105 research outputs found
First-principles study of Ti-doped sodium alanate surfaces
We have performed first-principles calculations of thick slabs of Ti-doped
sodium alanate (NaAlH_4), which allows to study the system energetics as the
dopant progresses from the surface to the bulk. Our calculations predict that
Ti stays on the surface, substitutes for Na, and attracts a large number of H
atoms to its vicinity. Molecular dynamics simulations suggest that the most
likely product of the Ti-doping is the formation of H-rich TiAl_n (n>1)
compounds on the surface, and hint at the mechanism by which Ti enhances the
reaction kinetics of NaAlH_4.Comment: 3 pages with 3 postscript figures embedded. Uses REVTEX4 and graphicx
macros. More information at http://www.ncnr.nist.gov/staff/taner/alanates
Effective-Hamiltonian modeling of external pressures in ferroelectric perovskites
The phase-transition sequence of a ferroelectric perovskite such as BaTiO_3
can be simulated by computing the statistical mechanics of a first-principles
derived effective Hamiltonian [Zhong, Vanderbilt and Rabe, Phys. Rev. Lett. 73,
1861 (1994)]. Within this method, the effect of an external pressure (in
general, of any external field) can be studied by considering the appropriate
"enthalpy" instead of the effective Hamiltonian itself. The legitimacy of this
approach relies on two critical assumptions that, to the best of our knowledge,
have not been adequately discussed in the literature to date: (i) that the
zero-pressure relevant degrees of freedom are still the only relevant degrees
of freedom at finite pressures, and (ii) that the truncation of the Taylor
expansion of the energy considered in the effective Hamiltonian remains a good
approximation at finite pressures. Here we address these issues in detail and
present illustrative first-principles results for BaTiO_3. We also discuss how
to construct effective Hamiltonians in cases in which these assumptions do not
hold.Comment: 5 pages, with 2 postscript figures embedded. Proceedings of
"Fundamental Physics of Ferroelectrics, 2002", R. Cohen and T. Egami, eds.
(AIP, Melville, New York, 2002). Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_effp/index.htm
Structurally Triggered Metal-Insulator Transition in Rare-Earth Nickelates
Rare-earth nickelates form an intriguing series of correlated perovskite
oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator
electronic phase transition, a concurrent structural phase transition and a
magnetic phase transition toward an unusual antiferromagnetic spin order.
Appealing for various applications, full exploitation of these compounds is
still hampered by the lack of global understanding of the interplay between
their electronic, structural and magnetic properties. Here, we show from
first-principles calculations that the metal-insulator transition of nickelates
arises from the softening of an oxygen breathing distortion, structurally
triggered by oxygen-octahedra rotation motions. The origin of such a rare
triggered mechanism is traced back in their electronic and magnetic properties,
providing a united picture. We further develop a Landau model accounting for
the evolution of the metal-insulator transition in terms of the $R cations and
rationalising how to tune this transition by acting on oxygen rotation motions.Comment: Submitted in Nature Communicatio
Giant Direct and Inverse Electrocaloric Effects in Multiferroic Thin Films
Refrigeration systems based on compression of greenhouse gases are
environmentally threatening and cannot be scaled down to on-chip dimensions. In
the vicinity of a phase transition caloric materials present large thermal
responses to external fields, which makes them promising for developing
alternative solid-state cooling devices. Electrocaloric effects are
particularly well-suited for portable refrigeration applications; however, most
electrocaloric materials operate best at non-ambient temperatures or require
the application of large electric fields. Here, we predict that modest electric
fields can yield giant room-temperature electrocaloric effects in multiferroic
BiCoO (BCO) thin films. Depending on the orientation of the applied field
the resulting electrocaloric effect is either direct (heating) or inverse
(cooling), which may enable the design of enhanced refrigeration cycles. We
show that spin-phonon couplings and phase competition are the underlying causes
of the disclosed caloric phenomena. The dual electrocaloric response of BCO
thin films can be effectively tuned by means of epitaxial strain and we
anticipate that other control strategies like chemical substitution are also
possible
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