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Register a new userElastic and magnetic properties of cubic Fe$_{4}$C from first-principles
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First-principles based on density functional theory is used to study the phase stability, elastic, magnetic, and electronic properties of cubic (c)-Fe$_4$C. Our results show that c-Fe$_{4}$C has a ferromagnetic (FM) ground state structure compared with antiferromagnetic (AFM) and nonmagnetic (NM)states. To study the phase stability of c-Fe$_4$C, BCC Fe$_4$C, FCC Fe$_4$C, and BCC Fe$_{16}$C, where C is considered at tetrahedral and octahedral interstitial sites, are also considered. Although, the formation energy of c-Fe$_4$C is smaller than BCC Fe$_4$C, but the shear moduli of c-Fe$_4$C is negative in the FM and AFM states indicating that c-Fe$_4$C is dynamically not stable in the magnetic (FM/AFM) states. However, NM state has positive shear moduli which illustrates that instability in c-Fe$_4$C is due to magnetism and can lead to soft phonon modes. The calculated formation energy also shows that c-Fe$_4$C has higher formation energy compared with the FCC Fe$_4$C indicating no possibility of c-Fe$_4$C in low carbon steels at low temperature. The magnetic moment of Fe in c-Fe$_4$C is also sensitive to lattice deformation. The electronic structure reveals the itinerant nature of electrons responsible for metallic behavior of c-Fe$_4$C.
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Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010] and [001] directions. The three methods agree well with each other, the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.
We expand our study on cubic BiFeO$_3$ alloys presented in [K. Koumpouras and I. Galanakis, textit{J. Magn. Magn. Mater} 323, 2328 (2011)] to include also the BiMnO$_3$ and Bi$_2$MnFeO$_6$ alloys. For the latter we considered three different cases of distribution of the Fe-Mn atoms in the lattice and six possible magnetic configurations. We show that Fe and Mn atoms in all cases under study retain a large spin magnetic moment, the magnitude of which exceeds the 3 $mu_B$. Their electronic and magnetic properties are similar to the ones in the parent BiMnO$_3$ and BiFeO$_3$ compounds. Thus oxygen atoms which are the nearest-neighbors of Fe(Mn) atoms play a crucial role since they mediate the magnetic interactions between the transition metal atoms and screen any change in their environment. Finally, we study the effect of lattice contraction on the magnetic properties of Bi$_2$MnFeO$_6$.
First principles study of structural, elastic, and electronic properties of the cubic perovskitetype BaHfO$_3$ has been performed using the plane wave ultrasoft pseudo-potential method based on density functional theory with revised Perdew-Burke-Ernzerhof exchange-correlation functional of the generalized gradient approximation (GGA-RPBE). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported in the literatures. The independent elastic constants (emph{C}$_{11}$, emph{C}$_{12}$, and emph{C}$_{44}$), bulk modules emph{B} and its pressure derivatives $B^{prime}$, compressibility $beta$, shear modulus emph{G}, Youngs modulus emph{Y}, Poissons ratio $ u$, and Lam{e} constants ($mu, lambda$) are obtained and analyzed in comparison with the available theoretical and experimental data for both the singlecrystalline and polycrystalline BaHfO$_3$. The band structure calculations show that BaHfO$_3$ is a indirect bandgap material (R-$Gamma$ = 3.11 eV) derived basically from the occupied O 2emph{p} and unoccupied Hf 5emph{d} states, and it still awaits experimental confirmation. The density of states (total, site-projected, and emph{l}-decomposed) and the bonding charge density calculations make it clear that the covalent bonds exist between the Hf and O atoms and the ionic bonds exist between the Ba atoms and HfO$_3$ ionic groups in BaHfO$_3$. From our calculations, it is shown that BaHfO$_3$ should be promising as a candidate for synthesis and design of superhard materials due to the covalent bonding between the transition metal Hf 5emph{d} and O 2emph{p} states.
The structural, elastic, magnetic properties, as well as electronic structure and chemical bonding picture of new oxide 3d1-perovskite BaVO3, recently synthesized, were systematically investigated involving the first-principles FLAPW-GGA calculations. The obtained results are discussed in comparison with available experimental data, as well as with those obtained before for isostructural and isoelectronic SrVO3 perovskite.
First-principles calculations through a FLAPW-GGA method for six possible polymorphs of ruthenium mononitride RuN with various atomic coordination numbers CNs: cubic zinc blende (ZB) and cooperite PtS-like structures with CNs = 4; cubic rock-salt (RS), hexagonal WC-like and NiAs-like structures with CNs = 6 and cubic CsCl-like structure with CN = 8 indicate that the most stable is ZB structure, which is much more preferable for RuN than the recently reported RS structure for synthesized RuN samples. The elastic and electronic properties of ZB-RuN were investigated and discussed in comparison with those for RS-RuN polymorph.
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