Technische Universität Bergakademie Freiberg

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Browsing Technische Universität Bergakademie Freiberg by Author "Gumeniuk, Roman"

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  • ItemOpen Access
    Controlling structural instability in the Cu$_{12-x}$Ni$_x$Sb$_4$S$_{13}$ ($x$\,=\,0, 0.05, 0.2, 0.5) tetrahedrites
    (Technische Universität Bergakademie Freiberg, 2026-01-08) Bolielyi, Oleksandr; Dobrozhan, Oleksandr; Levytskyi, Volodymyr; Tsirlin, Alexander A.; Pshenychnyi, Roman; Leithe-Jasper, Andreas; Gumeniuk, Roman
    Cu$_{12-x}$Ni$_x$Sb$_4$S$_{13}$ ($x\,=\,0, 0.05, 0.2, 0.5$) tetrahedrites were synthesized by the polyol method. Measurements of magnetic susceptibility, specific heat capacity as well as electrical and thermal transport properties indicated a structural phase transition (SPT) of 1$^{\mathrm{st}}$ order at $T_k\,=\,75-85$\,K, which is accompanied by strong changes in electronic density of states (EDOS) and entropy for $x\,=\,0, 0.05, 0.2$ whereas only weak changes were observed at $x\,=\,0.5$. However, further temperature dependent synchrotron high-resolution powder X-ray diffraction (HR PXRD) studies indicated compounds with Ni-content $x\,=\,0$ and 0.5 to remain body-centered cubic [space group (SG) $I\bar{4}3m$, $a\,\approx\,10.4(1)$\,\AA] down to 10\,K, whereas crystal structures of $x\,=\,0, 0.05$ and 0.2 became tetragonal, (SG $P\bar{4}c2$, $a_\mathrm{tetr}\,\approx\,a_\mathrm{cub}\sqrt{2}$, $c_\mathrm{tetr}\,\approx\,a_\mathrm{cub}$) below $T_k$. Comparing changes of EDOS at the Fermi level [$\Delta N^{(E_\mathrm{F})}$] deduced from magnetic and thermodynamic data with those calculated from density functional theory (DFT) assuming the validity of the rigid-band approximation, we found $\Delta N^{(E_\mathrm{F})}_\mathrm{theor}\,\approx\,0$ when compared with cubic and tetragonal arragements for Cu$_{11.5x}$Ni$_{0.5}$Sb$_4$S$_{13}$, which indicate the latter to be unstable. This study shows that SPT in tetrahedites is facilitated by the presence of minor amount of a dopant.
  • ItemPublic Metadata
    GRK 2802: Characterization of a recyclate-based MgO-steel as-sintered inert anode candidate after exposure to cryolite electrolysis
    (Technische Universität Bergakademie Freiberg, 2026-02-12) Yaroshevskyi, Serhii; Adamczyk, Alexander; Brachhold, Nora; Schmidt, Gert; Hubalkova, Jana; Gumeniuk, Roman; Charitos, Alexandros; Aneziris, Christos
    The replacement of consumable carbon anodes with oxygen-evolving inert, carbon free anodes is a key technological challenge for decarbonizing primary aluminum production. While metallic, ceramic, and cermet anodes have been extensively studied, the use of recycled raw materials remains largely unexplored. In this work, composite cermet anodes consisting of 60 vol% AISI 316L stainless steel and 40 vol% recycled MgO from spent refractories were developed and tested under galvanostatic Hall-Héroult electrolysis conditions in cryolite at 1000 °C. The as-sintered composites exhibited stable cell voltage (3.2-3.3 V) over 2 h of operation. Post-mortem SEM/EDS/EBSD analyses revealed a multilayered structure with protective Fe-oxide, Al/Cr spinel formation, corroded Mg-F-rich phases, and cryolite infiltration extending beyond 2 mm depth. While the 316L steel matrix showed relative stability, the recycled MgO fraction proved chemically reactive, generating porosity and acting as the primary pathway for melt intrusion and secondary Al2O3 precipitation. Complementary static contact corrosion tests confirmed rapid fluoridation of MgO, whereas steel grains remained intact.
  • ItemOpen Access
    Multiple superconducting transitions in Yb$_{3+x}$Co$_4$Sn$_{13-x}$
    (Technische Universität Bergakademie Freiberg, 2025-07-31) Meret, Orlob; Levytskyi, Volodymyr; Tsirlin, Alexander; Kundys, Bohdan; Sturm, Elena; Leithe-Jasper, Andreas; Gumeniuk, Roman
    Abstract of the accompanying article: In the Yb-Co-Sn system a Remeika phase Yb$_{3+x}$Co$_{4}$Sn$_{13-x}$ is found with solid solution extending within $0\le\,x\,\le0.5$. All these compounds crystallize with primitive cubic [space group $Pm\bar{3}n$, $a\approx9.54$ \AA] strongly disordered Sc$_{3}$Ir$_4$Si$_{13+x}$ type. Stoichiometric Yb$_3$Co$_4$Sn$_{13}$ is a superconductor with critical temperature $T_c\,=\,3.1(2)$\,K, lower- [$B_\mathrm{c1}=1.90(2)$\,mT] and upper [$B_\mathrm{c2}=2.16(5)$\,T] critical fields. The specific heat jump $\Delta c_p/\gamma T_c\,=\,1.72(9)$ together with the fact that $B_\mathrm{c2}(T_c)$ dependence is described by the Werthamer-Helfand-Hohenberg (WHH) model hint towards a conventional mechanism with a weak electron-phonon coupling. Although a more precise analysis is hampered by the presence of multiple superconducting transitions observed in the specific heat of this Remeika phase, the non-stoichiometric Yb$_{3.2}$Co$_4$Sn$_{12.8}$ reveals $T_c\,=\,2.4(2)$\,K and $B_\mathrm{c2}=4.79(9)$\,T exceeding the classical Pauli limit. Strong electron-phonon coupling in this superconductor is confirmed by the high value of the $\lambda_\mathrm{AD}=0.92(2)$ parameter determined from the Allen-Dynes formula. However, the electronic specific heat of Yb$_{3.2}$Co$_4$Sn$_{12.8}$ follows the exponential law [$c_\mathrm{el}(T)\propto e^{-\Delta(0)/k_\mathrm{B}T}$] and can be described by the $\alpha$-model [$\alpha\equiv\Delta(0)/k_\mathrm{B}T_c=2$] indicating this stannide to be a conventional BCS-superconductor. Both Yb$_{3+x}$Co$_{4}$Sn$_{13-x}$ ($x=0, 0.2$) compounds reveal complex phonon spectra with possible `rattling' behavior. They are also found to be metallic systems, some aspects of which can be described by a free electron gas model.