Session D20: Focus Session: Computational Design of New Materials -- Structure/Property Relationships

Ab initio construction of structure-property relationships in crystals

While the cluster expansion formalism is traditionally used to parametrize the configurational-dependence of scalar properties (such as the total energy), this talk introduces a generalization of this widely used formalism to tensorial quantities (such as elastic constants, phase-transformation-induced strains, permanent dielectric dipoles, etc). This new method generates a suitable orthogonal basis for the space of all mappings from lattice configurations to tensors. It also provides symmetry rules to determine which terms in the Tensorial Cluster Expansion are equivalent by symmetry and must therefore share a common coefficient. The proposed framework encompasses, as special cases, a number of existing tools, including the local cluster expansion (used for modeling the properties of point defects), the ``symmetrized'' cluster expansions (used for predicting tensorial properties of disordered phases), and transferable force constants (used for efficient lattice dynamics calculations). This formalism also provides a simple language to describe the coupling between symmetry-breaking phase transformations and materials properties.   

Defect-Induced Magnetism in BaZnF4

By first-principles density functional theory with generalized gradient approximation(GGA), oxygen(nitrogen)-doped BaZnF$_{4}$ with O(N) substituting F at concentrations 3.125\% has been demonstrated to exhibt a ferromagnetic behavior. The results show that the strong localization of defect states favors spontaneous spin polarization and local moment formation. Defect manipulation mediates long-range magnetic interaction, which opens a new route to design high-\textit{Tc} diluted magnetic semiconductors(DMS).   

Design of half-metallic antiferromagnets: transition metal chalcogenides and pnictides

Half-metallic antiferromagnts are the materials that exhibit half-metallicity and antifirromagnetism (compensated ferrimagnetism) simultaneously. Such materials are especially useful for spintronics devices since they have 100 \% spin-polarized Fermi surfaces despite of their robustness against a disturbance of external magnetic field. We found that (XY)Z$_2$, where X and Y are transition metal elements and Z is a chalcogens or a pnictigen, show half-metallic antiferromagnetism when the sum of effective d electron numbers of X and Y is 10. Examples are (CrFe)S$_2$ and (CrFe)Se$_2$. We report a systematic investigation of the electronic structure and transport properties of these materials calculated by the KKR-Green's function method combined with the Kubo-Greenwood formula.   

Transition metal based borohydrides for hydrogen storage

Using \textit{ab-initio} studies based on the density-functional theory, we have calculated binding energies per hydrogen molecule for decomposition reactions of transition metal borohydrides \textit{MH}$_{x}B_{12}H_{12}$ to \textit{MB}$_{12}$ structures, where $M$ corresponds to Sc\textit{, Ti, or V}. Depending on the valence of the transition metal, $x$ can be 1$, 2$, or $3.$ Crystal structures considered for \textit{MB}$_{12}$ included both hypothetical and those found in the international crystallographic structural database. On the other hand, the crystal structure considered for \textit{MH}$_{x}B_{12}H_{12}$ belongs to C2/c (space group 15) structure as reported in a previous study [V. Ozolins \textit{et al.} JACS, \textbf{131,} 230 (2009)]. Among the structures investigated, Titanium-based metal borohydride structure has the lowest binding energy per hydrogen molecule relative to the cubic TiB$_{12}$ structure ($\sim $0.37 eV/H$_{2})$. Our finding should be contrasted with the binding energy/H$_{2}$ for simple metal based borohydrides ($e.g.,$ \textit{CaB}$_{12}H_{12}$ ), which has a value of $\sim $ 1.5 eV/H$_{2}$, suggesting that transition metals play a significant role in lowering the H$_{2}$ binding energy in borohydrides.   

