arXiv subjects

Pavel Ondračka

Publications and source records attributed to Pavel Ondračka.

8 recordsLinked to original sources

arXiv2026

C 1s core-level fingerprints of reconstructed titanium vacancies in titanium carbide

Recent first-principles searches predict that a titanium vacancy in rocksalt TiC is not an empty octahedral site but can reconstruct through the formation of carbon Frenkel pairs and short C-C bonds. We determine the C 1s signatures of these local structures and assess whether they remain specific in the presence of disordered carbon. Site-resolved binding energies were calculated with a self-consistent core-hole method for an unreconstructed vacancy and four representative C-C-bonded reconstructions. The unreconstructed vacancy shifts its nearest-neighbour C 1s level by only -0.36 eV. In contrast, the largest positive shift in each reconstructed model ranges from 2.34 eV to 2.80 eV. The large shifts occur at carbon atoms participating in the reconstructed C-C network and correlate with markedly less negative Mulliken charges, providing candidate local fingerprints of vacancy reconstruction. However, an exploratory TiC/amorphous-C model shows that disordered carbon environments can extend into the same high-binding-energy range. Overall, the calculated shifts provide reference energies for experimental searches for reconstructed Ti vacancies in TiC.

cond-mat.mtrl-sci↗

arXiv2026

Effect of oxygen content on optical, structural, and dielectric properties of Al$_x$Ta$_y$O$_z$$ thin films

This study reports on the optical, structural, and dielectric properties of aluminum tantalum oxide (Al$_x$Ta$_y$O$_z$) thin films deposited at low temperature on silicon and steel substrates by pulsed direct current reactive magnetron sputtering of a target containing 80 at.% aluminum and 20 at.% tantalum in Ar/O$_2$ atmosphere. Oxygen flow rates ranging from 5.0 to 20 sccm corresponded to O content changes from 57.7 to 69.6 at.% and resulted in large differences in dielectric behavior, from films with no measurable dielectric strength to a dielectric strength of 231 V$μ$m$^{-1}$, respectively. Ab initio calculations were employed to explain the large property changes, and we show that a decrease in the dielectric strength can be linked to the formation of metal-metal bonds in the material, when the O content is less than what would correspond to a stoichiometric Ta$_2$O$_5$ and Al$_2$O$_3$ mixture. The electronic states corresponding to the metal--metal bonds are located in the band gap close to the top of the valence band, leading to an effective band gap reduction, which is directly supported by X-ray photoelectron spectroscopy valence band measurements and by a broad optical absorption in the visible region.

cond-mat.mtrl-sci↗

arXiv2025

Prediction and identification of point defect fingerprints in X-ray photoelectron spectra of TiN$_x$ with 1.18 $\le x \le$ 1.37

We investigate the effect of selected N and Ti point defects in $B$1 TiN on N 1s and Ti\,2p$_{3/2}$ binding energies (BE) by experiments and ab initio calculations. X-ray photoelectron spectroscopy (XPS) measurements of TiN$_x$ films with 1.18 $\le x \le$ 1.37 reveal additional N 1s spectral components at lower binding energies. Ab initio calculations predict that these components are caused by either Ti vacancies, which induce a N 1s BE shift of -0.54 eV in its first N neighbors, and/or N tetrahedral interstitials, which have their N 1s BE shifted by -1.18 eV and shift the BE of their first N neighbors by -0.53 eV. However, based on {\it ab initio} data the tetrahedral N interstitial is estimated to be unstable at room temperature. We, therefore, unambiguously attribute the N 1s spectral components at lower BE in Ti-deficient TiN$_x$ thin films to the presence of Ti vacancies. Furthermore, it is demonstrated that the vacancy concentration in Al-capped Ti-deficient TiN$_x$ can be quantified with the here proposed correlative method based on measured and predicted BE data. Our work highlights the potential of ab initio-guided XPS measurements for detecting and quantifying point defects in $B$1 TiN$_x$.

