and a little lithium; but nucleosynthesis reactions within stars manufactured all the other elements of the periodic table.

The first populations of stars had few heavy elements. Subsequent stars were created from material synthesized in these first stars, so they have more heavy elements. Since the Sun is from a later generation of stars, Earth has the panoply of elements that we see.

Nucleosynthesis of heavier elements is hard to do, and that's why these elements are rare. Supernovae perform what's called r-process nucleosynthesis to create neutron-rich nuclei. The r in r-process stands for rapid, since this type of nucleosynthesis proceeds by a rapid succession of neutron capture on a lighter seed nucleus. Not surprisingly, there's a companion slow process, the s-process. About half the heavier elements are thought to have been created by the s-process, and the rest by the r-process.

Tellurium is a rare element with a crustal abundance of just 1 μg/kg, or a part-per-billion. The rarest of the so-called rare earth elements have crustal abundances that are hundreds of times greater.

The isotopes of tellurium are quite interesting. The stable isotopes of tellurium (¹²²Te, ¹²⁴Te, ¹²⁵Te and ¹²⁶Te) comprise just a third of natural tellurium. The rest is radioactive isotopes with extremely long half lives. Although technically radioactive, ¹²⁸Te has a half life of 2.2 x 10²⁴ years, which is more than a hundred trillion times the age of the universe.[5]

The Hubble Telescope observation is the first time that tellurium has been detected in stars. Tellurium was detected in three stars, designated BD +17 3248, HD 108317, and HD 128279. These are ancient stars, nearly 12 billion years old, located a few thousand light-years away.[1-2]

The tellurium was detected using near-ultraviolet spectroscopy, and the ratio of tellurium to the heavy elements, barium and strontium, was the same for all three stars. This supports the theory that a rare type of supernova was responsible for the creation of the very heavy elements, including tellurium.[3]

Anna Frebel, an assistant professor of physics at MIT, and a co-author of the tellurium paper, had this to say:

"You can make iron and nickel in any ordinary supernova, anywhere in the universe... But these heavy elements seem to only be made in specialized supernovas. Adding more elements to the observed elemental patterns will help us understand the astrophysical and environmental conditions needed for this process to operate."[3]

Home, Sweet Home Artist's impression of our Milky Way Galaxy. (NASA image, via Wikimedia Commons)).

References:

1. Ian U. Roederer, James E. Lawler, John J. Cowan, Timothy C. Beers, Anna Frebel, Inese I. Ivans, Hendrik Schatz, Jennifer S. Sobeck and Christopher Sneden, "Detection of the Second R-Process Peak Element Tellurium in Metal-Poor Stars," The Astrophysical Journal Letters, vol. 747, no. 1 (March 1, 2012), doi:10.1088/2041-8205/747/1/L8.
2. Ian U. Roederer, James E. Lawler, John J. Cowan, Timothy C. Beers, Anna Frebel, Inese I. Ivans, Hendrik Schatz, Jennifer S. Sobeck and Christopher Sneden, "Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars," arXiv Preprint Server, February 10, 2012.
3. Jennifer Chu, "Rare Earth element found far, far away," MIT Press Release, February 17, 2012.
4. Usman Zafar Paracha,"Tellurium, Rare Earth Element, Observed in Three Ancient Stars," Technorati, February 21, 2012.
5. Tellurium page on Wikipedia.