A new family of chiral boron fullerenes and related planar boron sheet

Using the recent idea of balancing two-center and three-center bondings between boron atoms, a new class of stable chiral boron nanostructures (fullerenes and nanotubes) is designed. The structures of these fullerenes consist of triangular and hexagonal motifs similar to those seen in the most stable $\alpha$-boron sheet and the B$_{80}$ fullerene. The binding energy of the new sheet is only 0.02 eV lower than that of the $\alpha$-boron sheet. Our density functional calculations show that these new boron nanostructures are energetically competitive with the recently proposed $\alpha$-boron nanostructures. For example, B$_{60}$, the smallest member of this new 60n$^2$ boron fullerene family, is 0.7 eV more stable than exact boron equivalent of icosahedral C$_{60}$ fullerene.   

A study of doped semiconducting nanowires using linear-scaling density-functional theory

In recent years, the possibility of using semiconducting nanowires as building blocks for future nanoscale devices has generated considerable interest in understanding and theoretically predicting the effect of point defects in such structures. To this end, we present a fully {\em ab initio} study of both the neutral vacancy and gold substitutional defect in bulk silicon and SiNWs. We follow a systematic methodology for converging the defect formation energy using the supercell approach within plane-wave DFT [1]. Our results highlight the importance of using large supercells to accurately describe the long-ranged disturbance effects caused by point defects in crystal lattices. We show that the linear-scaling DFT code ONETEP [2] can be used to study defects in large systems with thousands of atoms without loss of accuracy, thereby allowing calculations on realistically sized NWs. We also compute maximally localized Wannier functions for the defect systems; these provide insight into the nature of the electronic bonds that are formed between atoms in the vicinity of the defect. Finally, we discuss phonon calculations on defect centres using ONETEP to determine the thermodynamic properties of these systems. [1] M. J. Probert and M. C. Payne, Phys. Rev. B {\bf 67}, 075204 (2003). [2] C.-K. Skylaris, P. D. Haynes, A. A. Mostofi, and M. C. Payne, J. Chem. Phys. {\bf 122}, 084119 (2005).   

The Atomic and Electronic Structures of GaN:ZnO Alloys

GaN:ZnO, a kind of nonisovalent alloys, currently holds the record of the water photo-splitting efficiency under visible light. The mechanism of the large band gap bowing of this alloy, however, is still not clear, due to the lack of knowledge of its detailed atomic structure. We developed a charge flow model based on electron counting rule, which describes the ab initio energies of different alloy atomic configurations of GaN:ZnO with an average error of only 7 meV/atom. This model Hamiltonian was used in Monte Carlo (MC) simulations to study the atomic structures of systems containing thousands of atoms. The equilibrium atomic structures from the MC simulations at different temperatures were then used to calculate their electronic structures. We found that at the experimental synthesis temperature of 1100 K, uniform alloy can be formed, albeit with a strong short range ordering. Consequently, their electronic structure is very different from the completely random alloy. The charge flow model was also applied to many other nonisovalent alloy systems with good accuracy.   

Small to medium atomic size mismatch leads to phase-separation yet very large mismatch can lead to spontaneous ordering.

Large atomic size-mismatch between compounds discourages their binding onto a common lattice because of the ensuing cost in strain energy. This central paradigm in the theory of isovalent alloys, is clearly broken by the occurrence of stable spontaneous long-range order in mixtures of alkali-halides with as much as ~40$\,\%$ size-mismatch (e.g LiF-CsF). Our theoretical analysis of these failures uncovered a different design principle for stable alloys: very large atomic size-mismatch can lead to spontaneous ordering if the large (small) components have the ability to raise (lower) their coordination number (CN) within the mixed phase. This heuristic design-principle leads us to explore via first-principles structure-search a few mismatched binary systems whose components have a propensity for CN disproportionation. We find ordered structures for BeO-BaO (37\,\% size-mismatch) and BeO-SrO (30\,\%), and ordering in LiCl-KCl (20\,\%). BN-InN (33\,\%) is lowers its positive formation enthalpy by $60\,\%$ with CN disproportionation. This design principle could be used to explore novel phases not expected to order according to the common paradigm of strain instability.   

