arXiv subjects

M. Mihalkovic

Publications and source records attributed to M. Mihalkovic.

16 recordsLinked to original sources

arXiv2013

Prediction of stable insulating intermetallic compounds

We explore the stability of structure exhibiting hybridization gaps across a broad range of binary and ternary intermetallic compositions by means of band structure and total energy calculations. This search reveals previously unknown metal-based insulators, some with large gaps exceeding 1 eV, such as Al2Fe and Al4IrRe. We confirm large gaps using a hybrid density functional including exact exchange, and predict a gap of 2.2 eV for AlMnSi in the Pearson type tP6 structure, which is a chemically ordered ternary variant of the prototype MoSi2 (Pearson type tI6) structure.

cond-mat.mtrl-sci↗

arXiv2011

Ab-initio tiling and atomic structure for decagonal ZnMgDy quasicrystal

We discover the detailed atomic structure of $d$-MgZnY, a stable decagonal quasicrystal alloy of the layered Frank-Kasper type, and related phases, using the "tiling and decoration" approach. The atoms have invariable sites in the rectangle and triangle tiles of a 10-fold-symmetric planar tiling. To discover the lowest-energy structures, we combine the methods of density functional theory (DFT) total energy calculations, empirical oscillating pair potentials (fitted to DFT), fitting effective Hamiltonians for tilings, and discovering optimum tiling structures using a nonlocal tile-reshuffling algorithm. We find a family of practically stable compounds with varying composition, including the decagonal quasicrystal and the known Mg$_4$Zn$_7$ phase these are more stable than competing icosahedral structures by a small margi

cond-mat.mtrl-sci↗

arXiv2008

Matching rules from Al-Co potentials in an almost realistic model

We consider a model decagonal quasicrystal of composition Al$_{80.1}$Co$_{19.9}$ -- closely related to actual structures, and using realistic pair potentials -- on a quasilattice of candidate sites. Its ground state, according to simulations, is a Hexagon-Boat-Star tiling satisfying Penrose's matching rules. In this note, we rationalize these results in terms of the potentials; the Al-Co second-neighbor potential well is crucial.

cond-mat.mtrl-sci↗

arXiv2008

Penrose Matching Rules from Realistic Potentials in a Model System

We exhibit a toy model of a binary decagonal Al-Co quasicrystal -- closely related to actual structures -- in which realistic pair potentials yield a ground state which appears to perfectly implement Penrose's matching rules, for Hexagon-Boat-Star (HBS) tiles of edge 2.45 A. The second minimum of the potentials is crucial for this result.

cond-mat.mtrl-sci↗

arXiv2008

Empirical oscillating potentials for alloys from ab-initio fits

By fitting to a database of ab-initio forces and energies, we can extract pair potentials for alloys, with a simple six-parameter analytic form including Friedel oscillations, which give a remarkably faithful account of many complex intermetallic compounds. As examples we show results for (crystal or quasicrystal) structure prediction and phonon spectrum for three systems: Fe--B, Al--Mg--Zn, and Al--Cu--Fe.

cond-mat.mtrl-sci↗

arXiv2007

Symmetry-broken crystal structure of elemental boron at low temperature

The crystal structure of boron is unique among chemical elements, highly complex, and imperfectly known. Experimentalists report the beta-rhombohedral (black) form is stable over all temperatures from absolute zero to melting. However, early calculations found its energy to be greater than the energy of the alpha-rhombohedral (red) form, implying beta cannot be stable at low temperatures. Furthermore, beta exhibits partially occupied sites, seemingly in conflict with the thermodynamic requirement that entropy vanish at low temperature. Using electronic density functional theory methods and an extensive search of the configuration space we find a unique, energy minimizing pattern of occupied and vacant sites that can be stable at low temperatures but that breaks the beta-rhombohedral symmetry. Even lower energies occur within larger unit cells. Alternative configurations lie nearby in energy, allowing the entropy of partial occupancy to stabilize the beta-rhombohedral structure through a phase transition at moderate temperature.

