Element germanium under pressure matches predictions of modern condensed matter theory

Although its name may make many people think of flowers, the element germanium is part of a frequently studied group of elements, called IVa, which could have applications for next-generation computer architecture as well as implications for fundamental condensed matter physics.

New research conducted by Xiao-Jia Chen, Viktor Struzhkin, and Ho-Kwang (Dave) Mao from Geophysical Laboratory at Carnegie Institution for Science, along with collaborators from China, reveals details of the element's transitions under pressure. Their results show extraordinary agreement with the predictions of modern condensed matter theory.

Germanium (atomic number 32) is used in fiber-optic systems, specialized camera and microscope lenses, circuitry, and solar cells. Under ambient conditions it is brittle and semiconducting. But under pressure, the element should exhibit superconductivity, meaning that there is no resistance to the flow of an electric current.

The team's research, published in Physical Review Letters, discovered that under pressure of 66 GPa (about 650,000 atmospheres), germanium undergoes a structural change from one type of solid material to another that is metallic -- meaning it conducts electricity. It then undergoes another structural change under pressure of 90 GPa (about 890,000 atmospheres). These findings matched theoretical predictions about the element's behavior under extreme pressure.

"A series of phase transitions was observed on compression of germanium that creates structures with increased density," Chen said. "We found extraordinary agreement between theory and experiment for the structures, energies, and compressional behavior. Though some of this behavior had been noted earlier, the agreement between the new highly accurate experimental results and theory really was quite remarkable."

The team's results show that superconductivity in this simple element is caused by phonons, or collective vibrations in the crystal structures that germanium assumes under pressure.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Carnegie Institution.

Journal Reference:

Xiao-Jia Chen, Chao Zhang, Yue Meng, Rui-Qin Zhang, Hai-Qing Lin, Viktor Struzhkin, Ho-kwang Mao. ß-tin›Imma›sh Phase Transitions of Germanium. Physical Review Letters, 2011; 106 (13) DOI: 10.1103/PhysRevLett.106.135502

Exploring the possibilities for zeolites: Team creates database of 2.6 million varieties of molecular sieves

 Some people collect stamps and coins, but when it comes to sheer utility, few collections rival the usefulness of Rice University researcher Michael Deem's collection of 2.6 million zeolite structures.

Zeolites are materials -- including some natural minerals -- that act as molecular sieves, thanks to a Swiss-cheese-like arrangement of pores that can sort, filter, trap and chemically process everything from drugs and petroleum to nuclear waste. Zeolites are particularly useful as catalysts -- materials that spur chemical reactions. There are about 50 naturally occurring zeolites and almost three times as many human-made varieties.

Deem's database, which is described in a new paper that will be featured on the cover of an upcoming issue of the Royal Society of Chemistry's journal Physical Chemistry Chemical Physics, hints at the untapped possibilities for making even more synthetic zeolites.

"For many catalytic applications only a single material has been found," said Deem, the John W. Cox Professor in Biochemical and Genetic Engineering and professor of physics and astronomy. "Expanding the diversity of the zeolite structures would be helpful to improve performance in existing applications, to explore novel functions and to answer basic scientific questions."

Zeolites are useful because of the particular way atoms are mixed and arranged in their porous interiors. Based on these arrangements, zeolites can cause chemicals to react in particular ways, and even subtle changes in the arrangements can alter the reactions that are spurred. Deem's database was created to explore the many zeolite structures that are physically possible, and he said several researchers are already using the information to identify zeolites that could be used for carbon sequestration and other applications.

"Computational methods can play a stimulatory role in the synthesis of new zeolite materials," Deem said. "That is the motivation; that is the challenge that brings us back to zeolites time and again."

In 2007, Deem and his students used both supercomputers and unused computing cycles from more than 4,300 idling desktop PCs to painstakingly calculate every conceivable atomic formulation for zeolites. They created a database of more than 3.4 million atomic formulations of the porous silicate minerals.

In the current study, Deem, Rice graduate student Ramdas Pophale and Purdue University computational analyst Phillip Cheeseman designed tools to examine and compare the physical properties of each entry. Using these tools, they pared down the larger set by removing potential redundancies as well as "low-energy" structures that would either be unstable or impossible to synthesize.

For each of the 2.6 million remaining structures in the database, the team carried out calculations to find specific physical and chemical properties -- including X-ray diffraction patterns, ring-size distributions and dielectric constants -- that could help guide researchers interested in synthesizing them or in finding a new type of zeolite for a specific application.

