A. Sathyamoorthy

Zr2FeHx system hydrided at low temperatures: structural aspects by Mössbauer and x-ray diffraction studies

Abstract The first successful hydriding of the high-temperature phase of Zr 2 Fe, which has a C-16 CuAl 2 -type structure, is reported. The intermetallic compound could be hydrided at temperatures down to about 250 K and hydrogen pressures less than about 0.5 atm to a composition Zr 2 FeH x ( x = 4.5) without changing its structure. The hydrides with x

On the properties of CeFe2Hx hydrided at low temperatures

Abstract The cubic Laves phase compound CeFe 2 could be hydrided down to 203 K and 0.5 atm hydrogen pressure, in a copper reactor allowing fast withdrawal of heat. The hydrides so formed at low temperatures contained an appreciably higher hydrogen concentration, as compared with earlier reported values. Their properties are also significantly different. No crystalline ternary hydride is formed down to 203 K. Simultaneous occupation of various available sites by hydrogen atoms, resulting in a composition CeFe 2 H x ( x ⩾4) of the hydrided fraction, coupled with the inability of iron atoms to diffuse rapidly, is believed to be responsible for the ternary amorphous phase formation. X-ray diffraction, Mossbauer studies with and without magnetic field, a.c. and d.c. magnetic susceptibility and electrical resistance measurements are reported on the hydrides. The amorphous ternary hydride formed at 223 K does not exhibit a simple ferromagnetic ordering, in contradiction to earlier reports, as shown by a broad maximum in a.c. susceptibility at about 250 K. This ordering can, however, be transformed to the normal ferromagnetic type by applying a small magnetic field of a few kilogauss. The temperature dependence of the electrical resistance of the same sample exhibits a semiconducting behaviour and the magnetic transition at about 250 K is also discernible.

ErFe2-H system: a new plateau and the structure of the new hydride phase

Abstract Pressure-composition isotherm and X-ray diffraction studies of ErFe2-H system have been carried out to resolve the reported controversies regarding the composition limits of various hydride phases. Further, the existence of a new plateau in ErFe2Hx for x values between 4 and 5 has been established and the structure of the new hydride phase with x ⋍ 5 is found to be orthorhombic.

High-pressure X-ray diffraction study of UMn2Ge2

Abstract Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered tetragonal ThCr2Si2 structure with space group I4/mmm, a=3.993 A and c=10.809 A under ambient conditions. Energy dispersive X-ray diffraction was used to study the compression behavior of UMn2Ge2 in a diamond anvil cell. The sample was studied up to static pressure of 26 GPa and a reversible structural phase transition was observed at a pressure of 1 6.1 GPa . Unit cell parameters were determined up to 12.4 GPa and the calculated cell volumes were found to be well reproduced by a Murnaghan equation of state with K 0 =73.5 GPa and K′=11.4. The structure of the high-pressure phase above 16.0 GPa is quite complicated with very broad lines and could not be unambiguously determined with the available instrument resolution.

Hydriding of the mixed-valent compound CeRhIn: structural and magnetization studies

Abstract Mixed-valent compound, CeRhIn, having the hexagonal Fe 2 P-type structure and a moderate value of the Kondo temperature ( T K ∼150 K), is found to form two distinct hydride phases. The higher hydride phase in CeRhInH y system with y ≅1.55 is found to be quite unstable and readily transforms to a lower hydride with y ≅0.55. From structural and magnetization studies, it is found that in the lower hydride CeRhInH 0.55 (i) the hexagonal structure is retained, with a slightly expanded unit cell, and (ii) the cerium continues to be in the mixed-valent state, though a definite shift towards Ce 3+ is indicated, as seen by a reduction in the T K value to ∼90 K. It is suggested that the hydride formation results in increasing the d-band occupancy of Rh, due to partial transfer of electrons from hydrogen. This, in turn, decreases the hybridization between the Ce-4f electrons and those on the neighboring Rh atoms, which is responsible for a shift of Ce valence towards trivalent state.

The hydriding behaviour of U(Fe1−xNix)Al system (0≤x≤0.75) and magnetic studies on U(Fe1−xNix)AlH0.8

