arXiv subjects

M. Mihalkovič

Publications and source records attributed to M. Mihalkovič.

8 recordsLinked to original sources

arXiv2026

Family of (NxH)-polytypes with La2WO6-related stoichiometry

Structure and chemical composition of non-stoichiometric $\mathrm{(N \times H)}$-polytypes, $\mathrm{N} \in \{3,\,4,\,5,\,6,\,7\}$, belonging to the family of La2WO6-related tungstates, are presented and the basic rules behind their construction are formulated. The polytypes are validated from the point of view of the internal energy per atom against all the competing La-O-W compounds by means of the ternary convex hull computed by DFT. According to DFT, the ground state of the La2WO6 family surprisingly turned out to be the as-yet hypothetical tungstate La2WO6.oP36, isostructural with the known compound Gd2WO6, closely related to the basic building H-block of the polytypes. One more candidate for an experimentally accessible low-energy structure, La2WO6.mC72, derived from the already observed Sm2MoO6, has been identified to lie only $ΔE \approx 5\:\mathrm{meV/at.} \cong 60\:\mathrm{K}$ above the convex envelope.

cond-mat.mtrl-sci↗

arXiv2024

Simulated structure and thermodynamics of decagonal Al-Co-Cu quasicrystals

Atomic structures of Al-Co-Cu decagonal quasicrystals (QCs) are investigated using empirical oscillating pair potentials (EOPP) in molecular dynamic (MD) simulations that we enhance by Monte Carlo (MC) swapping of chemical species and replica exchange. Predicted structures exhibit planar decagonal tilin g patterns and are periodic along the perpendicular direction. We then recalculate the energies of promising structures using first-principles density functional theory (DFT), along with energies of competing phases. We find that our $τ$-inflated sequence of QC approximants are energetically unstable a t low temperature by at least 3 meV/atom. Extending our study to finite temperatures by calculating harmonic vibrational entropy, as well as anharmonic contributions that include chemical species swaps and tile flips, our results suggest that the quasicrystal phase is entropically stabilized at temperatur es in the range 600-800K and above. It decomposes into ordinary (though complex) crystal phases at low temperatures, including a partially disordered B2-type phase. We discuss the influence of density and composition on QC phase stability; we compare the structural differences between Co-rich and Cu-rich quasicrystals; and we analyze the role of entropy in stabilizing the quasicrystal, concluding with a discussion of the possible existence of "high entropy" quasicrystals.

cond-mat.mtrl-sci↗

arXiv2011

Structure and stability of Al$_2$Fe

We employ first principles total energy and phonon calculations to address the structure and stability of Al$_2$Fe. This structure, which is reported as stable in the assessed Al-Fe phase diagram, is distinguished by an unusually low triclinic symmetry. The initial crystallographic structure determination additionally featured an unusual hole large enough to accommodate an additional atom. Our calculations indicate the hole must be filled, but predict the triclinic structure is unstable relative to a simpler structure based on the prototype MoSi$_2$. This MoSi$_2$ structure is interesting because it is predicted to be nonmagnetic, electrically insulating and high density, while the triclinic structure is magnetic, metallic and low density. We reconcile this seeming contradiction by demonstrating a high vibrational entropy that explains why the triclinic structure is stable at high temperatures. Finally, we note that Al$_5$Fe$_2$ poses a similar problem of unexplained stability.

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↗

arXiv2004

First-principles prediction of a decagonal quasicrystal containing boron

We interpret experimentally known B-Mg-Ru crystals as quasicrystal approximants. These approximant structures imply a deterministic decoration of tiles by atoms that can be extended quasiperiodically. Experimentally observed structural disorder corresponds to phason (tile flip) fluctuations. First-principles total energy calculations reveal that many distinct tilings lie close to stability at low temperatures. Transfer matrix calculations based on these energies suggest a phase transition from a crystalline state at low temperatures to a high temperature state characterized by tile fluctuations. We predict B$_{38}$Mg$_{17}$Ru$_{45}$ forms a decagonal quasicrystal that is metastable at low temperatures and may be thermodynamically stable at high temperatures.

cond-mat.mtrl-sci↗

arXiv2004

Cohesive energies of Fe-based glass-forming alloys

We calculate the cohesive energies of Fe-based glass-forming alloys in the B-Fe-Y-Zr quaternary system. Our {\it ab-initio} calculations fully relax atomic positions and lattice parameters yielding enthalpies of mixing at T=0K. We examine both the known equilibrium and metastable phases as well as a selection of plausible structures drawn from related alloy systems. This method, generally reproduces experimentally determined phase diagrams while providing additional information about energetics of metastable and unstable structures. In particular we can identify crystalline structures whose formation competes with the metallic glass. In some cases we identify previously unknown structures or observe possible errors in the experimental phase diagrams.

cond-mat.mtrl-sci↗

arXiv2003

Combined energy -- diffraction data refinement of decagonal AlNiCo

We incorporate realistic pair potential energies directly into a non-linear least-square fit of diffraction data to quantitatively compare structure models with experiment for the Ni-rich $d$(AlNiCo) quasicrystal. The initial structure models are derived from a few {\it a priori} assumptions (gross features of the Patterson function) and the pair potentials. In place of the common hyperspace approach to the structure refinement of quasicrystals, we use a real-space tile decoration scheme, which does not rely on strict quasiperiodicity, and makes it easy to enforce sensible local arrangements of the atoms. Inclusion of the energies provides information complementary to the diffraction data and protects the fit procedure from converging on spurious solutions. The method pinpoints sites which are likely to break the symmetry of their local environment.

cond-mat.mtrl-sci↗

arXiv2001

Total-energy-based prediction of a quasicrystal structure

Quasicrystals are metal alloys whose noncrystallographic symmetry and lack of structural periodicity challenge methods of experimental structure determination. Here we employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first principles considerations. We employ Monte Carlo simulations, taking as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition, and using a few features of the experimental Patterson function. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of $τ$, the golden mean.

cond-mat↗