arXiv subjects

Andrzej Szajek

Publications and source records attributed to Andrzej Szajek.

6 recordsLinked to original sources

arXiv2026

Interplay between crystal structure and magnetism in CeCrB$_4$

CeCrB$_4$ belongs to the family of MTB$_4$ ternary metal borides, some of whose magnetic phases have recently been identified as quantum spin dimers. Employing density functional theory within the GGA+$U$ framework, this work investigates the key ground-state properties of this system, including its ferromagnetic ground state, the mixed-valence nature of cerium, and the charge transfer from the cationic cerium--chromium layers to the anionic boron layers. In particular, we focus on elucidating the bonding mechanism within the structurally embedded Cr-Cr dimers. Specifically, we evaluate a previously proposed interpretation that the localized Cr $3d_{z^2}$ states are molecular-like bonding and antibonding states. The calculated dependence of the $3d_{z^2}$ electronic structure on the intra-dimer distance correlates with the level-splitting characteristics known for classical homonuclear diatomic molecules.

cond-mat.mtrl-sci↗

arXiv2025

Effect of substitution La by Mg on electrochemical and electronic properties in La$_{2-x}$Mg$_x$Ni$_7$ alloys: a combined experimental and ab initio studies

La-Mg-Ni-based alloys are promising negative electrode materials for 3rd generation of Ni-MH$_x$ batteries. In this work, we investigate the effect of Mg substitution on the electrochemical and electronic properties of La$_{2-x}$Mg$_x$Ni$_7$ materials. The mechanical alloying technique is used to produce a series of La$_{2-x}$Mg$_x$Ni$_7$ alloys ($x$ = 0.00, 0.25, 0.50 and 0.75). The X-ray diffraction measurements indicate multiphase character of the samples with majority (La,Mg)$_2$Ni$_7$ phases of hexagonal Ce$_2$Ni$_7$-type and rhombohedral Gd$_2$Co$_7$-type. Electrochemical measurements show how the maximum discharge capacity ($C_{max}$) increases with Mg concentration and that reach the highest value of 304 mAh/g for La$_{1.5}$Mg$_{0.5}$Ni$_7$ ($x$ = 0.5). The experimental efforts are followed by the density functional theory (DFT) calculations performed with the full-potential local-orbital minimum-basis scheme (FPLO). To simulate chemical disorder, we use the coherent potential approximation (CPA). The calculations are focused on the La$_{1.5}$Mg$_{0.5}$Ni$_7$ composition with the highest measured value of $C_{max}$. Additionally, several other structures are considered as reference points. We find that hexagonal and rhombohedral structures of La$_2$Ni$_7$ have almost identical total energies, which is in a good agreement with a coexistence of both phases in the samples. The calculated site preferences of Mg in both Ce$_2$Ni$_7$-type and Gd$_2$Co$_7$-type La$_{1.5}$Mg$_{0.5}$Ni$_7$ phases are consistent with the previous experimental data. Furthermore, the valence band of the nanocrystalline La$_{1.5}$Mg$_{0.5}$Ni$_7$ sample is investigated by X-ray photoelectron spectroscopy (XPS). The experimental XPS are interpreted based on the corresponding spectra calculated with DFT.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Gd and Co content on electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloys: a combined experimental and first-principles study

In this work, we investigate the effect of Gd and Co substitutions on the electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloy. Two series of La$_{1.5-x}$Gd$_x$Mg$_{0.5}$Ni$_7$ ($x$ = 0.0, 0.25, 1.0) and La$_{1.5}$Mg$_{0.5}$Ni$_{7-y}$Co$_y$ ($y$ = 0.0, 0.5, 1.5) alloys are produced using mechanical alloying technique. The X-ray diffraction indicates multiphase character of the samples, with the majority of hexagonal Ce$_2$Ni$_7$-type and rhombohedral Gd$_2$Co$_7$-type structures of (La,Mg)$_2$Ni$_7$ phase. Partial substitutions of La by Gd or Ni by Co in La$_{1.5}$Mg$_{0.5}$Ni$_7$ phase result in increase of cycle stability of the metal hydride (MH$_x$) electrodes. All considered alloys reach the maximum discharge capacity after three charging-discharging cycles. Two optimal compositions, in respect of electrochemical properties, are subsequently investigated by the X-ray photoelectron spectroscopy (XPS). The experimental analysis of the valence band is further extended by the density functional theory (DFT) calculations. We also discuss the effects of alloying and site preference of dopants on the position of the van Hove-type singularity, as observed in the electronic densities of states (DOS) in proximity of the Fermi level.

cond-mat.mtrl-sci↗

arXiv2025

Occupation deficiency in layered structures of UNi$_{x}$Sb$_2$ ($ 0 \leq x \leq 1 $) studied by density functional theory supercell calculations

The five crystal structures of selected UNi$_{x}$Sb$_2$ compositions are investigated by density functional theory supercell calculations. The considered phases are USb$_2$, UNi$_{0.33}$Sb$_2$, UNi$_{0.5}$Sb$_2$, UNi$_{0.66}$Sb$_2$, and UNiSb$_2$ ($x$ = 0, 1/3, 1/2, 2/3, 1). The occupation deficiency of Ni is modeled by removing the Ni layers from constructed supercells, followed by relaxation of the structures. A linear dependence of the lattice parameter $c$ versus Ni concentration $x$ is observed, same, fulfilling the empirical Vegard's law. The agreement between results of our calculations with the empirical data from literature confirms the validity of our approach of supercells with empty Ni layers, at least in predicting of lattice parameters. The calculated with orbital polarization magnetic moments on uranium atoms decrease from 1.70 $μ_{\mathrm{B}}$ to 1.61 $μ_{\mathrm{B}}$ with increasing Ni concentration $x$. In comparison to available empirical data of USb$_2$ and UNi$_{0.5}$Sb$_2$, the magnetic moments calculated with orbital polarization are less than 10% smaller.

cond-mat.mtrl-sci↗

arXiv2021

Electronic structure of YbFe$_4$Al$_8$ antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe$_4$Al$_8$ using X-ray photoelectron spectroscopy (XPS) and first-principles calculaions. The XPS valence band of YbFe$_4$Al$_8$ consists of two contributions coming from divalent (Yb$^{2+}$) and trivalent (Yb$^{3+}$) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4$d$ XPS spectra. We study several collinear antiferromagnetic models of YbFe$_4$Al$_8$ from the first-principles and for comparison we also consider LuFe$_4$Al$_8$ with a fully filled 4$f$ shell. We predict that only Fe sublattices of YbFe$_4$Al$_8$ carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mullliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4$f$ orbitals on Yb valence and Fe magnetic moments.

cond-mat.mtrl-sci↗

arXiv2019

Study on CePtIn4 grown in a platelet-like morphology

We present the results of our comprehensive structural investigation performed on single crystals of CePtIn4 grown from In flux in a platelet-like morphology. They were found to crystalize in an orthorhombic structure having space group Cmcm (no. 63) with the lattice parameters a = 4.52175 (15) Å, b = 16.7777 (5) Å, and c = 7.3903 (2) Å. The unit cell volume of such crystals is slightly larger than that of previously reported needle-shaped single crystals of CePtIn4. This feature seems to influence the low-temperature physical properties of the compound, such as enhanced ordering temperature (TN = 2.3 K versus 1.65 K, observed before) along with a sharp drop in the electrical resistivity at TN (that was absent for the needle-like CePtIn4). In addition to the experimental data, we present the results of our ab-initio calculations of the electronic structure of CePtIn4 with the platelet-like morphology.

cond-mat.str-el↗