F)twpPnqcP-qyy3q MAGNETIC PROPERTIES AND CRYSTAL STRUCTURE OF lU?NiAl AND UNiAl

Introduction RENiAl (RI3 = rare-earth metal) and UNiAl compounds crysta.llising in the hexagonal ZrNiA1-type structure (space group P~2nz ) can absorb up to 2 and 3 hydrogen (deuterium) atoms per formula unit, respectively. Hydrogenation leads to a notable lattice expansion and modification of magnetic properties. However, the impact of hydrogenation on magnetism is the opposite for 4f-and 5f-materials: TN(TC)is lowered in the case of rare-earth hydrides, while for UNiAIH(D)x it increases by an order of magnitude [1][2][3][4]. Here we present results of magnetic and structure studies performed of these compounds, focusing on the correlation between magnetic and structural variations and discussing possible reasons of the strilcing difference in effect of hydrogenation on rare-earth and actinide intermetallics. Experiment. Hydrides (deuterides) have been synthesised using a two-stage process. First the parent intermetallics were arc-melted from the constituent elements under Ar atmosphere, and he phase purity was veritled by means of X-rays diffraction. Secondly the material was crushed and hydrogenated, by means of activating the specimen in a vacuum of 10<Torr at 350"C for 1 hour, followed by exposure to H2(Dz) at 20 atm pressure for HoNiAl and 55atm pressure for UNiA1. The synthesis of HoNiA.lH2.0was initiated by raising the temperature in the reaction chamber to 50 'C. The amount of absorbed D(H) was determined by monitoring the decrease of pressure in a calibrated volume. In order to avoid sample decomposition material was stored in sealed quartz or glass ampulas. Magnetic measurements were performed using an Oxford Instruments Faraday balance and SQUID magnetometer from Quantum Design. DISCLAIMER Results and discussion. HoNiAl has the highest absorption capability among the whole 12ENiAl series. It forms , towards lower temperatures at higher fields demonstrates that HoNiAIHzo orders antiferromagnetically at TN= 6 K, while &e critical temperature of the parent compound is 13 K [5]. These two materials also display different magnetic phase diagrams: the hydride undergoes only one magnetic phase transition within the experimentally achievable temperature range (i.e. down to 1.8 K), while HoNiAl experiences spin re-orientation ffom amplitude-modulated ferromagnetic phase to a canted ferromagnetic structure at TN = 4.9 K [5,6]. The absence of the latter transition in the hydride maybe either due to its shift below 1.8 K or due to the modtilcation of the magnetic structure by the incorporation of hydrogen. For T> 11 K, the temperature dependence of magnetic susceptibility, AT), of HoNiAlH2.0 follows the Curie-Weiss law with~~~= 10.9 p~f.u. and@= -11& compared to 10.7 p~f.u. and 7.2 K for HoNiA1. We can conclude that the effective moment remains approximately unaffected by the hydrogenation, and the paramagnetic Curie temperature ,changes its sign, retaining the same order of magnitude. Field dependencies of magnetisation M(H) are quite different for HoNiAlH2.0 and HoNiAl: magnetisation of hydride reaches lower value (5.9 p.tif.u.) at the maximum field of 5 T, than that of pure compound (-7.6 ptif.u. [5]), the latter also has a remnant magnetisation, whereas M(H) of HoNiAlH2.0 shows zero remanence. Qualitatively new feature, the inflection point at 0.5 T, appears on the M vs. H dependence of hydride, while for HoNiAl it has not been observed up to~= 40 T [5]. UNiAl hydride was first obtained by Drulis et a2. [2], and the first magnetic characterisation was done by Zogal et al. [1]. Larger number of anomalies on XT) reported in [1] may be attributed to the presence of impurity phase, but the ordering temperature above 100 K looks realistic. We have studied both hydride and deutefide, uNiA1.Hz.sand " UNiAlD2.J, indicating antiferromagnetic order at 99 K and 94, respectively Non-uniform expansion in the basal plane, accompanying hydrogenation, leads to the orthorhombic distortion of the HoNiAl lattice (Table l.). Besides that the unit cell is