Martin Friák

The Czech Academy of Sciences | AVCR · Department of Structure of Materials

For undoped SnO2, room temperature ferromagnetism could be seen uniquely in 2-dimensional configurations, particularly in ultra-thin films (whose thickness is ideally below 100 nm). Both bulk samples and nano-powders of pristine SnO2 are diamagnetic, indicating that a 2D surface is a key point in shaping up the magnetic properties in SnO2. As a com...

We have performed a theoretical study of three polymorphs of Pb-supersaturated Sn-rich Pb-Sn alloys with the \(\alpha \)-Sn, \(\beta \)-Sn, and a simple-hexagonal \(\gamma \)-Sn structure employing quantum-mechanical calculations. We focused on structure–property relations in the case of lattice parameters, thermodynamic stability, elastic properti...

Experimentally it is shown that without any oxygen manipulation for TiO2, a strong room temperature ferromagnetism could be expected only in ultra-thin films, with the ideal thickness below 100 nm. Both bulks and nano-powders of TiO2 are diamagnetic, indicating that the surface and its nano-sublayers play very important roles in tailoring the magne...

We report on a combined experimental and theoretical study of stoichiometric Heusler-structure Ni2MnSn. Our detected Mössbauer spectrum of Sn atoms can be fitted as a combination of four components. As only a single component is expected in the defect-free Ni2MnSn, we have performed quantum-mechanical calculations of Ni2MnSn with selected point def...

Quantum-mechanical calculations have become an indispensable tool for computational materials science due to their unprecedented versatility and reliability. Focusing specifically on the Density Functional Theory (DFT), the reliability of its numerous implementations was tested and verified mostly for pure elements. An extensive testing of binaries,...

We have performed a theoretical study of three different allotropes of Pb-supersaturated Sn-rich Pb-Sn alloys with the α-Sn, β-Sn and a simple hexagonal γ-Sn structure employing quantum-mechanical calculations. Structure-property relations were analyzed in the case of the lattice parameters, thermodynamic stability, elastic properties and mechanica...

We have performed a theoretical study of three different allotropes of Pb-supersaturated Sn-rich Pb-Sn alloys with the α-Sn, β-Sn and a simple hexagonal γ-Sn structure employing quantum-mechanical calculations. Structure-property relations were analyzed in the case of the lattice parameters, thermodynamic stability, elastic properties and mechanica...

Point defects can play a very positive role - they can stabilize some phases which become significantly off-stoichiometri - see results of our theoretical study (quantum-mechanical calculations: formation energy, phonons, elasticity) in which we explain experimental findings related to the tau-phase Al2Ge2Mg

We present a combined experimental and theoretical study of surface-related magnetic states in TiO2. Our experiments on nano-sized thin films of pure TiO2 have suggested that the observed room-temperature magnetism originates from defects, in particular, from the surface of thin films as well as from point defects, such as oxygen vacancies located...

We present a Python package for the efficient generation of special quasi-random structures (SQS) for atomic-scale calculations of disordered systems. Both, a Monte-Carlo approach or a systematic enumeration of structures can be used to carry out optimizations to ensure the best optimal configuration is found for given cell size and composition. We...

We have performed a combined experimental and theoretical investigation of the vibrational properties of Ni2MnSn Heusler alloys. Sn-partial vibrational density of states (VDOS) of Sn119 was measured by nuclear resonant inelastic x-ray scattering at temperatures of 15 and 300 K, while magnetism and local environment of Sn was resolved by Sn119 Mössb...

We have performed a quantum-mechanical study of thermodynamic, elastic, magnetic and structural properties of four different ferrimagnetic states in Ni1.9375Mn1.5625Sn0.5 martensite. They are modeled by the four-layer modulated 4O structures with Mn-excess atoms randomly distributed in Ni and Sn sublattices. The Mn atoms at the Ni sublattice turn o...

We have performed a multi-methodological theoretical study of impact of thermal vibrations on the Mössbauer effect in the tetragonal β-phase of tin. We have seamlessly combined (i) atomic-scale numerical data in the form of mean square displacements of Sn atoms determined by quantum-mechanical calculations, (ii) continuum-level thermodynamic modeli...

We have performed a quantum-mechanical study of the influence of antiphase boundaries (APBs) on the temperature dependence of selected materials properties of Fe3Al. We show that the studied APBs very strongly affect thermal vibrations of Fe3Al and reduce the width of the band gap in phonon frequencies. Our results also show that the Fe3Al with APB...

Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transformation energetics between bcc and hcp ord...

The scanning electron microscopy, X-ray diffraction, positron annihilation spectroscopy, Mössbauer spectrometry, and measurements of magnetic characteristics by vibrating sample magnetometer complemented by theoretical simulations are applied in the present investigations of the Fe-Al-Co alloys with 25 at.% Al and Co substituting Fe in amount of 15...

