Type

Bassem Ourane, †, ‡ Etienne Gaudin,* Ridha Zouari, Samuel Couillaud, and Jean-Louis Bobet , † ‡ † † † CNRS, Universite de Bordeaux, ICMCB, 87 Avenue du Dr. A. Schweitzer, Pessac F-33608, France de Sfax, B.P. 1171, 3000 Sfax, Tunisie ‡ Faculte des Sciences de Sfax, Department de Chimie, Universite tube and melted in a high-frequency furnace. The tantalum tube was then enclosed in an evacuated quartz tube and annealed at 973 K for 10 days. The temperature was decreased to 300 K at 6 K•h -1 . 11 Unknown secondary phases were observed, and then other closed starting composition and annealing procedures were tested to reduce the amount of secondary phases.

ABSTRACT: The new intermetallic NdNiMg 5 was discovered during the study of the Mg-rich part of the Mg-Nd-Ni system. It was synthetized by melting of the constituent elements in a sealed tantalum tube with subsequent annealing. Its structure was determined by Xray diffraction on a single crystal. Crystal data:

Despite the inevitable presence of minor secondary phases that are di erent in amount and nature depending on the orthorhombic system, Cmcm, Z = 4, a = 4.4799( 2 observed with T N = 12 K, and the effective magnetic form blocks stacked along the c axis (Figure 1). These blocks moment μ is equal to 3.89(1) μ .

eff B

T he study of the Mg-rich part of a RE-TM-Mg ternary diagram (RE = rare earth and TM = transition metal) is of great interest for several practical applications and usually leads to the discovery of new interesting phases. For instance, the Mgrich part of the Mg-Zn-RE system was studied to find new light structural materials with good corrosion resistance. [1][2][3] Longperiod stacking/order (LPSO) phases with hexagonal closepacking (hcp) blocks of Mg were observed. 2,3 Face-centeredcubic (fcc) blocks made of Mg, Zn, and Y atoms were located between them. Many studies were also performed to find new candidates for solid hydrogen storage because of the low weight and cost of magnesium. [4][5][6][7][8][9] Only a few Mg-rich phases were reported in the literature, and to our knowledge, LaCuMg is the Nd-Mg distance of up to 3.4 Å, i.e., the sum of the metallic radii, there are 10 Mg atoms in the coordination sphere of Nd (Figure 3). The shortest Nd-Mg distance equal to 3.292 Å for the Mg1 position is similar to the shortest distance observed in Mg 3 Nd. 18 If such hexagonal layers are already observed in other structures, for instance, in the ZrBeSi type, 19 the peculiarity in this case is that the magnetic REs belong to the hexagonal planes.

The distance between two consecutive NiNd layers is equal to 6.893 Å (i.e., c/2). One can also notice that despite the layered character of this structure a high density of 3.50 g•cm -3 is observed. This may be explained by the high compactness of the Mg blocks, which are connected through very short Mg-Mg distances, and the space between these blocks is filled by Nd and Ni atoms.

As explained in the introduction, only a few ternary Mg-rich ordered compounds are reported in the literature. NdNiMg 5 is, to our knowledge, the richest one. The structures of the richest phases are often disordered and derive from those of the corresponding binary compounds such as observed for LaCuMg 8 7 or LaAg 1.44 Mg 10.56 . 20 LaCuMg 8 crystallizes with the La Mg -type structure, and its formula can be rewritten as

  ) Å, b ff = 9.9827(3) Å, c = 13.7854(10) Å, d calc = 3.49 g•cm . Its -3 experimental procedure, the magnetic transition at 12 K is systematically evidenced by magnetic or resistivity measurements. The full pattern matching of the powder pattern of the sample used for magnetic measurements (see below) is shown in the Supporting Information. The study of the secondary phases is under progress. structure is made of infinite layers of Mg atoms that form blocks stacked along the c axis. These blocks, with a closepacked array of Mg atoms, are separated by infinite NiNd layers and connected through short Mg-Mg bonds. In the NiNd layer, the Ni and Nd atoms form an ordered graphite-type network. Antiferromagnetic ordering is The structure of NdNiMg is made of in nite layers of Mg that fi

Figure 1 .

 1 Figure 1. View along the a axis of the structure of NdNiMg . The Mg-

Figure 2 .

