CrB-type Structure Research Articles

Magnetic order and spin-reorientation in HoGa

We have determined the magnetic structure of the intermetallic compound HoGa by high-resolution neutron powder diffraction. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure and the magnetic structure comprises ferromagnetic order of the Ho sublattice along the c-axis. The Curie temperature is 66(3) K. Upon cooling below 20 K, the Ho magnetic moments cant away from the c-axis towards the ab-plane. At 3 K, the Ho moment is 8.8(2) μB and the Ho magnetic moments point in the direction θ = 30(2)° and ϕ = 49(4)° with respect to the crystallographic c-axis. The observation of an ab-plane component at around 50° from the a-axis is in contrast with the suggested magnetic structure of ac order (θ = 32° and ϕ = 0°) reported by Delyagin et al. [1] on the basis of a 119Sn Mössbauer spectroscopy study of a Sn-doped HoGa sample. However, we find that these two sets of orientations are in fact indistinguishable by Mössbauer spectroscopy.

Electronic structure and chemical bonding of LiYSi

The electronic structure of the ternary silicide LiYSi (ZrNiAl type, P6¯2m N°189, a = 702.3, c = 421.2 pm) is examined from ab initio with an assessment of the properties of chemical bonding. The compound is found semi-conducting with a very small gap and the chemical bonding is found mainly between Y and Si as well as Li–Si with differentiated Li–Si1/Li–Si2. The structure with totally de-intercalated Li keeps the characteristics of LiYSi with a reduction of the c/a ratio and of the volume albeit with less stability than binary YSi with orthorhombic CrB type structure. The electronic structure calculations indicate the possibility of an at least partial delithiation Li1−xYSi while keeping the hexagonal structure.

Magnetocaloric Effect in ErSi Compound

The magnetic properties and magnetocaloric effects (MCEs) have been investigated in ErSi compound, which crystallizes in the orthorhombic CrB-type structure. This compound is antiferromagnetic (AFM) with a Neel temperature TN of 12.5 K. The measurements of the temperature and field dependences of the magnetization clearly show that the ErSi compound undergoes a field-induced metamagnetic transition from AFM to ferromagnetic (FM) state below TN. The isothermal magnetic entropy change (ΔSM) and refrigerant capacity (RC) of ErSi have been measured for various applied magnetic fields and their values are found to be -14.8J ·kg -1·K -1 and 210 J ·kg -1 for a field change of 0-5 T, respectively. The large MCE is attributed to the field-induced AFM to FM metamagnetism occurring in this compound.

Hydrogen interaction with IMC RNi (R = Sm, Tb, Gd, Dy)

The hydrogenation of GdNi, TbNi, DyNi and SmNi at room temperature and hydrogen pressure below 1 atm has been studied. Ternary hydrides GdNiH 3.2, TbNiH 3.4, DyNiH 3.4 and SmNiH 3.7 crystallize in orthorhombic CrB-type structure (S.G. Cmcm). The hydrides formation is accompanied with the strong lattice expansion. On the basis of neutron diffraction patterns refinement the atomic positions of D atoms and interatomic distances were determined for TbNiD 3.4 and DyNiD 3.4 samples. Deuterium atoms occupy three types of interstitial sites in the structure of TbNiD 3.4 and DyNiD 3.4: 4(c)-tetrahedron sites [R 3Ni], 8(f)-trigonal bipyramide sites [R 3Ni 2] and linear 4(b)-sites [Ni 2].

YNi and its hydrides: Phase stabilities, electronic structures and chemical bonding properties from first principles

Within density functional theory, establishing the equations of states of YNi in two different controversial structures in the literature, leads to determine the orthorhombic FeB-type as the ground state one with small energy difference. For YNiH 3 and YNiH 4 hydrides crystallizing in the orthorhombic CrB-type structure the geometry optimization and the ab initio determination of the H atomic positions show that the stability of hydrogen decreases from the tri- to the tetra- hydride. New states brought by hydrogen within the valence band lead to its broadening and to enhanced localization of metal density of states. The chemical bonding analysis shows a preferential Ni–H bonding versus Y–H.

Magnetic properties of the intermetallic compounds RNi (R=Gd, Tb, Dy, Sm) and their hydrides

Hydrogen interaction with RNi intermetallic compounds and the influence of hydrogen on magnetic properties of these compounds were investigated. Ternary hydrides GdNiH3.2, TbNiH3.4, DyNiH3.4 and SmNiH3.7 were prepared by hydrogenation of the initial alloys at room temperature and hydrogen pressure up to 0.1 MPa. Hydrides possess orthorhombic CrB-type structure (S.G. Cmcm). The formation of hydrides results in substantial expansion of the metallic sublattices, weakening of the ferromagnetic interactions and decreasing of the paramagnetic Curie temperatures. The article was translated by the author.