cond-mat.mtrl-sci↗

arXiv2006

Energy-based Structure Prediction for d(Al70Co20Ni10)

We use energy minimization principles to predict the structure of a decagonal quasicrystal - d(AlCoNi) - in the Cobalt-rich phase. Monte Carlo methods are then used to explore configurations while relaxation and molecular dynamics are used to obtain a more realistic structure once a low energy configuration has been found. We find five-fold symmetric decagons 12.8 A in diameter as the characteristic formation of this composition, along with smaller pseudo-five-fold symmetric clusters filling the spaces between the decagons. We use our method to make comparisons with a recent experimental approximant structure model from Sugiyama et al (2002).

cond-mat.mtrl-sci↗

arXiv2006

Structure prediction for cobalt-rich decagonal AlCoNi from pair potentials

A systematic, decoration-based technique to discover the atomic structure of a decagonal quasicrystal, given pair potentials and experimentally measured lattice constants, is applied to the ``basic'' cobalt-rich decagonal Al-Co-Ni quasicrystal. First lattice-gas Monte Carlo simulations are performed, assuming the atomic sites are vertices of a rhombus tiling with edge 2.45 A. This phase is found to be dominated by 13 A diameter decagon-shaped clusters, each with a pentagon of Co atoms at the center. These, and another smaller cluster, decorate vertices of a ``binary tiling'' with rhombus edge 10.4 A. Further simulations with a restricted site list show that Al arrangements on the borders of the 13 A decagon cluster form Hexagon, Boat, and Star tiles with edge 2.45 A; they indicate specific sites for Co versus Ni atoms, and how the structure adapts to small composition changes. In the second half of the paper, relaxation (augmented by molecular dynamics annealing) is used to obtain realistic structures. The dominant new feature is a set of linear ``channels'' attractive to Al atoms and running transverse to the layers. Each is typically occupied by three atoms in four layers, implying puckering and a spontaneous period doubling to c $\approx$ 8 A. Puckering favors pentagonal long range order of the cluster orientations. Our simulation captures most features of the related $W$-AlCoNi crystal, except for its pentagonal bipyramid motif.

cond-mat.mtrl-sci↗

arXiv2005

Co-rich decagonal Al-Co-Ni: predicting structure, orientational order, and puckering

We apply systematic methods previously used by Mihalkovic et al. to predict the structure of the `basic' Co-rich modification of the decagonal Al70 Co20 Ni10 layered quasicrystal, based on known lattice constants and previously calculated pair potentials. The modelling is based on Penrose tile decoration and uses Monte Carlo annealing to discover the dominant motifs, which are converted into rules for another level of description. The result is a network of edge-sharing large decagons on a binary tiling of edge 10.5 A. A detailed analysis is given of the instability of a four-layer structure towards $c$-doubling and puckering of the atoms out of the layers, which is applied to explain the (pentagonal) orientational order.

cond-mat.mtrl-sci↗

arXiv2004

Stability of Fe-based alloys with structure type C6Cr23

Bulk metallic glass forms when liquid metal alloys solidify without crystalization. In the search for Iron-based bulk glass-forming alloys of the metal-metalloid type (Fe-B- and Fe-C-based), crystals based on the structural prototype C6Cr23 often preempt the amorphous phase. Destabilizing this competing crystal structure could enhance glass-formability. We carry out first-principles total energy calculations of enthalpy of formation to identify third elements that can effectively destabilize C6Cr23. Yttrium appears optimal among transition metals, and rare earths also are suitable. Atomic size is the dominant factor.

cond-mat.mtrl-sci↗

arXiv2004

Equilibrium structure of decagonal AlNiCo

We investigate the high temperature decagonal quasicrystalline phase of Al$_{72}$Ni$_{20}$Co$_8$ using a quasilattice gas Monte-Carlo simulation. To avoid biasing towards a specific model we use an over-dense site list with a large fraction of free sites, permitting the simulation to explore an extended region of perpendicular space. Representing the atomic surface occupancy in a basis of harmonic functions directly reveals the 5-fold symmetric component of our data. Occupancy is examined in physical and perpendicular space.

