MS&E Colloquium: Prof. Richard G. Hennig, Cornell University
Date: April 24, 2009 from 2:00 pm to 3:00 pm EDT
Location: Columbia University
Morningside Campus
Interschool Lab - 750 CEPSR
Contact: For further information regarding this event, please contact Chad Gurley by sending email to cg2029@columbia.edu .
Info: Click Here to Visit Website.
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Prof. Richard G. Hennig, Cornell University

Slide 1
Creatio Ex Nihilo or the Ab Initio Prediction of
Crystal Structure Formation

The ancient Greek philosopher Parmenides of Elea stated the nothing could come from nothing or in its latin form "ex nihilo nihil fit." A few years later Leucippus of Miletus and his student Democritus speculated that matter is composed of indivisible components, atomV or atoms. In this talk I will show that blind random search method coupled to ab-initio relaxations on modern supercomputers can accurately predict how atoms arrange into crystal structure without any prior information about the system or in other words "creatio ex nihilo."

We apply the method to search for high-pressure crystal phases in the Li-Be system. High pressure can affect electronic structure and crystal packing, and in some cases even induce compound formation between elements that do not bond under ambient conditions. Our computational study for the Li-Be system shows that the reactivity of Li and Be is fundamentally altered by pressure. These two lightest of all metallic elements are immiscible at ambient conditions. Using structure search methods1,2.
we discover four stoichiometric LixBe1-x compounds that are stable over a range of pressures3. The electronic density of states of one of them displays a remarkable step-like feature and plateau at the bottom of the valence band, which is typical of a quasi-2D electronic structure and rather unexpected in a 3D crystal environment. We attribute this feature to large size differences between the ionic cores of Li and Be. Under increased pressure, the Li cores start to overlap and thereby expel valence electrons into quasi-two-dimensional layers characterized by delocalized free-particle-like states in the vicinity of Be ions. These alloys are also interesting from the perspective of superconductivity.  The Tc in the LiBe alloy is expected to be substantially higher than that of the component elements due to the increased density of states at the Fermi energy compared particularly to elemental Be.

1 .C. J. Pickard and R. J. Needs. High-Pressure Phases of Silane. Phys. Rev. Lett. 97, 045504 (2006)
2. D. G. Pettifor.
A Chemical Scale for Crystal-Structure Maps. Solid State Commun. 51, 31 (1984).
3. J. Feng, R. G. Hennig, N. W. Ashcroft and Roald Hoffmann.
Emergent reduction of electronic state dimensionality in dense ordered Li-Be alloys. Nature 451, 445 (2008).

Hosted by Prof. Chris Marianetti.