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Wellcome Dr. Yildirim's Research Website
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Our research addresses structural, magnetic, and transport properties of novel materials with
an eye toward practical applications. This is done by calculating the properties of real materials
using first-principles computational techniques and testing the results by neutron scattering
and other measurements. Systems of particular interest include molecular solids such as the
fullerenes and cubane, nanoporous materials such as metal-organic frameworks for hydrogen storage,
frustrated magnetic systems such as the Kagome lattice and cuprates, novel superconductors
such as iron-pnictides, doped fullerenes and magnesium diboride, and nanomaterials such as
nanotubes and molecular magnets.
RECENT RESEARCH HIGHLIGHTS:
The Unprecedented Giant Magneto-Elastic Coupling in New Iron-based superconductors
T. Yildirim, Phys. Rev. Lett., 102, 037003 (2009)
From first principles calculations we unravel surprisingly strong interactions between arsenic ions
in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic
moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant
reduction in the c-axis. For
CaFe2As2
system, this reduction is as large as 1.4 Ang. (~13%).
Since the large c-reduction has been recently observed only under high-pressure,
our results suggest that the iron
magnetic moment should be present in Fe-pnictides at all times at ambient pressure.
The giant coupling of the on-site Fe-magnetic moment with the As-As bonding
that we have discovered here may provide a mechanism for the superconductivity.
Read more:
T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009) and
arXiv:0807.3936v2
(PRL's extended version)
Nature and Tunability of H2-Binding in MOFs
W. Zhou and T. Yildirim, J. Phys. Chem. C, 112 , 8132 (2008)
MOF compounds with exposed transition-metal (TM) sites exhibit impressive heats of adsorption of H2,
thus the TM-H2 interaction was believed to be of the "Kubas-type". Our calculation shows that the H2 binding
in Mn4Cl-MOF is not of the expected Kubas-type. Instead, the major contribution to the overall binding
comes from the classical Coulomb interaction, which is not screened due to the open-metal site.
We also show that the orientation of H2 has a surprisingly large effect on the binding potential,
reducing the classical binding energy by almost 30%. Read more...
Strong Dependence of H2-Binding on Metal Ions in MOFs
W. Zhou, H. Wu, and T. Yildirim,
J. Am. Chem. Soc., 130, 15268 (2008),
We conducted a systematic study of the H2 adsorption on a series of isostructural
MOFs, M2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The experimental Qst for H2 of these MOFs range from 8.5 to
12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni.
The H2 binding energies derived from DFT calculations follow the same trend.
We also found a strong correlation between the metal ion radius, the M-H2 distance
and the H2 binding strength, which provides a viable, empirical method
to predict the relative H2 binding strength of different open metals.
Read more...
Metal-Ethylene Complexes for Hydrogen Storage
 In our recent studies
[Phys. Rev. B 76, 085434 (2007),
Phys. Rev. Lett. 97, 226102 (2006)],
we suggest that co-deposition of metals (i.e. Ti/Li) with small organic molecules such
as very cheap ethylene molecule (a ho-hum material that is the building block of the
most common plastic) into nanopores of low-density high surface materials could be
a very promising direction for discovering new materials with better storage properties.
We found to our surprise that the interaction of Ti with the C=C double bond of
ethylene molecule (i.e. C2H4 ) mimics what we found in C60.
Detailed first-principles calculations show that the complex resulting
from attaching a Ti atom to each ends of C2H4 (see figure) will reversibly
adsorb ten H2 molecules. The equivalent material gravimetric capacity of 14%,
if realized in practice, would readily exceed the 2015 DOE system goal.
Another advantage is that, unlike our previously predicted structures
involving fullerenes and nanotubes, the metal-ethylene complexes
have actually been synthesized and actively studied as catalytic
systems for several decades. Their potential for hydrogen storage
was first revealed by our theory and modeling work.
Some preliminary experimental results demonstrating 14 wt% H2 adsorption
at 300K on Ti-C2H4 complexes formed by laser ablation has been recently published in
Phys. Rev. Lett. 100, 105505 (2008).
Press Coverage: physorg.com,
greencarcongress.com,
azonano.com,
treehugger.com,
NikkeiBP_Nozawa.pdf,
cnpublications.net
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Last updated: Thursday 22nd of January 2009 12:23:09 PM |
Today's date: Saturday 04th of July 2009 12:59:05 AM |
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