Hydride phases, structure, and magnetic properties of theUNiAlHysystem

Abstract

The hydriding behavior of the heavy fermion compound UNiAl has been investigated. A range of hydride compositions with orthorhombic structure ͑space group Pnma͒ has been stabilized. As a result, a different nomenclature for the UNiAlH y hydride phases is proposed. X-ray diffraction and magnetic studies are reported on three compositions in this range of hydrides, viz., yϭ0.06, 0.14, and 0.58. All these compositions do not show magnetic ordering down to 2 K, though weak ferromagnetic correlations are apparent in their magnetic studies. Low effective moment values on uranium atoms in these cases, appreciably below the value for itinerant antiferromagnetic UNiAl, indicate increased itinerant character of U 5 f electrons. We also report magnetization studies on hexagonal UNiAlH y (yϭ0.7). It shows ferromagnetic ordering below 87 K. This means that in the hexagonal hydride phases, the nature of magnetic ordering changes from antiferromagnetic (yϭ0) to ferromagnetic (yϭ0.7) and again back to antiferromagnetic for yϭ2.3. It is shown that the moment on the uranium atoms in the hexagonal hydride phases ͑yХ0.7 and 2.0-2.5͒ is close to the free-ion local moment value.

Structure, magnetization and heat capacity of U(Ni0.85Co0.15)AlH2.1 system

“…Following our studies on the hydrogen absorption properties of ZrNiAl-type hexagonal U(Ni x Fe 1Àx )Al compounds [14][15][16], we report in the present work our results on the hydrogenation of U(Ni x Co 1Àx )Al for x ¼ 0.85. We have probed the structural and magnetic behavior of both the parent compound and its hydride by using X-ray powder diffraction, magnetization and heat capacity.…”

“…UPdIn absorbs hydrogen to the extent of 0.7 H/formula unit resulting in large anisotropic volume expansion [13]. In the UTAl series, UNiAl absorbs hydrogen in the range of 2-2.7 H/formula unit [14][15][16]. The ability to absorb hydrogen decreases drastically by Fe or Co substitution for Ni.…”

“…The hydriding characteristics of UNiAl has been thoroughly investigated. It has been shown that, depending on the hydrogen content, UNiAlH y hydrides can be stabilized either in the hexagonal or in the orthorhombic form depending on the hydrogen content [14]. The orthorhombic phase with hydrogen content y < 0.6 does not order magnetically down to w2 K. In the hexagonal phase region, the magnetic ordering was found to change from itinerant antiferromagnetic for pure UNiAl (T N ¼ 19 K) to ferromagnetic (below w90 K for y ¼ 0.7) and again back to antiferromagnetic (T N ¼ 95 K) for y w 2-2.5 [14].…”

“…It has been shown that, depending on the hydrogen content, UNiAlH y hydrides can be stabilized either in the hexagonal or in the orthorhombic form depending on the hydrogen content [14]. The orthorhombic phase with hydrogen content y < 0.6 does not order magnetically down to w2 K. In the hexagonal phase region, the magnetic ordering was found to change from itinerant antiferromagnetic for pure UNiAl (T N ¼ 19 K) to ferromagnetic (below w90 K for y ¼ 0.7) and again back to antiferromagnetic (T N ¼ 95 K) for y w 2-2.5 [14]. On the other hand, in U(Ni 1Àx Fe x )Al system, no H 2 absorption takes place for x > 0.3 and the only hydride phase that could be stabilized has a hydrogen content of 0.7 H/formula unit [15,16].…”

“…Among the UTX compounds, the UTAl, UTSi, UTSn and UTIn series have been studied for their hydrogenation properties [10][11][12][13][14][15][16]. The compounds UNiSi and UPdSi form hydrides with w1 H/formula unit [11], whereas UCoSi absorbs 0.7 H/formula unit [12].…”