cond-mat.mtrl-sci↗

arXiv2023

Bond formation at polycarbonate | X interfaces (X = Al$_2$O$_3$, TiO$_2$, TiAlO$_2$) studied by theory and experiments

Interfacial bond formation during sputter deposition of metal oxide thin films onto polycarbonate (PC) is investigated by ab initio molecular dynamics simulations and X-ray photoelectron spectroscopy (XPS) analysis of PC | X interfaces (X = Al$_2$O$_3$, TiO$_2$, TiAlO$_2$). Generally, the predicted bond formation is consistent with the experimental data. For all three interfaces, the majority of bonds identified by XPS are (C-O)-metal bonds, whereas C-metal bonds are the minority. Compared to the PC | Al$_2$O$_3$ interface, the PC | TiO$_2$ and PC | TiAlO$_2$ interfaces exhibit a reduction in the measured interfacial bond density by ~ 75 and ~ 65%, respectively. Multiplying the predicted bond strength with the corresponding experimentally determined interfacial bond density shows that Al$_2$O$_3$ exhibits the strongest interface with PC, while TiO$_2$ and TiAlO$_2$ exhibit ~ 70 and ~ 60% weaker interfaces, respectively. This can be understood by considering the complex interplay between the metal oxide composition, the bond strength as well as the population of bonds that are formed across the interface.

cond-mat.mtrl-sci↗

arXiv2023

Correlative Theoretical and Experimental Study of the Polycarbonate | X Interfacial Bond Formation (X = AlN, TiN, TiAlN) during Magnetron Sputtering

To understand the interfacial bond formation between polycarbonate (PC) and magnetron-sputtered metal nitride thin films, PC | X interfaces (X = AlN, TiN, TiAlN) are comparatively investigated by ab initio simulations as well as X-ray photoelectron spectroscopy. The simulations predict significant differences at the interface, as N and Ti form bonds with all functional groups of the polymer, while Al reacts selectively only with the carbonate group of pristine PC. In good agreement with simulations, experimental data reveal that the PC | AlN and the PC | TiAlN interfaces are mainly defined by interfacial C-N bonds, whereas for PC | TiN, the interface formation is also characterized by numerous C-Ti and (C-O)-Ti bonds. Bond strength calculations combined with the measured interfacial bond density indicate the strongest interface for PC | TiAlN followed by PC | AlN, whereas the weakest is predicted for PC | TiN due to its lower density of strong interfacial C-N bonds. This study shows that the employed computational strategy enables prediction of the interfacial bond formation between PC and metal nitrides and that it is reasonable to assume that the research strategy proposed herein can be readily adapted to other organic | inorganic interfaces.

cond-mat.mtrl-sci↗

arXiv2023

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.

cond-mat.mtrl-sci↗

arXiv2021

$\textit{Ab initio}$-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films

$\textit{Ab initio}$ calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti$_{1-δ}$O$_x$N$_{1-x}$). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by $\sim$0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.

cond-mat.mtrl-sci↗

arXiv2016

Accurate prediction of band gaps and optical properties of HfO$_2$

We report on optical properties of various polymorphs of hafnia predicted within the framework of density functional theory. The full potential linearised augmented plane wave method was employed together with the Tran-Blaha modified Becke-Johnson potential for exchange and local density approximation for correlation. Unit cells of monoclinic, cubic, and tetragonal crystalline, and a simulated annealing-based model of amorphous hafnia were fully relaxed with respect to internal positions and lattice parameters. The thus obtained band gaps of 5.76 eV, 5.88 eV, 6.54 eV, and 5.5 eV for monoclinic, cubic, tetragonal, and amorphous hafnia, respectively, are in excellent agreement with available experimental data. Dielectric functions were calculated and a shift of spectral weights to higher energies with respect to experimental data was predicted. This was attributed to the random phase approximation used to calculate the dielectric function, rather than inaccuracies in the band structure.

cond-mat.mtrl-sci↗