Magnetic properties and magnetocaloric effect of GdGa compound

Polycrystalline GdGa compound was prepared by arc-melting method. X-ray powder diffraction reveals that GdGa crystallizes in the CrB-type structure. The Curie temperature of the compound is 183 K. A distinct peak associated with spin reorientation transition was shown around 100 K on the magnetization versus temperature Curve measured in a field of 0.05T. Isothermal magnetic entropy change (vertical bar Delta S(M)vertical bar) was estimated based on the magnetization isotherms. The maximum value of vertical bar Delta S(M)vertical bar is 4.81 J kg(-1) K(-1) under the apphed field changing from 0T to 5.0 T. The relative cooling power (RCP(S)) of GdGa compound is about 576 J kg(-1) under an applied field change of 5 T. The specific heat of the compound has been estimated within the framework of Debye and mean field approximations to evaluate the adiabatic temperature change (Delta T(ad)). (C) 2008 Elsevier B.V. All rights reserved.

Structure and magnetic properties of the pseudobinary solid solution TbGa1-xGex(0≤x≤0.4)

Polycrystalline samples of TbGa1-xGex(x≤0.4) have been prepared by arc-melting. X-ray powder diffraction reveals that all the samples crystallize in a single phase of CrB-type structure and space group Cmcm. The lattice constants of the compounds decrease linearly with the increasing Ge content. The compounds form a solid solution in the Ge content range of 0≤x≤0.4. The asymptotic Curie points and the effective paramagnetic moments of the compounds have been determined by thermomagnetic measurements. The ordering temperatures were derived from ac susceptibility data. The XRD patterns at various temperatures reveal that there is no structure phase transition in the samples with x=0.2 and 0.3 in the temperature range of 113—273K.

Infinite Gold Zig‐Zag Chains as Structural Motif in Ca3Au3In – A Ternary Ordered Variant of the Ni4B3 Type

AbstractNew auride Ca3Au3In was synthesized from the elements in a sealed tantalum tube in a high‐frequency furnace. Ca3Au3In was investigated by X‐ray powder and single crystal diffraction: ordered Ni4B3 type, Pnma, a = 1664.1(6), b = 457.3(2), c = 895.0(3) pm, wR2 = 0.0488, 1361 F2 values, and 44 variables. The three crystallographically independent boron positions of the Ni4B3 type are occupied by the gold atoms, while the four nickel sites are occupied by calcium and indium in an ordered manner. All gold atoms have trigonal prismatic coordination, i.e. Ca6 prisms for Au1 and Au2 and Ca4In2 prisms for Au3. While the Au3 atoms are isolated, we observe Au1–Au1 and Au2–Au2 zig‐zag chains at Au–Au distances of 292 and 284 pm. These slabs resemble the CrB type structure of CaAu. Consequently Ca3Au3In can be considered as a ternary auride. Together the Au2, Au3 and indium atoms build up a three‐dimensional [Au2In] polyanionic network (281–293 pm Au–In) in which the chains of Au1 centered trigonal prisms are embedded. The crystal chemical similarities with the structures of Ni4B3, CaAuIn, and CaAu are discussed.

Electronic structure and magnetism of PrNi xPt 1− x compounds

We have studied influence of the Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi x Pt 1− x compounds. Polycrystalline samples with x = 1, 0.9, 0.75, 0 were prepared and characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The analysis of XRPD data confirmed that the orthorhombic CrB-type structure owned by the parent binary compounds remains conserved through the entire series. The samples were subsequently investigated by specific heat ( C p ), magnetization ( M) and ac susceptibility measurements in the temperature range 2–350 K and in magnetic fields up to 9 T. All compounds were found to order ferromagnetically. The T C values monotonously increase with increasing Ni content. To inspect the crystal-field (CEF) effects and magnetocaloric properties specific-heat data were analyzed in detail and the magnetic contribution to the specific heat together with the magnetic entropy have been determined. The results of first principles electronic structure calculations of the PrNi and PrPt confirmed that besides the stable Pr magnetic moments due to localized 4f-electrons only a very small magnetic moments of at most 0.2 μ B is induced at the Ni (Pt) site due to the polarized 3d-electron states (5d-electron states) hybridizing with the Pr 5d-electron states, i.e. the Ni (Pt) moment plays only minor role in the total balance of the magnetic moments in these compounds.