Electronic Structure of Arenene-Transition Metal-Graphene Complexes

The interaction between the electronic structures of transition metals and pi-conjugated species leads to the formation of organometallics, of interest in particular for their catalytic properties. With the availability of high-quality graphene, new hybrid structures with controllable electronic properties become attractive for electronics. We report calculations of `sandwich` structures containing transition metals between aromatic molecules and graphene. First we identify the adsorption geometries of metal atoms with emphasis on the minimal clustering regimes. Then we find the adsorption structures and electronic properties of aromatic molecules on the metal-graphene substrate. The adsorption of aromatics is greatly enhanced by the metal atoms due to strong hybridization between their pi-orbitals with the metal d-states. We study the ground-state electronic and magnetic properties of these systems. We also analyze their electronic excitations that can be related to spectroscopy data.   

Inter-surface interactions in a 3-dimensional topological insulator : Bi$_2$Se$_3$ thin film

Recently much attention has focused on 3-dimensional strong topological insulators as a new quantum state of matter, such as Bi$_2$Se$_3$ and Bi$_2$Te$_3$. One of their intriguing features is a topologically protected surface state whose quasiparticle dispersion shows a Dirac cone. Due to lack of backscattering and robustness against disorder and interaction, surface states have the potential to be perfect conducting channels which carry not only charge but also spin currents. Here, we present a theoretical study of electronic structures and surfaces of thin film Bi$_2$Se$_3$ using the highly precise FLAPW method\footnote {Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)}. Our calculated results focus on the interaction between surface states on opposing sides of the slab. The gap opening from the inter-surface interaction can be easily explained by simple symmetry arguments considering both time-reversal and spatial inversion. For a 6 quintuple layer slab ($\sim$6 nm), a 1.06 meV gap at the $\overline{\Gamma}$ point survives due to the inter-surface interactions, and we discuss how to preserve the massless excitations despite this inter-surface interaction.   

Prediction of a new crystalline lithium phosphorus oxynitride -- Li$_2$PO$_2$N

Lithium phosphorus oxynitride materials have been investigated for many years, especially in relation to the thin film electrolye LiPON, developed at Oak Ridge National Laboratory.\footnote{ J. B. Bates {\em{et al}}, {\em{Solid State Ionics}} {\bf{53-56}} 647-654 (1992).} We have carried out first principles simulations of related crystalline materials in an attempt to understand the sources of stability and mechanisms of Li ion conductivity in these materials. Starting with crystalline LiPO$_3$ which has twisted phosphate chains,\footnote{E. V. Marashova, {\em{ Crystallography Reports}} {\bf{46}} 942-946 (2001).} we consider the possibility of modifying the structure by substituting N and Li for O. The optimized structures are computed to have regularized phosphate chains which form planar -P-N-P-N- backbones. To the best of our knowledge, the new predicted crystals, which we call $s_1$-Li$_2$PO$_2$N with a 24 atom unit cell and $s_2$-Li$_2$PO$_2$N with a 12 atom unit cell, have not yet been observed experimentally. We suggest several possible exothermic reaction pathways to synthesize these crystals.   

Highly Efficient (Cs$_{8}$V) Superatom based Spin-polarizer

Quantum transport through molecules and the possibility to manipulate spin has generated tremendous excitement. Here, we demonstrate unusual spin transport through a molecule of two Cs$_{8}$V magnetic superatoms. Calculations based on density functional theory and non-equilibrium Green's function methods find a much higher current for the spin-down charge carriers relative to the spin-up carriers in the model Au-(Cs$_{8}$V)-(Cs$_{8}$V)-Au device system with almost 100{\%} spin polarization, indicating a highly efficient spin polarizer. The new behavior is rooted in strong coupling of the localized magnetic core on V and the itinerant electrons of the Cs shell atoms leading to nearly full spin-polarization.