New Theory Explains Quick Switch in Switchable Mirrors

March 19, 1997

Cincinnati -- University of Cincinnati physicist Fu-Chun Zhang and collaborators who include UC graduate student Kwai-Kong Ng have developed a theory to explain how a shiny mirror made from rare earth elements can suddenly change into a transparent window. The unusual materials were discovered in 1996 and have many potential applications, including the development of "smart" windows for energy savings.

Zhang will explain his theory Wednesday, March 19 during the annual meeting of the American Physical Society in Kansas City, Missouri. It covers a class of new materials consisting of thin films of rare earth elements such as yttrium and lanthanum combined with hydrogen.

Ronald Griessen of the Netherlands led a group which first produced these materials and demonstrated that they can change from a metallic mirror to an insulating window in a matter of seconds.

Zhang says standard physics theory does not explain the switching phenomenon from mirror to window, because it is a single-particle theory and does not account for the strong electron correlation in the hydrides. Instead, Zhang considers a many-particle theory where strong correlation is built in.

"The answer turns out to be quite simple. Hydrogen is the smallest atom in the world, so the hydrogen is like a ping pong ball. It's very small, and the rows of the metallic atoms are very big like basketballs. The ping pong ball can move in and out of the basketball lattice very easily."

The rare earth hydrides are also unusual, because unlike semiconductors, they can take in an enormous number of electrons at the switching point. Those electrons change their behavior when the hydrogen atoms enter the lattice. One positively charged proton plus one negatively charged electron equals one hydrogen atom. Once inside the lattice, however, the hydrogen atoms attract a second electron. That sets the stage for the big switch.

"The electric charge does not change much, but the electron state changes dramatically," explained Zhang.

The electrons become localized around hydrogen atoms, no longer free to move around. When enough become locked into place, light which would have bounced off the mobile electrons is now able to move directly through the material. Instead of reflecting light (the mirror state), the material now transmits light (the window state).

The switch was depicted in dramatic fashion last year on the cover of Nature (March 21, 1996). In the mirror state, a thin film of ytrrium hydride reflected the image of a knight from a chess set. After the switch to the window state, a chessboard could be clearly seen behind the knight.

Zhang says his theory can explain the switch in both the lanthanum and yttrium-based materials, and should be useful in studying other rare earth hydrides as well. An early version of his theory was published earlier this year in Physical Review Letters. The co-authors were Ng, Professor T.M. Rice at the Swiss Federal Institute of Technology in Zurich and Professor V.I. Anisimov of the Russian Academy of Sciences.
-end-


University of Cincinnati

Related Hydrogen Articles from Brightsurf:

Solar hydrogen: let's consider the stability of photoelectrodes
As part of an international collaboration, a team at the HZB has examined the corrosion processes of high-quality BiVO4 photoelectrodes using different state-of-the-art characterisation methods.

Hydrogen vehicles might soon become the global norm
Roughly one billion cars and trucks zoom about the world's roadways.

Hydrogen economy with mass production of high-purity hydrogen from ammonia
The Korea Institute of Science and Technology (KIST) has made an announcement about the technology to extract high-purity hydrogen from ammonia and generate electric power in conjunction with a fuel cell developed by a team led by Young Suk Jo and Chang Won Yoon from the Center for Hydrogen and Fuel Cell Research.

Superconductivity: It's hydrogen's fault
Last summer, it was discovered that there are promising superconductors in a special class of materials, the so-called nickelates.

Hydrogen energy at the root of life
A team of international researchers in Germany, France and Japan is making progress on answering the question of the origin of life.

Hydrogen alarm for remote hydrogen leak detection
Tomsk Polytechnic University jointly with the University of Chemistry and Technology of Prague proposed new sensors based on widely available optical fiber to ensure accurate detection of hydrogen molecules in the air.

Preparing for the hydrogen economy
In a world first, University of Sydney researchers have found evidence of how hydrogen causes embrittlement of steels.

Hydrogen boride nanosheets: A promising material for hydrogen carrier
Researchers at Tokyo Institute of Technology, University of Tsukuba, and colleagues in Japan report a promising hydrogen carrier in the form of hydrogen boride nanosheets.

World's fastest hydrogen sensor could pave the way for clean hydrogen energy
Hydrogen is a clean and renewable energy carrier that can power vehicles, with water as the only emission.

Chemical hydrogen storage system
Hydrogen is a highly attractive, but also highly explosive energy carrier, which requires safe, lightweight and cheap storage as well as transportation systems.

Read More: Hydrogen News and Hydrogen Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.