Chris A. Marianetti

First-principles prediction of redox potentials in transition-metal compounds with LDA+ U

First-principles calculations within the local density approximation (LDA) or generalized gradient approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+U method with a self-consistent evaluation of the U parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li xMPO4 (M = Mn, Fe Co, Ni), layered LixMO2 (x = Co, Ni) and spinel-like LixM2O4 (M = Mn, Co), can be reproduced accurately.

Failure Mechanisms of Graphene under Tension

Recent experiments established pure graphene as the strongest material known to mankind, further invigorating the question of how graphene fails. Using density functional theory, we reveal the mechanisms of mechanical failure of pure graphene under a generic state of tension at zero temperature. One failure mechanism is a novel soft-mode phonon instability of the K1 mode, whereby the graphene sheet undergoes a phase transition and is driven towards isolated hexagonal rings resulting in a reduction of strength. The other is the usual elastic instability corresponding to a maximum in the stress-strain curve. Our results indicate that finite wave vector soft modes can be the key factor in limiting the strength of monolayer materials.

Phase transformations and volume changes in spinel LixMn2O4

Abstract First-principles methods have been used to calculate the phase diagram and volume expansion of spinel Li x Mn 2 O 4 as a function of lithium content. The calculations confirm the experimentally observed two phase region between x =1 and 2 and the ordered phase at x =1/2. In addition, the expected large step in voltage at x =1 is obtained. It is shown that these phenomena are qualitatively determined by the interactions of Li with each other and the Mn 2 O 4 host and only quantitatively influenced by the more subtle electronic effects such as Jahn–Teller distortions, charge ordering and magnetic excitations. The two-phase region between x =1 and 2 is found to be driven by strong repulsive interactions between lithium ions occupying adjacent tetrahedral 8a and octahedral 16c sites. The origin of the large volume change upon transforming from LiMn 2 O 4 to Li 2 Mn 2 O 4 is also investigated from first principles. The possible sources of the volume change are identified to be the intercalated lithium, the Jahn–Teller distortion, and the introduction of anti-bonding e g electrons into the Mn d-orbitals. The latter effect is found to be dominant. Some speculation is offered on how the large volume change upon lithiation of manganese dioxide can be prevented.

Site-selective Mott transition in rare-earth-element nickelates

A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare-earth-element nickelate perovskites arises from a site-selective Mott phase, in which the d electrons on half of the Ni ions are localized to form a fluctuating moment while the d electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of charge order. A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.

Phase separation in Li x FePO 4 induced by correlation effects

We report on a significant failure of LDA and GGA to reproduce the phase stability and thermodynamics of mixed-valence Li

S = 1 2 chains and spin-Peierls transition in TiOCl

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First-principles alloy theory in oxides

FePO

Physical adsorption and charge transfer of molecular Br2 on graphene.

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Three-dimensional metallic and two-dimensional insulating behavior in octahedral tantalum dichalcogenides

compounds. Experimentally, Li

Role of Hybridization in NaxCoO2 and the Effect of Hydration

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