Mystery of two-dimensional quasicrystal formation from metal oxides solved

The structure of two-dimensional titanium oxide inhibits at high temperatures by adding barium; instead of regular hexagons, rings of four, seven, and ten atoms are occasionally created in that order. A team at Martin Luther University Halle-Wittenberg (MLU) made this discovery in collaboration with researchers from the Max Planck Institute (MPI) for Microstructure Physics, the Université Grenoble Alpes and the National Institute of Standards and Technology (Gaithersburg, USA), through it. solving the mystery of two-dimensional quasicrystal formation from metal oxides. Their findings were published in the journal “Nature Communications”.

Hexagons are frequently found in nature. The most famous example is honeycomb, but graphene or various metal oxides, such as titanium oxide, also form this structure. “Hexagons are an ideal pattern for periodic arrangements,” explains Dr Stefan Förster, researcher in the Surface and Interface Physics group at MLU’s Institute of Physics. “They fit together so perfectly that there are no gaps.” In 2013, this group made an astonishing discovery when they deposited an ultrathin layer containing titanium oxide and barium on a platinum substrate and heated it in an ultrahigh vacuum to about 1,000 degrees Celsius. The atoms arranged themselves into triangles, squares and diamonds that clustered into even larger symmetrical shapes with twelve edges. A structure with 12-fold rotational symmetry was created, instead of the expected 6-fold periodicity. According to Förster, “quasicrystals have been created that have an aperiodic structure. This structure is made of basic atomic clusters that are highly ordered, even if the systematics behind this ordering are difficult for the observer to discern.” The physicists from Halle were the first worldwide to demonstrate the formation of two-dimensional quasicrystals in metal oxides.

The mechanisms underlying the formation of such quasicrystals have remained enigmatic since their discovery. The physicists at MLU have now solved this mystery in collaboration with researchers from the Max Planck Institute for Microstructure Physics Halle, the Université Grenoble Alpes and the National Institute of Standards and Technology (Gaithersburg, USA). Using extensive experiments, energetic calculations and high-resolution microscopy, they showed that high temperatures and the presence of barium create a network of titanium and oxygen rings with four, seven and ten atoms respectively. “The barium both breaks up the atomic rings and stabilizes them,” explains Förster, who heads the joint project. “One barium atom is embedded in a ring of seven, two in a ring of ten.” This is possible because the barium atoms interact electrostatically with the platinum support, but do not form a chemical bond with the titanium or oxygen atoms.

With their latest discovery, the researchers did more than just clarify a fundamental question of physics. “Now that we have a better understanding of the formation mechanisms at the atomic level, we can try to produce such two-dimensional quasicrystals on demand in other application-relevant materials such as metal oxides or graphene,” says Förster. “We are excited to learn whether this special arrangement will produce entirely new and usable properties.”