contracted along the c-axis but due to higher multiplicity of the a-axis the total-volume is increased: AV7V= 5.8 %. The latter value is lower then the maximum volume increase in the lWNiAIHXseries observed for SmNiAIH1.2:8.7 % [4]. In spite of notably lower deuteriurn content in HoNiAID0.g7the unit cell volume remains appro~ately the same AVIV= 6.0 %, but the orthorhombic distortion is weaker and b/a ratio approaches +3, typical for hexagons ymmetry. Hydrogenation of UNiAl leads to similar non-uniform deformation of the unit cell but its symmetry remains unchanged, and the volume increase is twice higher than in for HoNiAlH2.00 Due to higher coherent scattering cross-section of deuterium neutron diffraction studies have been performed on HoNiAIDo.g7 Collusions. Hydrogenation has noticeable effect on magnetic properties and crystal structure of RENiAl compounds and UNiA1. It leads to the change of the ordering temperatures and expansion of the unit cell in both cases. While the crystal lattice modification has some common features in 4j-and 5~-compounds, i.e. expansion in the basal plane, contraction along the c-axis, positive value of AWV, magnetic uro~erties are chamzed in the omosite 6 manner. The ordering temperature of UNiAlH2.3is higher by ahnost 100 K compared to its parent compound,~shifts to more negative values, indicating stronger antiferromagnetic interaction. However, we do not have any reliable estimate for the magnitude of ordered moments~u. All these changes can be attributed to the lattice expansion, which leads to decrease of the 5~-ligand and 5~-5~hybridisation. On the contrast to UNiAl, HoNiAlH2.0has lower ordering temperature than HoNi.Al, it also has different type of magnetic ordering: AFM vs. FM in parent compound. & changes its sign after hydrogenation, but retains the same order of magnitude, indicating rather modification of the type of magnetic exchange than its strength. The mechanism leading to . these effects in HoNiAlH2.0should be different from that in UNiAlH2.3. As in the rest of RENiAl series [3,4] it can be ascribed to the weakening of the RKKY exchang~interaction, responsible for the magnetic ordering in REIW41's due to the decrease of the conduction electron density. Geometrical effect plays a secondary role since 4~-states responsible for. magnetism are located quite far from EF. The minor importance of the lattice expansion is reflected, for example, in the absence of the direct correlation between AV7Vand the decre~e of the ordering temperature. For instance ATC= -46 K and AV7V= 5.9% for GdNiASH1.ss [3,4], and ahnost the same increase of volume in HoNiAlH2.0 leads to-the reduction of the ordering temperature by 7 K only. The wedcening of the exchange interaction cannot be attributed either to the symmetry change under hydrogenation because GdNiAIH1.35and GdN~l.Ob having orthorhombic and hexagonal unit cell, respectively, show ahnost identical TN. The role of the symmetry changes maybe quite crucial for the magnetic structures for the incorporation on hydrogen affects local symmetry, thus, altering possibilities for certain arrangement of magnetic moments. &apos

The sodium-hydrogen system /

Work performed at the Chemical Research Laboratory, Metal Hydrides Incorporated, Beverly, Massachusetts."Date Declassified: December 7, 1955"--P. [2] of cover."Oak Ridge National Laboratory work performed under Contract No. W-7405-Eng-26, Subcontract 390.""January 1, 1954."Includes bibliographical references (p. 19).Mode of access: Internet

Hydride Reduction by a Sodium Hydride-Iodide Composite

Sodium hydride (NaH) is widely used as a Brønsted base in chemical synthesis and reacts with various Brønsted acids, whereas it rarely behaves as a reducing reagent through delivery of the hydride to polar π electrophiles. This study presents a series of reduction reactions of nitriles, amides, and imines as enabled by NaH in the presence of LiI or NaI. This remarkably simple protocol endows NaH with unprecedented and unique hydride-donor chemical reactivity.MOE (Min. of Education, S’pore)Published versio