Magnetic and thermodynamic properties and MCE of intermetallic compounds PrNi 1−xPt x

We have studied the influence of Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi 1− x Pt x pseudobinary compounds. The orthorhombic CrB-type structure owned by PrNi was found to remain conserved when substituting Pt for Ni while the unit cell volume linearly increased. The polycrystalline PrNi 1− x Pt x samples were investigated by measuring the specific heat ( C P ) and magnetization ( M) as function of temperature and magnetic field. All studied compounds were found to order ferromagnetically; the Curie temperature T C is linearly decreasing with increasing Pt content. The magnetocaloric effect (the isothermal magnetic entropy change Δ S mag) was determined consistently from the specific heat and magnetization data. The maximum values of Δ S mag are the same for the boundary compounds PrNi and PrPt but become somewhat reduced in solid solutions.

Be-Mo 2元系状態図の熱力学的解析

A thermodynamic analysis of the Be-Mo system has been carried out using the CALPHAD method. The formation energies of the four intermetallic phases of this system obtained from first-principles calculations were utilized to compensate for the lack of experimental information on the phase boundaries and thermodynamic properties of this binary system. The optimized thermodynamic parameters reproduced the characteristic features of the phase diagram quite well. Our first-principles calculations show that the stable BeMo3 phase may not have the Cr3Si-type cubic structure reported in a previous study, because of its lower stability. The first-principles calculations also imply that BeMo with a CrB-type orthorhombic structure may be the stable phase in the ground state.

Effect of chemical and external pressure on the structure of intermetallic compound CeNi

Neutron powder diffraction was employed to study the structural modifications of the intermediate-valence compound CeNi at room temperature induced by either chemical or external pressure. For the first time we were able to record the diffraction pattern resulting from the pressure-induced first-order phase transition occurring in CeNi at 300 K. At pressure P = 2 GPa we observe the coexistence of two phases while only single pressure-induced phase is visible at P = 5 GPa. The results obtained are indicative of a higher symmetry of the collapsed structure as compared to the CrB-type ambient pressure structure of CeNi. The critical pressure range around 2 GPa is found to be agreement with the previous estimation derived from the thermopower measurements.

A conjoint XRD–ND analysis of the crystal structures of austenitic and martensitic Ti 0.64Zr 0.36Ni hydrides

The crystal structures of the hydrides of austenitic and martensitic Ti 0.64Zr 0.36Ni alloy have been investigated by conjoint X-ray diffraction (XRD)–neutron diffraction (ND) analysis. Austenitic Ti 0.64Zr 0.36Ni alloy with cubic CsCl-type structure preserves its metal sublattice structure after deuteration. It forms a Ti 0.64Zr 0.36NiD 1.5 deuteride with D-atoms occupying half of the octahedrally coordinated 3 d sites. On the contrary, the monoclinic TiNi-type structure of martensitic Ti 0.64Zr 0.36Ni alloy is modified after deuterium absorption. At P D 2 = 1 0 3 Pa and T = 298 K , two deuterides coexist with orthorhombic CrB-type structure for the metal sublattice and compositions Ti 0.64Zr 0.36NiD ( β-deuteride) and Ti 0.64Zr 0.36NiD 2.6 ( γ-deuteride). For the β-monodeuteride, deuterium atoms are tetrahedrally coordinated by (Ti,Zr) atoms. For the γ-deuteride, D-atoms fully occupy tetrahedrally coordinated (Ti,Zr) 3Ni 8 f sites and partially occupy pyramidal (Ti,Zr) 3Ni 2 4 c sites. At higher pressures, deuterium solution occurs in the γ-phase with a partial occupancy of octahedrally coordinated (Ti,Zr) 2Ni 4 4 a sites.

Hydriding and dehydriding properties of CaSi

The hydriding and dehydriding properties of CaSi were investigated both theoretically and experimentally. First-principles calculations suggested that CaSiH n is thermodynamically stable. Experimentally, the p – c isotherms clearly demonstrated plateau pressures in a temperature range of 473–573 K and the maximum hydrogen content was 1.9 weight % (wt.%) under a hydrogen pressure of 9 MPa at 473 K. The structure of CaSiH n is different from those of ZrNi hydrides, although CaSi has the CrB-type structure as well as ZrNi.

