Large H Absorption in Nd 2 Ni 2 In; Magnetism in a New Structure Type
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Large H absorption in Nd2Ni2In; magnetism in a new structure type Ladislav Havela1, Silvie Mašková1, Stanislav Daniš1, Olha Stelmakhovych1,2 and Khrystyna Miliyanchuk2 1 Department of Condensed Matter Physics, Charles University, Prague 2, Czech Republic 2 Faculty of Chemistry, Ivan Franko National University, Lviv, Ukraine
ABSTRACT Several hydrides of Nd2Ni2In with various H concentrations up to 6 H atoms/f.u. have been synthesized. Nd2Ni2In crystallizes in the tetragonal Mo2FeB2 structure type which can be changed upon hydrogenation to an orthorhombic structure, space group Pbma. The unit-cell volume increases by 15.5 % for Nd2Ni2InH4.5 and by 21.6 % for Nd2Ni2InH6. The triangular rareearth lattice can give rise to magnetic frustration in case of antiferromagnetic interactions in Nd2Ni2In. The frustration is removed upon hydrogenation due to the orthorhombic distortion. The magnetic ordering temperature decreases with H concentration from 8 K in Nd2Ni2In down to 3.5 K in Nd2Ni2InH6. INTRODUCTION Nd2Ni2In belongs to the large family of RE2T2X (RE = rare-earth, T = transition metal, X = p-metal) compounds [1] crystallizing in the Mo2FeB2 structure type (space group P4/mbm). The structure is a layered structure with two types of layers, which alternate along the c-axis. The first layer contains only RE atoms, while the second one contains T and X atoms. The RE atoms are arranged in triangular motif (Figure 1), which may bring a geometrical frustration into the system. For a certain type of inter-site coupling, the structure is an example of the so-called twodimensional Shastry-Sutherland lattice [2]. Hydrogenation properties of this compound were studied previously [3]. Hydrogenation at room temperature and 500 mbar pressure of hydrogen led to the absorption of approximately 4 H/f.u. and crystal structure modifications. The structure undergoes an orthorhombic distortion (space group Pbam) due to the hydrogen absorption. Investigation of crystallographic parameters of the existing interstitials showed that favourable interstitial positions for hydrogen atoms allow the accommodation of up to 6 H/f.u. In this paper, we describe the successful synthesis of hydrides with up to 6 H/f.u. and their crystal structures and magnetic properties.
Figure 1. Projection of the Nd2Ni2In structure onto the xy plane. Nd, Ni and In atoms are drawn as grey, filled and open circles, respectively. Atoms connected by thin lines lie on the planes at z = 0 and by thick lines at z = ½. The dotted lines mark the basis of the tetragonal structure. EXPERIMENT Nd2Ni2In was prepared by arc melting of pure elements under Ar atmosphere. The button was melted two times to ensure homogeneity. The alloy was subsequently crushed into submillimeter pieces. Then the standard procedure of hydrogenation was performed. So as to remove surface contaminants, the powder was heated to 250 °C in dynamic vacuum (10-6 mbar). After introducing the hydrogen gas (4 bar), sample started to absorb hydrogen readily. The hydrogen absorption was registered b
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