Siberian Federal University: For the First Time Scientists Observe a Unique Rearrangement of Hybridization in a Lithium-Borate Crystal at Extremely Low Pressure
Scientists from Russia and China have been the first in the world to discover and describe the change in the structure in a lithium-borate crystal under the influence of pressure. The triangular coordination of oxygen atoms to boron (BO3) inside the crystal, known to chemists, changed to tetrahedral (BO4) at a pressure of 2.85 GPa. Previously, such rearrangements of atomic coordinations were deemed impossible at such low pressures since they are connected with a change in the hybridization of molecular orbitals. According to the scientists, this precedent can open up broad prospects for changing already known materials and lead to the emergence of new technologies.
Hybridization is a process that scientists have hypothetically predicted, and knowledge about which is growing. It is considered that hybridization occurs with electron orbitals in atoms during the formation of bonds — in particular, during the creation of molecules and crystals. Different orbitals mix and form several of the same. For example, one s-orbital and two p-orbitals of a carbon atom can mix and form three hybrid sp2 orbitals that are equal, but located at an angle of 120° to each other. If many carbon atoms with such hybridization combine into a hexagonal grid, we obtain graphene. Several layers of graphene, formed on top of each other, become graphite. And if graphite is subjected to high pressure, there will be a chance that one s-orbital and three p-orbitals of a carbon atom form four hybrid sp3 orbitals . These orbitals will be located in space at an angle of 109.5 ° to each other — this shape resembles a tetrahedron. When many carbon atoms with this hybridization combine with each other, forming a three-dimensional framework, you will get the most durable material in the world — diamond. Thus, it is clear that hybridization is very important and is still difficult to manage in different materials.
“Our group’s work is fundamental. Previously, the transformation of sp1 or sp2 into sp3 was observed exclusively in crystals at enormous pressures of about 10 GPa. This pressure can be reduced by very high temperatures and (or) the presence of expensive catalysts. For the first time in the history of chemistry, we showed the transition of sp2 to sp3 hybrid orbitals at a relatively low pressure of 2.85 gigapascals. Until now, scientists around the world had no idea that this was even possible. We believe that this discovery will radically change many technologies and lead to the creation of new devices. For example, hybridization is an excellent way to compact compound structures and achieve unique physical and chemical properties such as superhardness, superconductivity, and ultrahigh energy density,” said Maxim Molokeev, associate professor of the Basic Department of Physics of Solid Body and Nanotechnologies of the School of Engineering Physics and Radio Electronics, SibFU.
To achieve this effect, the scientists conducted X-ray experiments using synchrotron radiation on a crystal of lithium metaborate (LiBO2). The crystal was subjected to various pressures (from 0 to 15 GPa) in diamond anvils. At 2.85 GPa, the scholars found that the X-ray pattern changed dramatically. The experts determined the structure of this phase based on the obtained X-ray pattern. The structure was solved by Maxim Molokeev, SibFU scientist.
“It is quite difficult to solve the structure from the powder experiment in general; however, it turned out to be possible. I was lucky to be the first who saw this structure, and I was bewildered. It was quite different from the original structure of LiBO2 because a different coordination appeared in the new phase, and BO4 tetrahedra appeared. The density of the compound sharply and abruptly increased by approximately 23%, which is linked to a change in the packing of atoms. This is also a rare case that has not been observed before in solids. Many checks as well as testing other structural models only confirmed the unusual structural type of the boron crystal, which turned out to be absolutely new and previously unexplored,” noted Maxim Molokeev.
The density jumps in a solid material recorded during the experiment are unique. The researchers suggested that during the pressure on the sample in the anvil, the crystallites experienced stress transfer from the ionic lattice of lithium ions to the BO3 molecular groups. The external pressure applied to the crystal probably adds up to this local stress and leads to a trigger transition of hybrid orbitals in boron from sp2 to sp3. Next, the boron coordination changes from BO3 triangles to BO4 tetrahedra.
The experts emphasize that this mechanism still requires further study, and as a result, a new excellent tool for changing the hybridization of atoms in other crystals may open up. The ability to control molecular orbitals by small means (for example, low pressure) cannot be overestimated.