cond-mat.mtrl-sci↗

arXiv2004

Order-disorder transition in the Cd-Ca cubic approximant

Recent experiments discovered an order-disorder transition occuring at low temperatures in large unit 1/1 cell cubic approximants of the stable Cd-based binary alloy quasicrystals. The transition is related to correlations among orientational degrees of freedom whose separations are around 12 Å. We analyze the interactions between the degrees of freedom using {\em ab-initio} calculations for Cd-Ca alloys and derive an equivalent antiferromagnetic Ising model which shows a similar phase transition. However, the calculated transition temperature is higher than observed experimentally, indicating that the actual structure and its order-disorder transition are more complex than originally proposed. A side-benefit of our study is the discovery of a canonical-cell decoration model for the Cd-Ca icosahedral phase.

cond-mat.mtrl-sci↗

arXiv2004

Quasicrystal approximants with novel compositions and structures

We identify several new quasicrystal approximants in alloy systems in which quasicrystals have not been previously reported. Some occur in alloys with large size contrast between the constituent elements, either containing small Boron atoms, or large Ca/Eu atoms, leading to quasicrystal structures quite different from currently known systems where the size contrast is smaller. Another group of the approximants are layered Frank--Kasper structures, demonstrating competition between decagonal and dodecagonal ordering within this family of structures.

cond-mat.mtrl-sci↗

arXiv2002

Tile Hamiltonians for Decagonal Phases

A tile Hamiltonian (TH) replaces the actual atomic interactions in a quasicrystal with effective interactions between and within tiles. We study Al-Co-Ni and Al-Co-Cu decagonal quasicrystals described as decorated Hexagon-Boat-Star (HBS) tiles using {\em ab-initio} methods. A dominant term in the TH counts the number of H, B and S tiles, favoring tilings of H and B only. In our model for Al-Co-Cu, chemical ordering of Cu and Co along tile edges defines tile edge arrowing. Unlike the edge arrowing of Penrose matching rules, however, the energetics for Al-Co-Cu do not force quasiperiodicity. Energetically favored structures resemble crystalline approximants to which the actual quasicrystalline compounds transform at low temperature.

cond-mat↗

arXiv2002

Temperature-dependent "phason" elasticity in a random tiling quasicrystal

Both ``phason'' elastic constants have been measured from Monte Carlo simulations of a random-tiling icosahedral quasicrystal model with a Hamiltonian. The low-temperature limit approximates the ``canonical-cell'' tiling used to describe several real quasicrystals. The elastic constant K2 changes sign from positive to negative with decreasing temperature; in the ``canonical-cell'' limit, K2/K1 appears to approach -0.7, about the critical value for a phason-mode modulation instability. We compare to the experiments on i-AlPdMn and i-AlCuFe.

cond-mat.mtrl-sci↗

arXiv2000

Structure determinations for random-tiling quasicrystals

How, in principle, could one solve the atomic structure of a quasicrystal, modeled as a random tiling decorated by atoms, and what techniques are available to do it? One path is to solve the phase problem first, obtaining the density in a higher dimensional space which yields the_averaged_ scattering density in 3-dimensional space by the usual construction of an incommensurate cut. A novel direct method for this is summarized and applied to an i(AlPdMn) data set. This averaged density falls short of a true structure determination (which would reveal the typical_unaveraged_ atomic patterns.) We discuss the problematic validity of inferring an ideal structure by simply factoring out a ``perp-space'' Debye-Waller factor, and we test this using simulations of rhombohedral tilings. A second, ``unified'' path is to relate the measured and modeled intensities directly, by adjusting parameters in a simulation to optimize the fit. This approach is well suited for unifying structural information from diffraction and from minimizing total energies derived ultimately from ab-initio calculations. Finally, we discuss the special pitfalls of fitting random-tiling decagonal phases.

cond-mat.mtrl-sci↗