0324ctt silica pressure lo res

[Image above] Credit: J J; Flickr CC BY-NC-ND 2.0

Silica—a.k.a., silicon dioxide—is one of the most abundant natural compounds here on Earth.

And yet scientists are just learning some secrets of this common yet complex material—a team from the Carnegie Institute for Science recently discovered new rare forms of the compound that appear under extreme pressures at room temperature.

Silica has some interesting crystalline phases beyond the familiar and well-understood forms of quartz, cristobalite, and tridymite. It forms coesite—in which a silicon atom is surrounded by four oxygens—under high pressure and at high temperatures.

At even greater pressures, silica takes the even denser form of stishovite, in which a silicon atom is surrounded by six oxygens. And, at still higher pressures, experiments show that silica forms even denser post-stishovite.

In the absence of high pressure, silica forms many others polymorphs as temperatures increase.

Until now, scientists thought that an intermediate, amorphous phase existed between these distinct phases.

Using single-crystal synchrotron X-ray analysis, however, the Carnegie scientists identified five new crystalline phases of silica that form after the material transitions from coesite.

According to a Carnegie press release, “The team, including Carnegie’s Qingyang Hu, Jinfu Shu, Yue Meng, Wenge Yang, and Ho-Kwang, ‘Dave’ Mao, demonstrated that under a range from 257,000 to 523,000 times normal atmospheric pressure (26 to 53 gigapascals), a single crystal of coesite transforms into four new, co-existing crystalline phases before finally recombining into a single phase that is denser than stishovite, sometimes called post-stishovite, which is the team’s fifth newly discovered phase. This transition takes place at room temperature, rather than the extreme temperatures found deep in the earth.”

0324ctt silica chart lo res

Simulation of the structural transition from coesite to post-stishovite. The silicon atoms (blue spheres) surrounded by four oxygen atoms (red spheres) are shown as blue tetrahedrons. The silicon atoms surrounded by six oxygen atoms are shown as green octahedrons. The intermediate phases are not filled in with color, showing the four stages that are neither all-blue like coesite nor all-green like post-stishovite. Credit: Ho-Kwang Mao

In addition to identifying the phases, the scientists built computational models that allowed them to calculate the transition paths from one phase to the next, precisely tracking how silica rearranges and reinvents itself at each step.

“Scientists have long debated whether a phase exists between the four- and six-oxygen phases,” Mao says in the press release. “These newly discovered four transition phases and the new phase of post-stishovite we discovered show the missing link for which we’ve been searching.”

Because the new silica phases form at high pressures, the results provide insight into transitions and states of matter within the Earth’s interior. Silica’s transition could provide insight into phase transitions in other crystalline materials, too.

The paper, published in Nature Communications, is “Polymorphic phase transition mechanism of compressed coesite” (DOI: 10.1038/ncomms7630).

0324ctt silica graph lo res

Free-energy landscape showing the transition pathway from coesite (blue) to high pressure post-stishovite (green), with the new phases indicated by arrows. Credit: Ho-Kwang Mao.