Deem said the new database has been deposited in the publicly available Predicted Crystallography Open Database.

The research was funded by the National Science Foundation.

Story Source:

The above story is reprinted from materials provided by Rice University.

Journal Reference:

Ramdas Pophale, Phillip A. Cheeseman, Michael W. Deem. A database of new zeolite-like materials. Physical Chemistry Chemical Physics, 2011; DOI: 10.1039/C0CP02255A

First polymer solar-thermal device heats home, saves money

A new polymer-based solar-thermal device is the first to generate power from both heat and visible sunlight -- an advance that could shave the cost of heating a home by as much as 40 percent.

Geothermal add-ons for heat pumps on the market today collect heat from the air or the ground. This new device uses a fluid that flows through a roof-mounted module to collect heat from the sun while an integrated solar cell generates electricity from the sun's visible light.

"It's a systems approach to making your home ultra-efficient because the device collects both solar energy and heat," said David Carroll, Ph.D., director of the Center for Nanotechnology and Molecular Materials at Wake Forest University. "Our solar-thermal device takes better advantage of the broad range of power delivered from the sun each day."

Research showing the effectiveness of the device appears in the March issue of the peer-reviewed journal Solar Energy Materials and Solar Cells.

A standard, rooftop solar cell will miss about 75 percent of the energy provided by the sun at any given time because it can't collect the longest wavelengths of light -- infrared heat. Such cells miss an even greater amount of the available daily solar power because they collect sunlight most efficiently between 10 a.m. and 2 p.m.

"On a rooftop, you have a lot of visible sunlight and heat from the infrared radiation," Carroll said. "The solar-cell industry has for the most part ignored the heat."

The design of the new solar-thermal device takes advantage of this heat through an integrated array of clear tubes, five millimeters in diameter. They lie flat, and an oil blended with a proprietary dye flows through them. The visible sunlight shines into the clear tube and the oil inside, and is converted to electricity by a spray-on polymer photovoltaic on the back of the tubes. This process superheats the oil, which would then flow into the heat pump, for example, to transfer the heat inside a home.

Unlike the flat solar cells used today, the curve of the tubes inside the new device allows for the collection of both visible light and infrared heat from nearly sunrise to sunset. This means it provides power for a much greater part of the day than does a normal solar cell.

Because of the general structure and the ability to capture light at oblique angles, this is also the first solar-thermal device that can be truly building-integrated -- it can be made to look nearly identical to roofing tiles used today.

Tests of the solar-thermal device have shown 30 percent efficiency in converting solar energy to power. By comparison, a standard solar cell with a polymer absorber has shown no greater than 8 percent conversion efficiency.

The research team will build the first square-meter-size solar-thermal cell this summer, a key step in getting the technology ready for market.

Story Source:

The above story is reprinted from materials provided by Wake Forest University, via EurekAlert!, a service of AAAS.

Chemists produce first high-resolution RNA 'nano square'

Chemists at UC San Diego have produced the first high resolution structure of a nano-scale square made from ribonucleic acid, or RNA.

The structure was published in a paper in this week's early online edition of the Proceedings of the National Academy of Sciences by a team of chemists headed by Thomas Hermann, an assistant professor of chemistry and biochemistry at UCSD.

The scientists said the ability to carry structural information encoded in the sequence of the constituent building blocks is a characteristic trait of RNA, a key component of the genetic code. The nano square self-assembles from four corner units directed by the sequence that was programmed into the RNA used for preparing the corners.

Hermann said the RNA square has potential applications as a self-assembling nano platform for the programmed combination of molecular entities that are linked to the corner units.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - San Diego. The original article was written by Kim McDonald.

Journal Reference:

Sergey M. Dibrov, Jaime Mclean, Jerod Parsons, Thomas Hermann. Self-assembling RNA square. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1017999108

Search for advanced materials aided by discovery of hidden symmetries in nature

 A new way of understanding the structure of proteins, polymers, minerals, and engineered materials will be published in the May 2011 issue of the journal Nature Materials. The discovery by two Penn State University researchers is a new type of symmetry in the structure of materials, which the researchers say greatly expands the possibilities for discovering or designing materials with desired properties.

