https://phys.org/news/2018-07-molecular-boron-based-nanoclusters.html

"Boron lanthanides are an important class of materials used in electronics and other applications, but nanoclusters of boron lanthanides have not been studied," said Lai-Sheng Wang, a professor of chemistry at Brown and senior author of a paper describing the work. "We have just started to investigate these nanoclusters, and here we show that they can have an interesting 'inverse sandwich' structure with the right combination of boron and lanthanide atoms."

The structure—a ring of bonded boron atoms with a single lanthanide atom bonded to each side—emerged in clusters made from eight boron atoms and two atoms of either lanthanum or praseodymium (both members of the lanthanide group on the periodic table). Sandwich structures—complexes in which two planar aromatic hydrocarbon molecules surround a single metal atom—are well known in chemistry and their discovery earned a Nobel Prize in 1973. Inverse sandwich structures are known to form in uranium-organic molecular complexes, Wang says, but this is the first time the structure has been seen in boron lanthanides.