 2 Figure 2. (a) View of one Mg block with some empty octahedra drawn in orange and tetrahedra in gray or cyan. (b) View of the NiNd layers along the c axis.

  Figure 4. Temperature dependence of the magnetization M measured under an applied magnetic field μ 0 H = 0.05 T determined in the zerofield-cooling (ZFC) and field-cooling (FC) processes. The inset displays the reciprocal susceptibility measured under an applied magnetic field μ H = 3 T with a Curie-Weiss fit (red line).Figure 3. Environment of the Ni1 and Nd1 positions with 90% probability ellipsoids.

Figure 3 .

 3 Figure 4. Temperature dependence of the magnetization M measured under an applied magnetic field μ 0 H = 0.05 T determined in the zerofield-cooling (ZFC) and field-cooling (FC) processes. The inset displays the reciprocal susceptibility measured under an applied magnetic field μ H = 3 T with a Curie-Weiss fit (red line).Figure 3. Environment of the Ni1 and Nd1 positions with 90% probability ellipsoids.

The experimental effective magnetic moment μ eff calculated is 3.89(1) μ B . The main contribution to magnetism is expected to arise from the RE. Because the 5d and 6s electrons are delocalized, the contribution of the RE to magnetism originates from the localized 4f electrons. The theoretical effective magnetic moment expected for Nd is then μ eff th = g J [J(J + 1)] 1/2 μ B = 3.62 μ B (4f 3 configuration with S = 3 / 2 , L = 6, J = 9 / 2 , and g J = 8 / 11 ). The experimental value is slightly higher, and this may be attributed to (i) a small magnetic moment carried by Ni atoms, suggesting that their 3d bands are not completely filled, (ii) a contribution from conduction electrons, and (iii) the presence of a secondary phase. Neutron experiments are planned to address this question. Magnetization data were taken while the sample was being warmed. In a first step, the sample was prior cooled by zero field cooling (ZFC), and in a second step, it was prior cooled by applied field cooling (FC). Nevertheless, in both cases, the results are the same, and both M/H versus T curves are superposed (Figure 4). A cusplike behavior indicating antiferromagnetic ordering is clearly seen with a Neél temperature T N = 12 K. Complementary physical characterizations are also in progress: magnetization versus field, specific heat, transport, and mechanical properties measurements.

S * Supporting Information

Crystallographic data in CIF format, table with interatomic distances, and an X-ray diffraction pattern. This material is available free of charge via the Internet at http://pubs.acs.org. *E-mail: gaudin@icmcb-bordeaux.cnrs.fr. Fax: +33 5 4000 2761.

Notes

The authors declare no competing financial interest.

(1) Inoue, A.; Kawamura, Y.; Matsushita, M.; Hayashi, K.; Koike, J. J. Mater. Res. 2001, 16, 1894. ( (10) The composition of the phases in the sample was checked by microprobe analysis using a Cameca SX-100 instrument. Analysis was performed on the basis of intensity measurements of the Nd Lα, Ni Kα, and Mg Kα X-ray emission lines, which were compared with those obtained for the high-purity elements Nd, Ni, and Mg used as reference compounds.

(11) Starting materials for preparation of the samples were neodymium pieces (ZLX Tech, >99.9%), a nickel rod (Strem Chemicals, >99.9%), and a magnesium rod (Alpha Aesar, >99.8%).

(12) Crystal data: orthorhombic system, Cmcm, Z = 4, a = 4.4799(2) Å, b = 9.9827(3) Å, c = 13.7854(10) Å, d calc = 3.49 g•cm -3 . The refinement of the crystal structure was made using single-crystal X-ray diffraction data. The single crystal was isolated from a crushed block and selected by optical microscopy. Reflection data were collected at room temperature on an Enraf-Nonius Kappa CCD using Mo Kα radiation. A Gaussiantype absorption correction was applied, with the shape of the single crystal being determined with the video microscope of the diffractometer. A total of 2181 reflections were measured up to θ = 30°. Data processing and all refinements were performed with the Jana2006 program package. 22 All of the atom positions were located using the SUPERFLIP program. 23 At the end of the refinement, the reliability factors were R[F 2 > 2σ(F 2 )] = 0.021 and R w (F 2 ) = 0.056. S = 1.34 for 26 parameters, 516 independent reflections, and with a residual electron density in the range of -0.95 to +0.63 e

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