Papers by Keyword: Site Occupancy

Abstract: The synergetic effects of the substitution of Ti and Si for Fe on the crystallographic structure and magnetic properties of Nd₂Fe_17-x-yTi_xSi_y compounds have been comprehensively investigated by means of x-ray diffraction, neutron diffraction and magnetic measurements. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure. For a given Ti content, the a-axis and the unit cell volume V of Nd₂Fe_17-x-yTi_xSi_y decrease linearly with increasing silicon content, while the c-axis behaves complicatedly dependent on different Ti content. The site occupancies of Ti and Si in the crystallographic sites significantly change compared to what is observed in the corresponding singly substituted compounds. The T_C of doubly substituted Nd₂Fe_16.5-yTi_0.8Si_y and Nd₂Fe_16.5-yTi_0.5Si_y is higher than that of singly substituted Nd₂Fe_16-ySi_y for a lower Si content while the converse behavior is observed for a higher Si content. For a given Ti content, the T_C of Nd₂Fe_17-x-yTi_xSi_y compounds increases with increasing Si content and the M_s first increases and then decreases. The M_s of Nd₂Fe_17-x-yTi_xSi_y decreases with the increase of Ti content.

Abstract: Site occupancy of Y in the γ′-Co₃(Al,W) was predicted theoretically by first-principles calculations based on density functional theory. By computing total energy as a function of applied strain, the elastic constants of quaternary Co₃(Al,W) were also predicted. The results suggest that Y preferentially occupies the W sites in Co₃(Al,W). The calculation of heat of formation shows that the occupancy of Y on the W sites decreases the phase stability of Co₃(Al,W). The theoretical calculation also shows that the L1₂ Co₂₄Al₄W₃Y compound is ductile in nature.

Abstract: The site preference of ternary additions in Ni3X-type GCP compounds was determined from the direction of solubility lobe of the GCP phase on the experimentally reported ternary phase diagrams. In Ni3Nb (D0a), Co and Cu preferred the substitution for Ni-site, Ti, V and W the substitution for Nb-site, and Fe the substitution for both sites. In Ni3V (D022), Co preferred the substitution for Ni-site, Cr the substitution for both sites, and Ti the substitution for V-site. In Ni3Ti (D024), Fe, Co, Cu, and Si preferred the substitution for Ni-site, Nb, Mo and V the substitution for Ti-site. The thermodynamic model, which was based on the change in total bonding energy of the host compound by a small addition of ternary solute, was applied to predict the site preference of ternary additions. The bond energy of each nearest neighbor pair used in the thermodynamic calculation was derived from the heat of compound formation by Miedema’s formula. The agreement between the thermodynamic model and the result of the literature search was excellent. Both transition and B-subgroup elements have two possibilities, i.e., the case of substitution for Ni-site or the case for X-site, depending on the relative value of two interaction energies.

Abstract: The effect of rare earth (Ln = Gd, Dy, Y and Yb) and Mg ions on the microstructure and reliability of BaTiO3-based monolithic ceramic capacitors (MLCs) with Ni electrodes was investigated. The X-ray diffraction results about the lattice volume of sintered specimens suggested that Gd and Dy ions predominatly substituted into the Ba-site, Yb ions gave exclusive substitution at the Ti-site, while Y ions occupied either the Ba- and Ti-site. The reliability of ceramic capacitors was increasing with increasing the ionic radius of the rare earths in this study. The nonlinearity coefficient() etsimated from the leakage currents and the lifetime measured from the highly accelerated lifetime testing (HALT) showed a negative correlation which was observed only from Dy and Y ions doped specimens. The quantity of Ln and Mg in the grains tended to increase with increasing the Ln ionic radius. In order to improve the reliability and the insulation property of BaTiO3 based MLCs with Ni electrodes, it is important that acceptor ions at the Ti-site compensate donor ions which are rare earth ions at the Ba-site, so the overall quantity of the dopants required for the charge compensation with acceptor and donor ions increases accordingly.

Abstract: Site occupancies of ternary additions (Ti, V, and W) in the C15 ZrCr2 and NbCr2 Laves phases were predicted theoretically by first-principles calculations based on density functional theory. The results suggest that Ti preferentially occupies the Zr and Nb sites in ZrCr2 and NbCr2, respectively. V and W substitute the Cr sites in both ZrCr2 and NbCr2. The calculations of heats of formation also show that the occupancy of W on the Cr sites and of Ti on the Zr sites stabilize ZrCr2. For NbCr2, the occupancy of V on the Cr sites and of Ti on the Nb sites increases the phase stability.