Abstract The hydriding behaviour of the solid-solution series of compounds, U(Fe 1− x Ni x )Al, has been investigated. In the parent pseudoternary compounds, as Ni is gradually substituted for Fe, the magnetic correlations grow. As a result, the magnetic properties change from exchange enhanced Pauli-paramagnetic and spin fluctuating ( x =0), via weakly magnetic with no long range magnetic ordering down to 4 K (0 x ≤0.3) to ferromagnetic (0.35≤ x ≤0.75) and eventually to antiferromagnetic for x ≥0.9. The present hydriding report covers compositions up to the ferromagnetic regime. It is found that hydrogen absorption does not take place for x below ≅0.7. The lowest nickel containing composition, which forms a well defined hydride phase, viz., U(Fe 0.3 Ni 0.7 )Al, absorbs y =0.8 H atoms per formula unit. This matches exactly with the lower hydride phase of pure UNiAl. No other hydride phase is formed either above or below y =0.8. The magnetization studies show that U(Fe 0.3 Ni 0.7 )AlH 0.8 has much higher values of the magnetic ordering temperature ( T C ), paramagnetic Curie temperature ( θ P ) and the uranium magnetic moment, relative to the unhydrided composition. These findings are in consonance with the large increase in the 57 Fe Mossbauer isomer-shift value, on hydriding. It is suggested that the electron charge transfer from H to the Fe 3d band (inferred from the Mossbauer studies) weakens the 5f–3d hybridization, thus enhancing the ferromagnetic correlations. The observed large increase in the a -axis cell parameter also implies a reduction in U–U hybridization, further justifying the observed increase in T C , θ P and U moment values.

Role of hydride phases in the catalytic activity of Zr2Ni for the dehydrogenation of methanol

Abstract The intermetallic compound Zr 2 Ni showed high catalytic activity for methanol dehydrogenation in the temperature region of 375–500 K when it was subjected to a series of pretreatments under HF, NaOH and hydrogen. The reaction followed a two-step process, one of them being responsible for the initial higher catalytic activity. The pretreatments were found to generate secondary phases, such as fluorides and hydrides and also nickel clusters at the catalyst surface, the presence of which was found to be vital for the observed catalytic activity. The bulk of the intermetallic compound was, however, converted into a single-phase hydride after the above mentioned pretreatments. The removal of stored hydrogen and the oxidation of nickel sites during the reaction led to the progressive loss of catalyst activity. It is proposed that the secondary hydride phases act as support to the catalytically active nickel clusters, thus helping in better metal dispersion.

Hydrogen-induced amorphisation in the Ce(Fe1−xAlx)2 system

Hydriding behaviour of cubic Laves phase pseudo-binary Ce(Fe 1− x Al x ) 2 system has been studied for the iron-rich (0 ⩽ x ⩽ 0.1) as well as for the aluminium-rich compositions (0.7 ⩽ x ⩽ 0.85) at temperatures in the range of 223–298 K, and hydrogen pressure (≲ 0.5 atm), ensuring fast withdrawal of exothermic heat. The maximum hydrogen absorption capacity is found to decrease with increasing aluminium concentration. The results of X-ray diffraction and Mossbauer studies are reported. This system shows the unique feature of stabilising the amorphous ternary, for the Fe-rich region and the crystalline ternary hydride phases for the Al-rich compositions. The role of the maximum hydrogen absorption capacity, the rate and sequence of H atom occupancy and the volume associated with each hydrogen atom is discussed in relation to the amorphous ternary vis-a-vis the crystalline ternary hydride phase formation. It is shown that by restricting the maximum hydrogen absorption capacity such that the H-atom occupancy is confined to the g-type interstitial sites only, the crystalline ternary hydride phase can be stabilised in this system.

U(Fe1−xNix)AlHy system: new hydride phases, structural and magnetic properties

Abstract We report the hydriding characteristics of U(Fe1−xNix)AlHy, with x=0.85. It is found that (i) U(Fe0.15Ni0.85)Al can be hydrided to a maximum of y≈2.4 and the hydride shows antiferromagnetic ordering with T N ≈70 K , and (ii) new single phase orthorhombic hydrides can be stabilized for x⩾0.85. From our hydriding and magnetic studies on all the compositions in this series of compounds, including our earlier work, it is found that irrespective of the magnetic state of the parent composition, the hydrides with y⩾2 result in U-5f-electron localization and antiferromagnetic ordering. Structural and magnetic studies have also been carried out on the orthorhombic hydride phases and these are found not to order down to 2 K .

Structure and magnetic studies on UNiAlD2.2

Heavy fermion itinerant antiferromagnetic UNiAl is one of the very few U-containing compounds which absorbs H2/D2 without disproportionation. The present neutron diffraction studies on UNiAlDy (y=2.2) are directed towards resolving controversies with regard to the occupancy of Ni atoms and the associated interstitial sites for (H/D) atoms, as well as the nature of magnetic ordering in the higher hydride phase with y⩾2. The fit to the neutron diffraction data is found to improve considerably if the Ni atoms originally lying in the U-atoms’ plane in UNiAl get shifted to the Ni–Al atoms’ plane in the deuteride. This is in agreement with an earlier neutron diffraction report on a deuteride sample of similar composition [T. Yamamoto et al., J. Alloys Compd. 269, 162 (1998)] and our x-ray structural studies on UNiAlH2.3 [P. Raj et al., Phys. Rev. B 63, 94414 (2001)], but differs from those of Bordallo et al., [H. N. Bordallo et al., Physica B 276–278, 706 (2000)] and of Kolomiets et al. [A. V. Kolomiets et al.,...