Glass reveals secrets under pressure

Octahedral structure observed and detected for the first time

Structure of germanium glass ANL
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Glass is a material of a special kind. Now, under pressure, it has revealed one of its long-kept secrets. American researchers have for the first time succeeded in producing a glass of dense, purely octahedral crystals and analyzing its structure - something that chemists have been looking for for decades.

Researchers at the Argonne National Laboratory (ANL) have explored the mystery of the glass through a range of methods. "Little is known about the structure of glass under pressure, " explains Chris Benmore, materials researcher at ANL. "Although it is so important. We use it in our cars and homes and use it for a variety of industrial applications, but we do not know how its atomic structure behaves under pressure. "

Glass is considered difficult to analyze because it has a disordered structure and no periodically arranged crystals. Although it changes its structure under pressure, it elastically returns to its original state as soon as the pressure subsides - and is then no longer observable.

Germanium glass in the neutron beam

Therefore, the researchers had to use special "tricks", including X-rays and neutron beams, to capture this volatile state. They also used a special glass: "Silicon is the most important and best-known glass, " explains Benmore, "but we studied the softer germanium glass because it is structurally analogous to silicon glass, but converts to octahedron shape at much lower pressures than this. Germanium also shows a stronger contrast in the neutron and X-ray diffractometer and the details of its structure appear more pronounced. "Under normal pressure conditions germanium glass has the typical tetrahedral glass structure: Four oxygen atoms surround a germanium atom and divide their corners to form tiny, nanometer-sized cages form.

In a specially designed pressure chamber, glass samples were subjected to pressures of up to five gigapascals, which is equivalent to 50, 000 times the normal air pressure. Neutron beams steered by the pressure chamber responded with distraction to changes in the structure, revealing it. Using this setup, scientists were able to observe for the first time how the tetrahedra of the glass collapsed under pressure and pressed oxygen atoms with increasing density into the sides of neighboring "cages". display

Five times oxygen around a germanium

More precisely, the researchers were able to observe the process under X-rays. When a one-millimeter glass sample was compressed at about 60, 000 to 100, 000 times the atmospheric pressure, they discovered a hitherto unknown transition state in which five oxygen atoms form around the germanium atom immediately before the octahedra form. The question remained as to whether this transition was abrupt or continuous.

Clarification should also provide a dynamic simulation of the processes based on the observations and data. The computer first calculates the molecular structures of the substance and calculates the possible principles of form. "The simulations agreed with our data and revealed an anomaly that allows the existence of a deformed five-coordinate germanium in a limited pressure range, " explains Benmore. "This has given us the proof that germanium glass deforms continuously, contradicting the current two-staged model."

The researchers were also able to marvel at the octahedral state of the glass live when they exposed the glass to even higher pressures of up to 150, 000 atmospheres. However, the angles of the inner structures were not 90 and 180 degrees, as in a perfect octahedron, but near 90 and 165 degrees. "We will continue to study this dense glass, as it has never been characterized before, " concludes Benmore. Because of the high pressures, it is a real challenge, especially as some glass researchers thought the glass would crystallize immediately when it becomes octahedral.

(Argonne National Laboratory (ANL), 14.12.2004 - NPO)