Structural Evolution of Ammonia Borane for Hydrogen Storage

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225 K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4 mm above 225 K. At 16 K, the BD3-ND3 unit stacks along the c axis with a tilt angle of about 16° between the N-B bond and the c axis. As the temperature is increased, the BD3-ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition

Crystal and Electronic Structures of LiNH₂

The crystal structure of LiNH2 was reinvestigated using powder neutron diffraction with high sensitivity. The compound crystallizes in the tetragonal space group I4 with lattice parameters α = b= 5.034 42 (24) Å, c = 10.255 58 (52) Å. It is found that H atoms occupy 8g1(0.2429, 0.1285, 0.1910) and 8g2 (0.3840, 0.3512, 0.1278) sites. The bond lengths between the nearest nitrogen and hydrogen atoms are 0.986 and 0.942 Å, respectively. The bond angle between H-N-H is about 99.97°. These results are significantly different from those of previous experiments. The electronic structure was calculated according to the revised structural data. The calculated density of states and charge density distribution show strong ionic characteristics between the ionic Li+ cation and the covalent bonded [NH2]- anion

Crystal and Electronic Structures of the Complex Hydride Li₄BN₃H₁₀

The crystal structure of Li4BN3H10 was investigated using powder neutron diffraction with high sensitivity. The compound crystallizes in the cubic space group 213 with lattice parameters a=10.645 19(52) Å with an ordered arrangement of [NH2]−1 and [BH4]−1 anions in a molar ratio of 3:1. The bond lengths between the nearest nitrogen and hydrogen atoms are 1.04(4) and 1.14(4) Å. The bond angle between H(1)-N-H(2) is about 126(6)°, while those between H(3)-B-H(3) and H(3)-B-H(4) are about 109(6)°-110(7)°. There are three different Li sites surrounded by [NH2]−1 and [BH4]−1 anions in distorted tetrahedral configurations. The Li(3)-B and Li(3)-N bond distances are about 1.72(3) and 2.32(2) Å, respectively, while the Li(1)-N and Li(2)-N distances are both around 2.09 Å. The strong bonding of Li(3) to the [BH4]−1 and the weaker Li(3)-[NH2]−1 bond are evidenced by the presence of the LiBH4 moiety in a projection of the crystal structure onto the a-b plane. First-principle calculations have been performed based on the structural data. Analyses of the density of states and charge density indicate that H(1) and H(2) strongly interact with N, and H(3) and H(4) interact with B to form [NH2]−1 and [BH4]−1, respectively. It is confirmed that Li(1) and Li(2) are strongly bonded to N and Li(3) is strongly bonded to B. These results are significantly different from some of the previous studies

Magnetic Properties of the MnBi Intermetallic Compound

A MnBi alloy containing over 90 wt% low-temperature phase (LTP) has been obtained by high-temperature sintering and magnetic purification. The coercivity of the bonded MnBi magnet increases with increasing temperatures. A coercivity of 2.0 T has been achieved at 400 K. The maximum energy product (BH)max; of the magnet is 7.7 MGOe (61 kJ/m3) and 4.6 MGOe (37 kJ/m3) at room temperature and 400 K, respectively. Neutron diffraction and magnetic data reveal a spin reorientation, which gives rise to low anisotropy fields and coercivity at lower temperatures for the LTP MnBi alloy

Observation of Novel Disordered Rhombohedral R₂Fe₁₇ (R=Rare Earth) Based Compounds

A recent study of (SmGd)Fe 14Si3 showed an anomalous x-ray diffraction pattern [P. C. Ezekwenna, G. K. Marasinghe, J.-H. Nam, W. J. James, W. B. Yelon, M. Ellouze, and Ph. I\u27Héritier, J. Appl. Phys. 87, 6716 (2000)]. Although all observed peaks could be indexed to the rhombohedral 2:17 phase (R-3m) many lines were strongly reduced in intensity. Subsequently, the same phenomenon was seen in neutron diffraction patterns from Nd2-xDysFe17-ySiy alloys and conventional refinement using the rhombohedral phase left significant residuals. Fourier mapping revealed additional density along the c-axis chains of Fe dumbbells and rare earth atoms. The perfect a-b-c stacking of the CaCu5 layers with regular 1/3 replacement of the rare earth atoms by Fe dumbbells appears to be broken. Detailed analysis shows that excess Fe dumbbells are incorporated into the structure and some reverse substitution is seen. In addition, the weak intensities are associated with a near equivalence of the 9d and 18h Fe sites, as in the parent CaCu5 phase. The relationship of this new structure to the other CaCu5 derived phases is described. The high Fe concentration creates the possibility of high TC and higher remanence than in the ordered 2:17 compounds

Study of Structural and Magnetic Properties of Iron-Rich Mixed Rare-Earth NdDyFe (17-y-x)CoxSiy Compounds