The research is expected to have broad relevance in many development efforts involving physical, chemical, biological, or engineering disciplines including, for example, the search for advanced ferroelectric ferromagnet materials for next-generation ultrasound devices and computers. The paper describing the research will be posted early online by the journal on 3 April 2011, prior to its publication in the journal's May 2011 print edition.

Before the publication of this paper, scientists and engineers had five different types of symmetries to use as tools for understanding the structures of materials whose building blocks are arranged in fairly regular patterns. Four types of symmetries had been known for thousands of years -- called rotation, inversion, rotation inversion, and translation -- and a fifth type -- called time reversal -- had been discovered about 60 years ago. Now, Gopalan and Litvin have added a new, sixth, type, called rotation reversal. As a result, the number of known ways in which the components of such crystalline materials can be combined in symmetrical ways has multiplied from no more than 1,651 before to more than 17,800 now. "We mathematically combined the new rotation-reversal symmetry with the previous five symmetries and now we know that symmetrical groups can form in crystalline materials in a much larger number of ways," said Daniel B. Litvin, distinguished professor of physics, who coauthored the study with Venkatraman Gopalan, professor of materials science and engineering.

The new rotation-reversal symmetry enriches the mathematical language that researchers use to describe a crystalline material's structure and to predict its properties. "Rotation reversal is an absolutely new approach that is different in that it acts on a static component of the material's structure, not on the whole structure all at once," Litvin said. "It is important to look at symmetries in materials because symmetry dictates all natural laws in our physical universe."

The most simple type of symmetry -- rotation symmetry -- is obvious, for example, when a square shape is rotated around its center point: the square shows its symmetrical character by looking exactly the same at four points during the rotation: at 90 degrees, 180 degrees, 270 degrees, and 360 degrees. Gopalan and Litvin say their new rotation-reversal symmetry is obvious, as well, if you know where to look.

The "eureka moment" of the discovery occurred when Gopalan recognized that the simple concept of reversing the direction of a spiral-shaped structure from clockwise to counterclockwise opens the door to a distinctly new type of symmetry. Just as a square shape has the quality of rotation symmetry even when it is not being rotated, Gopalan realized that a spiral shape has the quality of rotation-reversal symmetry even when it is not being physically forced to rotate in the reverse direction. Their further work with this rotation-reversal concept revealed many more structural symmetries than previously had been recognized in materials containing various types of directionally oriented structures. Many important biological molecules, for example, are said to be either "right handed" or "left handed," including DNA, sugars, and proteins.

"We found that rotation-reversal symmetry also exists in paired structures where the partner components lean toward each other, then away from each other in paired patterns symmetrically throughout a material," Gopalan said. These "tilting octahedral" structures are common in a wide variety of crystalline materials, where all the component structures are tightly interconnected by networks of shared atoms. The researchers say it is possible that components of materials with rotation-reversal symmetry could be engineered to function as on/off switches for a variety of novel applications.

The now-much-larger number of possible symmetry groups also is expected to be useful in identifying materials with unusual combinations of properties. "For example, the goal in developing a ferroelectric ferromagnet is to have a material in which the electrical dipoles and the magnetic moments coexist and are coupled in the same material -- that is, a material that allows electrical control of magnetism -- which would be very useful to have in computers," Gopalan said. The addition of rotation-reversal symmetry to the materials-science toolbox may help researchers to identify and search for structures in materials that could have strong ferroelectric and ferromagnetic properties.

Gopalan and Litvin said a goal of their continuing research is to describe each of the more than 17,800 different combinations of the six symmetry types to give materials scientists a practical new tool for significantly increasing the efficiency and effectiveness in finding novel materials. The team also plans to conduct laboratory experiments that make use of their theoretical work on rotation-reversal symmetry. "We have done some predictions, we will test those predictions experimentally," Litvin said. "We are in the very early stages of implementing the results we have described in our new theory paper." Gopalan said, for example, that he has predicted new forms for optical properties in the commonplace quartz crystals that are used widely in watches and electronic equipment, and that his group now is testing these predictions experimentally.

The National Science Foundation provided financial support for this research through its Materials Research Science and Engineering Centers program.

Story Source:

The above story is reprinted  from materials provided by Penn State, via EurekAlert!, a service of AAAS.

Journal Reference:

Venkatraman Gopalan, Daniel B. Litvin. Rotation-reversal symmetries in crystals and handed structures. Nature Materials, 2011; DOI: 10.1038/nmat2987