UNDER PRESSURE — The interior structure of germania was revealed when researchers squeezed a sample between these two piston anvils, shown with postdoctoral appointee Qiang Mei, up to 50,000 times atmospheric pressure.
PRESSURED TO CHANGE – The images above show how the structure of germanium-dioxide glass changes as it is subjected to increasing greater pressure. The structural units are about 1 nanometer, or one-billionth of a meter, across. The top image shows the materials basic tetrahedral structure at normal pressures. In the second image, the tetrahedral structure begins to collapse when pressures reach about 50,000 times atmospheric pressure. In the bottom image, at 60,000 to 100,000 times atmospheric pressure, the structure becomes octahedral.
Glass is a mysterious material, but when researchers apply pressure, it reveals secrets.
Using a variety of techniques, researchers at Argonne National Laboratory saw for the first time ever, the atomic structure of a dense, purely octahedral glass that has eluded scientists for decades. They also witnessed a continuous structural change in the glass, disproving the theory that tetrahedral glasses go through a distinct transition between low- and high-density phases.
“Little is known about the structure of glass under pressure” said materials scientist Chris Benmore, “even though it is quite important. We put it in our cars and homes, and use it in many industrial applications, but how does the atomic structure react to extreme pressures?”
Scientist Chris Tulk from Oak Ridge National Laboratory created novel pressure cells for both instruments in conjunction with other pressure-cell experts from the Carnegie Institution.
“Silica is the most important and most widely used glass,” said Benmore, “but we studied the softer germania (Ge) because it is a structural analog to silica and transforms to the octahedral form at much lower pressures than silica. Germania also provides a greater contrast in the neutron and the X-ray studies, so the details appear more clearly.”
At ambient pressure, Ge has an archetypal tetrahedral network glass structure. Four oxygen atoms enclose a germanium atom and share corners to create cages that are only a nanometer across.
The researchers began their experiments at the IPNS. Neutrons reveal structural and dynamic properties of materials, and they are sensitive to lighter elements such as oxygen.
Two piston anvils inside the IPNS’s Glass, Liquid, and Amorphous Materials Diffractometer squeezed a 100-cubic millimeter sample of germania dioxide (GeO2) to pressures up to five gigapascals, or 50,000 times atmospheric pressure.
IPNS revealed the mechanism of how GeO2’s tetrahedra collapse under pressure. Oxygen atoms were seen being squeezed into the sides of neighboring tetrahedra as the cages collapsed and the glass density increased.
In contrast to the IPNS, the APS reveals germanium atoms more clearly and can test smaller samples, which allows studies at higher-pressures. As a 1 cubic millimeter GeO2 sample was pushed from 60,000 to 100,000 times ambient pressure, researchers witnessed the tetrahedral cages collapsing and an average of five oxygen atoms organizing around the germanium atom before the octahedral glass was formed. This average coordination number of five still did not clearly resolve the question of whether this phase change in germania is continuous or discontinuous.
Researchers thought they may have seen a gradual mixture of five- and then six-fold germanium atoms in the structure as the pressure increased, but the result was still unclear. So they called on their colleagues at the National Research Council of Canada to perform molecular dynamic simulations in which a computer calculates molecular structure and behavior from first principles. “The simulations agreed with our data and revealed a germanate anomaly, that allows a distorted five-fold coordination of germanium to exist over a limited pressure range,” Benmore explained. “This provided evidence that germania glass transforms continuously, which disagrees with the popular two-state model.”
As researchers pressurized a GeO2 sample to 150,000 times ambient pressure, they witnessed a dense, disordered octahedral – eight-sided – structure inside glass for the first time. The angles of the internal structures were not the 90 and 180 degrees of a perfect octahedron; instead, the angles were near 90 and 165 degrees.
“We’ll continue to study this dense glass since it has never before been characterized,” said Benmore. “It is a challenge because of the pressures needed. Also, some glass scientists thought the glass would immediately crystallize if it became octahedral.”
This research, which has appeared in Physical Review of Letters, Vol. 93, No. 11, was highlighted in the Editors’ Choice in the October 1, 2004 issue of Science.
This is not the first time the team of Benmore and Tulk has shown that the two-state polyamorphic theories have been wrong. In 2003, Benmore, Tulk and colleagues discovered new metastable ice forms when studying ice under pressure. These new forms appear to contradict the widely held belief that the phase change in amorphous ice is discontinuous. — Evelyn Brown
Catherine Foster | EurekAlert!
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
28.04.2017 | Event News
20.04.2017 | Event News
18.04.2017 | Event News
28.04.2017 | Medical Engineering
28.04.2017 | Earth Sciences
28.04.2017 | Life Sciences