Quantum computers are reaching one crucial milestone after another. Motivated by their progress in quantum chemistry, we performed an extensive series of simulations of quantum-computer runs that were aimed at inspecting the best-practice aspects of these calculations. In order to compare the performance of different setups, the ground-state energy...

We have performed an ab initio study of disordered ferrimagnetic Ni1.9375Mn1.5625Sn0.5 martensite. Employing the supercell approach combined with the special quasi-random structure concept for modeling of disordered states we have determined thermodynamic, magnetic, structural, elastic and vibrational properties of the studied material. Its atomic...

New approaches into computational quantum chemistry can be developed through the use of quantum computing. While universal, fault-tolerant quantum computers are still not available, and we want to utilize today’s noisy quantum processors. One of their flagship applications is the variational quantum eigensolver (VQE)—an algorithm for calculating th...

Exposure to particulate air pollution has been associated with a variety of health problems. One of the main sources of metal-rich airborne particulate pollution in roadside environments are brake-wear emissions. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Mössbauer spectroscopy (MS), X-ray powder diffraction...

Quantum computers are reaching one crucial milestone after another. Motivated by their progress in quantum chemistry, we have performed an extensive series of simulations of quantum-computer runs that were aimed at inspecting best-practice aspects of these calculations. In order to compare the performance of different set-ups, the ground-state ener...

Pressure study of the Heusler Ni2MnSn-based alloys revealed divergent values of the pressure effects on magnetic properties within a group of the off-stoichiometric alloys. An increase in temperature of the structural martensitic transition in the Mn-rich alloys under pressure is adequate for volume changes that accompanied the structural transitio...

Single crystal of the stoichiometric Ni2MnSn alloy (cubic L21 crystal structure) was prepared by the Czochralski method. The values of the magneto-crystalline anisotropy constant K1 have been determined at temperature 10 K under ambient and high hydrostatic pressures, K1 = + 0.17 × 10⁴ and +1.96 × 10⁴ J/m³ (0.7 GPa), respectively. The pressure-indu...

Quantum computing brings a promise of new approaches into computational quantum chemistry. While universal, fault-tolerant quantum computers are still not available, we want to utilize today's noisy quantum processors. One of their flagship applications is the variational quantum eigensolver (VQE) -- an algorithm to calculate the minimum energy of...

In this study, we performed a quantum mechanical examination of thermodynamic, structural, elastic, and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered B2-type lattice with and without antiphase boundaries (APBs) with (001) crystallographic orientation. Fe2CoAl was modeled using two different 54-atom supercell...

We have performed an ab initio study of vacancy-induced changes in thermodynamic, structural and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered (i) two-sublattice B2 phase or (ii) single-sublattice A2 phase. The two polymorphs of slightly non-stoichiometric Fe2CoAl (Fe27Co14Al13) were modeled by two different...

We performed a quantum mechanical study of segregation of Cu atoms toward antiphase boundaries (APBs) in Fe3Al. The computed concentration of Cu atoms was 3.125 at %. The APBs have been characterized by a shift of the lattice along the ⟨001⟩ crystallographic direction. The APB energy turns out to be lower for Cu atoms located directly at the APB in...

We performed a quantum-mechanical molecular-dynamics (MD) study of Fe3Al with and without hydrogen atoms under conditions of uniaxial deformation up to the point of fracture. Addressing a long-lasting problem of hydrogen-induced brittleness of iron-aluminides under ambient conditions, we performed our density-functional-theory (DFT) MD simulations...

Although the general instability of the iron nitride γ′-Fe4N with respect to other phases at high pressure is well established, the actual type of phase transitions and equilibrium conditions of their occurrence are, as of yet, poorly investigated. In the present study, samples of γ′-Fe4N and mixtures of α Fe and γ′-Fe4N powders have been heat-trea...

We have performed a quantum-mechanical study of a series of stoichiometric Ni2MnSn structures focusing on pressure-induced changes in their magnetic properties. Motivated by the facts that (i) our calculations give the total magnetic moment of the defect-free stoichiometric Ni2MnSn higher than our experimental value by 12.8% and (ii) the magnetic s...

We performed a quantum-mechanical study of the effect of antiphase boundaries (APBs) on structural, magnetic and vibrational properties of Fe 3 Al compound. The studied APBs have the {001} crystallographic orientation of their sharp interfaces and they are characterized by a 1/2 111 shift of atomic planes. There are two types of APB interfaces form...

To shed a new light on the complex microstructural evolution in the Ti–Al–Mo system, we employ ab initio calculations to study bcc-fcc structural transformations of ordered βo−TiAl(+Mo) and disordered β−TiAl(+Mo) to ordered γ−TiAl(+Mo) and hypothetically assumed disordered γdis−TiAl(+Mo) alloys, respectively. In particular, tetragonal (Bain's path)...

