"For decades it was believed that MgO is the only
thermodynamically stable magnesium oxide, and it was widely believed to be one
of the main materials of the interiors of the Earth and other planets,"
said Qiang Zhu, the lead author of this paper and a postdoctoral student in the
Oganov laboratory.
"We have predicted that two new
compounds, MgO2 and Mg3O2,
become stable at pressures above one and five million atmospheres,
respectively. This not only overturns standard chemical intuition but also
implies that planets may be made of totally unexpected materials. We have
predicted conditions (pressure, temperature, oxygen fugacity) necessary for
stability of these new materials, and some planets, though probably not the
Earth, may offer such conditions," added Oganov.
In addition to their general chemical
interest, MgO2 and Mg3O2 might be important planet-forming
minerals in deep interiors of some planets. Planets with these compounds would
most likely be the size of Earth or larger.
The team explained how its paper predicted
the structures in detail by analyzing the electronic structure and chemical
bonding for these compounds. For example, Mg3O2 is forbidden within "textbook
chemistry," where the Mg ions can only have charges "+2," O ions
are "-2, and the only allowed compound is MgO. In the
"oxygen-deficient" semiconductor Mg3O2, there
are strong electronic concentrations in the "empty space" of the
structure that play the role of negatively charged ions and stabilize this
material. Curiously, magnesium becomes a d-element (i.e. a transition metal)
under pressure, and this almost alchemical transformation is responsible for
the existence of the "forbidden" compound Mg3O2.
The findings were made using unique
methods of structure prediction, developed in the Oganov laboratory.
"These methods have led to the discovery of many new phenomena and are
used by a number of companies for systematically discovering novel materials on
the computer -- a much cheaper route, compared to traditional experimental
methods," said Zhu.
"It is known that MgO makes up about
10 percent of the volume of our planet, and on other planets this fraction can
be larger. The road is now open for a systematic discovery of new unexpected
planet-forming materials," concluded Oganov.
Structures of the newly predicted magnesium oxides: On the left, MgO2;
on the right, Mg3O2. Green – Mg atoms, red – O atoms. Isosurfaces show regions
of high electron localization.
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