Complexity of Simple Metals, Or Orbital Alchemy

Newswise — Under pressure, chemistry turns upside down. There is increasing evidence that pressure turns simple metals, such as Na or Ca, into transition metals with large occupancy of d-orbitals, and this “alchemical” transformation is accompanied by exotic phenomena. Furthermore, chemistry of these elements is strongly affected by core electrons, traditionally thought to be chemically inactive. In 2009, Ma, Eremets and Oganov discovered a startling demetallization of sodium under pressure, turning this archetypal silvery metal into a red transparent crystal.

For calcium, high-pressure structural transformations remained unknown, with experiment and theory contradicting each other. Experimentally, pressure makes calcium the best known elemental superconductor. In an article published in the prestigious journal Proceedings of the National Academy of Sciences (US), Artem R. Oganov, Professor of Geosciences and Physics at Stony Brook University, and his colleagues investigate this mysterious element. With the help of Oganov’s powerful evolutionary methodology, 8 crystal structures were predicted to be stable for calcium in the pressure range 0-600 GPa. The largest stability field, 134-564 GPa, belongs to a complex non-periodic host-guest structure, with calcium atoms “schizophrenically” playing two very different structural roles. While the causes of formation of such a structure remain mysterious, “orbital alchemy” is certainly one of them. Calculations reconcile previous results and demonstrate the accuracy with which superconducting properties can be predicted, even across so many complex structural changes.

In another paper, published simultaneously in Physical Review Letters, the study of exotic metals under pressure is brought another step ahead. Graduate student Yu Xie and his supervisors Artem R. Oganov and Yanming Ma, Professor of Physics from Jilin University (China), studied the high-pressure behavior of CaLi2, a controversial yet seemingly simple material. They found a unique stable crystal structure consisting of “textbook” parts - alternating graphene-type layers of calcium and lithium atoms, pierced by one-dimensional chains of lithium atoms. Such a material is likely to possess interesting electronic properties – and, indeed, it is a superconductor with properties finally explaining experimental data. Through such studies researchers hope to learn more about chemical bonding, the state of matter in planetary interiors, and the ways to make useful materials with special electronic, optical or mechanical properties.

1. Oganov A.R., Ma Y.M., Xu Y., Errea I., Bergara A., Lyakhov A.O. (2010). Exotic behavior and crystal structures of calcium under pressure. Proc. Natl. Acad. Sci. 107, 7646-7651. 2. Xie Y., Oganov A.R., Ma Y. (2010). Novel structures and high pressure superconductivity of CaLi2. Phys. Rev. Lett. 104, 177005.