We present a quantum-mechanical study of silver decahedral nanoclusters and nanoparticles containing from 1 to 181 atoms in their static atomic configurations corresponding to the minimum of the ab initio computed total energies. Our thermodynamic analysis compares T = 0 K excess energies (without any excitations) obtained from a phenomenological a...

Three Fe-Al-based alloys, a binary with 22 at. % of Al, a ternary with 22 at. % of Al and 7 at. % of Ti and a quaternary with 22 at. % of Al, 4 at. % of Ti and 4 at. % of Nb prepared by arc melting to small button type ingots were studied by small punch test and small punch creep test in order to obtain the high temperature tensile and creep proper...

Creep of an alloy based on the intermetallic compound Fe2AlCo was studied by compressive creep tests in the temperature range from 873 to 1073 K. The stress exponent n and the activation energy of creep Q were determined using the multivariable regression of the creep-rate data and their description by means of sinh equation (Garofalo equation). Th...

We have performed a quantum-mechanical study of a B2 phase of Fe 70 Al 30 alloy with and without antiphase boundaries (APBs) with the {001} crystallographic orientation of APB interfaces. We used a supercell approach with the atoms distributed according to the special quasi-random structure (SQS) concept. Our study was motivated by experimental fin...

We present a quantum-mechanical study of thermodynamic, structural, elastic, and magnetic properties of selected antiphase boundaries (APBs) in Fe 3 Al with the D0 3 crystal structure with and without Cr atoms. The computed APBs are sharp (not thermal), and they have {001} crystallographic orientation. They are characterized by a mutual shift of gr...

Superlattice architecture represents an effective strategy to improve performance of hard protective coatings. Our model system, MoN/TaN, combines materials well-known for their high ductility as well as a strong driving force for vacancies. In this work, we reveal and interpret peculiar structure-stability-elasticity relations for MoN/TaN combinin...

To facilitate the understanding of Invar effects and design of FeNiCo-base Invar alloys characterized by low thermal expansion coefficient (TEC), we investigated the magnetic and thermal expansion behavior of an equiatomic prototype medium entropy alloy FeNiCo and a non-equiatomic (super Invar) Fe 63 Ni 32 Co 5 (at. %) reference alloy by means of e...

We introduce a new web-based tool called MELASA (Multi-phase ELAStic Aggregates), open-access available at https://melasa.cerit-sc.cz, for computations and visualizations of anisotropic elastic properties of lamellar (nano-)composites. MELASA implements a linear-elasticity method by Grimsditch and Nizzoli (1986), originally developed for superlatti...

Results of creep tests of two Fe-27 at. % Al-based alloys with additions of 2.7 and 4.8 at. % of niobium conducted in the temperature range from 650 °C to 900 °C in the authors’ laboratory are presented. The purpose of the study is to supplement previous work on Fe-Al-Nb alloys to obtain a more complete overview of creep properties from the dilute...

We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe 71 Al 22 Ti 7 , we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into a matrix). The chemical composition of both phases,...

The aim of the paper is to address fracture problems in nanoscale-sized cracked components using a simplified form of the strain gradient elasticity theory aided by ab initio calculations. Quantification of the material length scale parameter l1 of the simplified form of the strain gradient elasticity theory plays a key role in the analysis. The pa...

Superlattice architecture represents an effective strategy to improve performance of hard protective coatings. Our model system, MoN/TaN, combines materials well-known for their high ductility as well as a strong driving force for vacancies. In this work, we reveal and interpret peculiar structure-stability-elasticity relations for MoN/TaN combinin...

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is prim...

The temperature and magnetic field dependences of magnetization of the Cantor CrMnFeCoNi alloy were studied at ambient and high (up to 1 GPa) external pressure. The low-temperature spin-glass-like behavior and the paramagnetic behavior at temperatures above 100 K with effective paramagnetic moment of the alloy, meff = 2.71 μB/f.u., were observed. A...

Combining first-principles and experimental techniques, we establish Young’s modulus de- pendence of AlN/TiN superlattices on the film-forming phases, their thicknesses and crystallo- graphic orientations. The disparate character of cleavage properties within different layers of the superlattice—providing a hint about crack initiation processes—is...

The structural, mechanical, thermodynamic and electronic properties of intermetallic compounds Al 3 Ti and Al 3 V in L1 2 structure in the pressure range of 0-100 GPa have been investigated using first-principles method. The calculated structural parameters and energy information at ambient pressure are consistent with the available experimental an...

https://damask.mpie.de Editor's choice: https://www.sciencedirect.com/science/article/pii/S0927025618302714?via%3Dihub Crystal Plasticity (CP) modeling is a powerful and well established computational materials science tool to investigate mechanical structure–property relations in crystalline materials. It has been successfully applied to study...