Relationship between polymorphism and hydrogenation properties in Ti 0.64Zr 0.36Ni alloy

The influence of the parent crystal structure on the hydrogenation of Ti 0.64Zr 0.36Ni alloy has been investigated. Austenitic Ti 0.64Zr 0.36Ni alloy with cubic CsCl-type structure preserves its metal sublattice structure after hydrogenation. It forms Ti 0.64Zr 0.36NiH 1.6 hydride ( P H 2 = 1 MPa and T = 373 K) through a cell-volume expansion of 10.4%. On the contrary, the monoclinic TiNi-type structure of martensitic Ti 0.64Zr 0.36Ni alloy is modified after hydrogen absorption. At P H 2 = 0.06 MPa and T = 373 K, two hydrides coexist with orthorhombic CrB-type structure and compositions Ti 0.64Zr 0.36NiH and Ti 0.64Zr 0.36NiH 2.2, which induces a cell-volume expansion of 5.36 and 18.3%, respectively. The stability of these crystal structures is discussed by comparison with other AB intermetallic compounds (A = Ti, Zr, Hf) and B = (Fe, Co, Ni).

Ab initio investigations of TlI-type compounds under high pressure

Abstract In this work, we present a theoretical study (based on DFT-calculations) in a wide pressure range of the structural and electronic properties and the stability of compounds crystallising in a TlI- or CrB-type structure. Both structure types have the characteristic structural feature of zigzag chains with unusual short homonuclear distances. The main focus of this study is to elucidate the nature of bonding within these zigzag chains at ambient and elevated pressure. For this purpose we discuss the evolution of the distances within the zigzag chains with pressure, the transition pressure of the phase transition to a CsCl-type arrangement (high-pressure phase) and compressibilities of the low- and high-pressure phases. For a better understanding of the structure and bonding, the band structures of these compounds are evaluated. The calculations are complemented by an orbital analysis using the crystal orbital Hamilton population (COHP) and an analysis of the electronic density topology with the electron localisation function (ELF). Our study indicates that there is a bonding electron pair in compounds crystallising in the CrB-type structure and that the nature of the electron pair does not change significantly at elevated pressure up to the phase transition. However, the “character” of the additional electron pair in the In-monohalides (TlI-type structure) changes with increasing pressure from nonbonding to bonding. The phase transition to a CsCl- type structure implies a fundamental change to nonbonding stereochemically inert electron pairs for all compounds.

On the structure and magnetic properties of CeNi with minor Cu substitutions

The introduction of Cu in the CeNi compound maintains the same crystallographic CrB-type structure up to CeNi 0.85Cu 0.15. The crystallographic structure of several compositions in the neighbourhood of this limit has been determined from single crystals in a 4-circle diffractometer. The thermal expansion coefficients of CeNi 0.9Cu 0.1 behave similar to those of CeNi. The Cu substitution disturbs the intermediate valence behaviour of CeNi, inducing non-zero magnetic moments at low temperatures and suggesting a cluster-glass behaviour.

Calculation of the density of states of transition metal monosilicides by a first-principle pseudopotential method using plane-wave basis

The density of states (DOS) of transition metal monosilicides of the following types of structures have been calculated; FeSi, CrB, FeB and MnP. The DOS curves of FeSi and CoSi in an equilibrium structure have a gap or quite low values at the Fermi level. The DOS of MnSi is split at the Fermi level by spin-polarization. The Fermi levels of ScSi, YSi (IIIa Group) with a CrB type structure, LaSi (IIIa Group) with an FeB type structure, and TiSi (IVa Group) with an FeB type structure are located just in the dip or in the region of relatively low DOS values. The Fermi level of equilibrium for RhSi and IrSi with a MnP-type structure lies in the region out of high DOS values but that of hypothetical CoSi with the same type of structure does not. Thus, the principle of ‘ the DOS at the Fermi level would be, hopefully, smaller in an energetically-favored structure ' is valid in these silicides.

Anomalous thermal expansion in Gd-based intermetallics

A study of the thermal expansion of Gd-based intermetallic compounds is presented. The goal was to investigate the thermal expansion in magnetic rare-earth systems without the influence of the crystal field. In order to separate the magnetoelastic effect from that of the crystal field. Gd compounds have been selected. The anisotropic thermal expansion of selected Gd-based compounds with various crystal structures has been studied by means of X-ray diffraction. In compounds with lower symmetry, e.g. GdCu 2 (CeCu 2-type structure) and GdNi (CrB-type structure), a huge anomalous thermal expansion has been found below their magnetic transition temperatures.