RESUMO
Carbon steel is strong primarily because of carbides with the most well-known one being θ-Fe3C type cementite. However, the formation mechanism of cementite remains unclear. In this study, a new metastable carbide formation mechanism was proposed as ω-Fe3C â ω'-Fe3C â θ'-Fe3C based on the transmission electron microscopy (TEM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal ω-Fe3C fine particles are distributed in the martensite twinning boundary alone, while two metastable carbides (ω' and θ') coexist in the quenched pearlite. These two carbides both possess orthorhombic crystal structure with different lattice parameters (aθ' = aω' = aω = [Formula: see text]aα-Fe = 4.033 Å, bθ' = 2 × bω' = 2 × cω = [Formula: see text]aα-Fe = 4.94 Å, and cθ' = cω' = [Formula: see text]aω = 6.986 Å for aα-Fe = 2.852 Å). The θ' unit cell can be constructed simply by merging two ω' unit cells together along its bω' axis. Thus, the θ' unit cell contains 12 Fe atoms and 4 C atoms, which in turn matches the composition and atomic number of the θ-Fe3C cementite unit cell. The proposed theory in combination with experimental results gives a new insight into the carbide formation mechanism in Fe-C martensite.
RESUMO
The structural, electronic and magnetic properties of CaCu(3)Co(4)O(12) were studied by use of the full-potential linearized augmented plane wave method. The calculated results indicate that CaCu(3)Co(4)O(12) is stable both thermodynamically and mechanically. Both GGA (generalized gradient approximation) and GGA+U methods predict that CaCu(3)Co(4)O(12) is metallic. The ferromagnetic configuration is only slightly more stable in energy compared with the non-magnetic configuration (3.7 meV), suggesting that they are competitive for being the ground state. Co is in the low spin state (S = 1/2).
RESUMO
The electronic and magnetic properties of CaCu3Cr4O12 and CaCu3Cr2Sb2O12 are investigated by the use of the full-potential linearized augumented plane wave (FPLAPW) method. The calculated results indicate that CaCu3Cr4O12 is a ferrimagnetic and half-metallic compound, in good agreement with previous theoretical studies. CaCu3Cr2Sb2O12 is a ferrimagnetic semiconductor with a small gap of 0.136 eV. In both compounds, because Cr4+ 3d (d2) and Cr3+ 3d (d3) orbitals are less than half filled, the coupling between Cr-Cu is antiferromagnetic, whereas that between Cu-Cu and Cr-Cr is ferromagnetic. The total net spin moment is 5.0 and 3.0 microB for CaCu3Cr4O12 and CaCu3Cr2Sb2O12, respectively. In CaCu3Cr4O12, the 3d electrons of Cr4+ are delocalized, which strengthens the Cr-Cr ferromagnetic coupling. For CaCu3Cr2Sb2O12, the doping of nonmagnetic ion Sb5+ reduces the Cr-Cr ferromagnetic coupling, and the half-filled Cr3+ t2g (t2g3) makes the chromium 3d electrons localized. In addition, the ordering arrangement of the octahedral chromium and antimony ions also prevents the delocalization of electrons. Hence, CaCu3Cr2Sb2O12 shows insulating behavior, in agreement with the experimental observation.
RESUMO
The influence of the Mn-O-Mn bond angle on the magnetic and electronic properties of YBaMn(2)O(5) was studied by density functional theory, which was implemented in the CASTEP code. In practical calculation, both G- and A-type antiferromagnetic (AFM) orderings were considered. The calculated results indicated that G-type is more stable than A-type, in agreement with both experiment and previous theoretical study. It is also interesting to note that a transition from G-type to A-type at an Mn-O-Mn angle of ca. 170 degrees was found upon increasing Mn-O-Mn angle. Therefore, the calculation suggested that what is essential to stabilize the G-type AFM state is the reduction of the Mn-O-Mn bond angle. For both magnetic orderings, the compound changes from semiconductor to metal with the increase of Mn-O-Mn angle.
