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Register a new userThe effect of magnetism and temperature on the stability of Cr, V, Al carbide MAX phases
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The stability of Cr, V, Al carbide MAX phases, materials of interest for a variety of magnetic as well as high temperature applications, has been studied using density-functional-theory first-principles calculations. The enthalpy of mixing predicts these alloys to be unstable towards unmixing at 0 K. The calculations also predict, however, that these phases would be thermally stabilised by configurational entropy at temperatures well below the values used for synthesis. The temperature Ts below which they become unstable is found to be quite sensitive to the presence of magnetic moments on Cr ions, as well as to the materials magnetic order, in addition to chemical order and composition. Allowing for magnetism, the value of Ts for helf V and half Cr with chemically disordered Cr and V atoms, is estimated to be between 516 K and 645 K depending on the level of theory, while, if constrained to spin-paired, Ts drops to 142 K. Antiferromagnetic spin arrangements are found to be favoured at low temperatures, but they are most likely lost at synthesis temperatures, and probably at room temperature as well. However, the combination of antiferromagnetic frustration and configurational disorder should give rise to interesting spin textures at low temperatures.
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Anomalies in the temperature dependences of the recoil-free factor, f, and the average center shift, <CS>, measured by 57-Fe Mossbauer Spectroscopy, were observed for the first time in the archetype of the sigma-phase alloys system, Fe-Cr. In both cases the anomaly started at the temperature close to the magnetic ordering temperature, and in both cases it was indicative of lattice vibrations hardening. As no magnetostrictive effects were found, the anomalies seem to be entirely due to a spin-phonon coupling. The observed changes in f and in <CS> were expressed in terms of the underlying changes in the potential, Delta E_p, and the kinetic energy, Delta E_k, respectively. The former, with the maximum value larger by a factor of six than the latter, decreases, while the latter increases with T. The total mechanical energy change, Delta E, was, in general, not constant, as expected for the Debye-like vibrations, but it resembled that of Delta E_p. Only in the range of 4-15 K, Delta E was hardly dependent on T.
In the present work, we systematically studied the effect of Al doping on the phase formation of iron nitride (Fe-N) thin films. Fe-N thin films with different concentration of Al (Al=0, 2, 3, 6, and 12 at.%) were deposited using dc magnetron sputtering by varying the nitrogen partial pressure between 0 to 100%. The structural and magnetic properties of the films were studied using X-ray diffraction and polarized neutron reflectivity. It was observed that at the lowest doping level (2 at.% of Al), nitrogen rich non-magnetic Fe-N phase gets formed at a lower nitrogen partial pressure as compared to the un-doped sample. Interestingly, we observed that as Al doping is increased beyond 3at.%, nitrogen rich non-magnetic Fe-N phase appears at higher nitrogen partial pressure as compared to un-doped sample. The thermal stability of films were also investigated. Un-doped Fe-N films deposited at 10% nitrogen partial pressure possess poor thermal stability. Doping of Al at 2at.% improves it marginally, whereas, for 3, 6 and 12at.% Al doping, it shows significant improvement. The obtained results have been explained in terms of thermodynamics of Fe-N and Al-N.
Significant discrepancies have been observed and discussed on the lattice stability of Cr between the predictions from the ab initio calculations and the CALPHAD approach. In the current work, we carefully examined the possible structures for pure Cr and reviewed the history back from how Kaufman originally determined the Gibbs energy of FCC-Cr in the 1970s. The reliability of Cr lattice stability derived by the CALPHAD and ab initio approaches was systematically discussed. It is concluded that the Cr lattice stability based on the CALPHAD approach has large uncertainty. Meanwhile, we cannot claim that the ab initio HFCC-Cr is error-free as FCC-Cr is an unstable phase under ambient conditions. The present work shows that the ab initio HFCC-Cr can be a viable scientific approach. As both approaches have their limitations, the present work propose to integrate the ab initio results into the CALPHAD platform for the development of the next generation CALPHAD database. The Fe-Cr and Ni-Cr binary systems were chosen as two case studies demonstrating the capability to adopt the ab initio Cr lattice stability directly into the current CALPHAD database framework.
Crystal and magnetic structures of the $x=0.2$ member of La$_{rm 0.8-x}$Tb$_{rm x}$Ca$_{0.2}$CoO$_3$ perovskite series have been determined from the powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below $T_C=55$ K. The moments evolve at first on Co sites, and ordering of Ising-type Tb$^{3+}$ moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear F$_x$ for cobalt subsystem, while it is canted F$_x$C$_y$ for terbium ions. The rare-earth moments align along local Ising axes within textit{ab}-plane of the orthorhombic $Pbnm$ structure. The behavior in external fields up to $70-90$ kOe has been probed by the magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb$^{3+}$ Ising character is the observation of a low-temperature region of anomalously large linear term of heat capacity and its field dependence. Similar behaviours are detected also for other terbium dopings $x=0.1$ and 0.3.
MAX phases are a family of layered, hexagonal-structure ternary carbides or nitrides of a transitional metal and an A-group element. What makes this type of material fascinating and potentially useful is their remarkable combinations of metallic and ceramic characteristics; as well as the indispensable role in top-down synthesis of their 2D counterparts, MXenes. To enhance the efficiency in the successful search for potential novel MAX phases, the main efforts could go toward creating an informationprediction system incorporating all MAX phases databases, as well as generally valid principles and the high-quality regularities. In this work, we employ structure mapping methodology, which has shown its merit of being useful guides in materials design, with Hume-Rothery parameters to provide guiding principles in the search of novel MAX phases. The formable/non-formable data on MAX phases can be ordered within a twodimensional plot by using proposed expression of geometrical and electron concentration factors.
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