ABSTRACT
The intermetallic compounds GdFe1-xCrxSi, x = 0-0.8, GdFe1-xVxSi, x = 0-0.4, and GdFe1-xNixSi, x = 0-0.4 with a tetragonal CeFeSi (P4/nmm) structure type have been synthesized. The Curie temperature, TC, sharply increases from 130 K to 255 K and 250 K for the GdFe1-xCrxSi and GdFe1-xVxSi compounds and decreases to 104 K for GdFe1-xNixSi. Within the framework of the model of effective d-f exchange interaction in R3Ìd intermetallics, these changes in TC can be caused by the corresponding changes in the density of states. The electronic structure, magnetic moments and types of magnetic orderings of the GdFe1-xTxSi, T = Cr, V, Ni intermetallic compounds were calculated using the DFT+U theoretical method. For the GdFe1-xNixSi system, the transformation of a ferromagnet with the composition x = 0 into an antiferromagnet with the composition x = 0.3 was established experimentally and using first-principles calculations. The correlation of the ferromagnetic or antiferromagnetic type of the magnetic state in the GdFe1-xNixSi compounds with the value of the lattice parameter c to greater or less than the critical value c = 6.72 Å for the GdCoSi antiferromagnet has been experimentally established. The magnetic structures of the antiferromagnets GdFe0.7Ni0.3Si and GdCoSi were found to be different. GdFe0.7Ni0.3Si is characterized by a collapse in the magnetocaloric effect via a change in the isothermal magnetic entropy ΔSM(TC). The compounds GdFe0.4Cr0.6Si with -ΔSM(TC) = 2.37 J kg-1 K-1 at TC = 255 K and RC = 82.07 J kg-1 and GdFe0.7V0.3Si with -ΔSM(TC) = 2.06 J kg K-1 at TC = 250 K and RC = 96.02 J kg-1 in a field changing to 17 kOe could be of practical interest due to the TC being close to room temperature.
ABSTRACT
The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.
ABSTRACT
In order to study the mobility of hydrogen in nanostructured Laves-phase hydrides, we have measured the proton nuclear magnetic resonance (NMR) spectra and the proton spin-lattice and spin-spin relaxation rates in two nanostructured systems prepared by ball milling: ZrCr(2)H(3) and TaV(2)H(1+δ). The proton NMR measurements have been performed at the resonance frequencies of 14, 23.8 and 90 MHz over the temperature ranges 11-424 K (for coarse-grained samples) and 11-384 K (for nanostructured samples). Hydrogen mobility in the ball-milled ZrCr(2)H(3) is found to decrease strongly with increasing milling time. The experimental data suggest that this effect is related to the growth of the fraction of highly distorted intergrain regions where H mobility is much lower than in the crystalline grains. For the nanostructured TaV(2)H(1+δ) system, the ball milling is found to lead to a slight decrease in the long-range H mobility and to a suppression of the fast localized H motion in the crystalline grains.