A series of NdDyFe(17-y-x)CoxSiy solid solutions with = 2 and 3 and = 0.5 1.0 and 1.5 were prepared by induction melting stoichiometric amounts of high-purity elements. The postannealed samples consist of two phases belonging to the space groups R3 m and P63 mmc . The lattice parameters and the unit cell volumes were calculated from the refinements of the magnetic and structural unit cells using the FULLPROF version of the Rietveld program. For a fixed content of Co, the maximum Curie temperatures (305 C to 405 C) were observed in samples with = 1 and having two phases, a disordered rhombohedral (DR) structure and a disordered hexagonal (DH) structure. An increase in the Curie temperature of 70 C per atom of cobalt is observed in NdDyFe(17-y-x)CoxSiy with y = 1 and x\u3c 3, suggesting that with a suitable choice of rare earths this DR phase may be a promising candidate for high-energy product permanent magnets. The magnetization versus temperature (M versus T) plots of the solid solutions, which consist of two phases, exhibit only a single magnetic ordering transition temperature

Large Scale Growth and Magnetic Properties of Fe and Fe₃O₄ Nanowires

Fe and Fe3O4 nanowires have been synthesized by thermal decomposition of Fe(CO)5, followed by heat treatments. The Fe wires are formed through the aggregation of nanoparticles generated by decomposition of Fe(CO)5. A core-shell structure with an iron oxide shell and Fe core is observed for the as-prepared Fe wires. Annealing in air leads to the formation of Fe2O3/Fe3O4 wires, which after heat treatment in a N2/alcohol atmosphere form Fe3O4 wires with a sharp Verwey [Nature (London) 144, 327 (1939)] transition at 125 K. The Fe3O4 wires have coercivities of 261 and 735 Oe along the wire axis at RT and 5 K, respectively. The large increase of coercivity at 5 K as compared to RT is due to the increase of anisotropy resulting from the Verwey transition

Structural, Magnetic, and Transport Properties of Zr-substituted La₀.₇ Sr₀.₃ Mn O₃

Zr-substituted perovskites La0.7Sr0.3 Mn1-x Zrx O3 with 0 ≤ x ≤ 0.20 were investigated by neutron diffraction (ND), magnetization, electric resistivity, and magnetoresistance measurements. ND refinements reveal that substituted Zr4+ goes only to the Mn site. Because of its large size, this leads to a Zr-solubility limit at x ≤ 0.10. The x ≤ 0.10 samples exhibit a rhombohedral structure (R3c) from 10 K to room temperature. For the x ≤ 0.10 samples, the cell parameters a and c, and volume increase continuously with increasing Zr content. In addition, the structural distortion of the Mn O6 octahedra increases with increasing Zr content. Zr-substituted La0.7 Sr0.3 Mn1-x Zrx O3 exhibits metallic behavior at low temperature. The field dependent resistivity suggests that electron-electron scattering is dominant and a two-magnon scattering process emerges with increasing temperature. The contribution from the two-magnon scattering in resistivity becomes larger with increasing Zr content. A maximum magnetoresistance (MRmax) of 35% is obtained for the x=0.03 sample at H=5 T. The MRmax shifts to a higher temperature region upon application of an external magnetic field

The Effect of Cu-Doping on the Magnetic and Transport Properties of La₀.₇Sr₀.₃MnO₃

The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1xCuxO3 (0\u3c=x\u3c=0.20) have been studied using neutron diffraction, magnetization, and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R[overline 3]c space-group from 10 K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the MnO bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x\u3e=0.15 samples. The x=0.15 sample shows the highest MR([approximate]80%), which might result from the co-existence of Cu3-Cu2+ and the dilution effect of Cu-doping on the double exchange interactio

Neutron and Magnetic Studies of La₀.₇Sr₀.₃Mn₁₋ₓCrₓO₃(x ≤ 0.7): A Homogeneous Charge-Ordered System

Structural and magnetic properties of La0.7Sr0.3Mn1-xCrxO3 (0 \u3c x ≤ 0.7) have been studied in order to determine the effect of substitution of Cr3+ for Mn3+. The data consist of neutron and x-ray powder-diffraction and magnetization measurements. We previously suggested these systems transition from ferromagnetic to antiferromagnetic ordering with the intermediate concentrations containing coexisting ferromagnetic and antiferromagnetic domains. Upon further detailed examination, we find that the neutron data can be fit using a single homogeneous long-range magnetically ordered state and compositionally dependent charge ordering. The magnetic structures are controlled by the competition between Mn-Mn, Mn-Cr, and Cr-Cr interactions (double exchange and superexchange). The metal to semimetal and semimetal to insulator transitions can be quantitatively described as due to the localization effect of superexchange. The presence of charge ordered states in the insulating region arises from the favorable energetics of Mn4+-O-Cr3+ superexchange bonds relative to Mn3+-O-Cr3+ bonds