FeB-type Structure Research Articles

Eine neue Modifikation von Lanthanmonogermanid – lT-LaGe /A New Modification of Lanthanummonogermanide – lT-LaGe

Abstract Single crystals of the low temperature modification of LaGe are obtained as a byproduct in the reaction of La metal, LaBr3 and Ge powder at 1000 °C as silver colored, moisture sensitive needles. lT-LaGe crystallizes in space group Cmcm with a = 4.5590(10), b = 13.766(2), c = 6.745(2)Å . The Ge atoms form cis-trans-cis-trans-chains with dGe−Ge = 2.621(1) and 2.799(1) Å in contrast to Ge zigzag chains found in the high temperature modification of LaGe crystallizing in the FeB structure type. In both structures the Ge atoms are surrounded by trigonal prisms of lanthanum atoms with CN = 6+1 but different connection of the prisms.

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.

Magnetic structure of GdCu through the martensitic structural transformation: A neutron-diffraction study

We report investigations on the magnetic structure through the martensitic structural transformation in the GdCu system obtained by means of neutron-diffraction experiments. At room temperature, the as-cast bulk samples adopt a CsCl-type crystallographic structure, but when the temperature is lowered a martensitic structural transformation $\mathrm{CsC}\stackrel{\ensuremath{\rightarrow}}{l}\mathrm{FeB}$ takes place at around 250 K propagating down to 120 K. After a thermal cycle through the forward and the reverse transformation, at room temperature the percentage of both phases is found to be \ensuremath{\sim}25% for the CsCl-type structure and \ensuremath{\sim}75% for the FeB-type one. In contrast, in powdered samples the CsCl-type phase is stable at any temperature. A comparative neutron thermodiffractometric study in both types of samples allows us to separate and investigate the magnetic behavior of these phases. The magnetic structure of the CsCl-type phase below ${T}_{N}^{\mathrm{CsCl}}=150\mathrm{K}$ is most consistent with a simple antiferromagnetic one with a propagation vector ${\mathbf{Q}}^{\mathbf{CsCl}}=(\frac{1}{2},\frac{1}{2},0),$ the magnetic moments lying along the c direction. However, for the FeB-type structure below ${T}_{N}^{\mathrm{FeB}}=45\mathrm{K},$ the situation is more complex: a helimagnetic structure with a propagation vector ${\mathbf{Q}}^{\mathbf{FeB}}=(0,\frac{1}{4},\frac{1}{4})$ is proposed. Furthermore, it is concluded that while $R\mathrm{Cu}$ cubic magnetic structures could be understood within a simple isotropic free-electron Ruderman-Kittel-Kasuya-Yosida model, an exchange anisotropy is needed in the orthorhombic ${\mathrm{GdNi}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ compounds to account for the evolution of the magnetic structures. Finally, an insight into the mechanism of the martensitic transformation is also discussed.

Coexistence of localized and (induced) itinerant magnetism and heat-capacity anomaliesinGd1−xYxNi alloys

The results of magnetization (M), electrical resistivity (\ensuremath{\rho}), and heat-capacity (C) measurements for the alloys, ${\mathrm{Gd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Y}}_{\mathrm{x}}$Ni, are reported. The x-ray-diffraction patterns indicate that there is a change in the crystal structure from CrB-type to FeB-type orthorhombic structure for increasing x near x=0.4. GdNi has been known to order ferromagnetically at 70 K and the observed x dependence of ${\mathrm{T}}_{\mathrm{C}}$ and ${\mathrm{\ensuremath{\theta}}}_{\mathrm{p}}$ proves that the strength of Gd-Gd exchange interaction is enhanced by a factor of about 1.5 in the FeB structure. The M data suggest that there is an excess moment for the Gd-rich alloys. The observed magnetization behavior is taken as an evidence to show that there is coexistence of localized magnetism (from Gd) and itinerant ferromagnetism of induced-type, presumably from Ni 3d band. There is an enhancement of C with decreasing temperature above ${\mathrm{T}}_{\mathrm{C}}$ over a wide temperature range presumably due to short-range order effects in these alloys.

Magnetic properties of a new series of Nd-based compounds NdNi1−xCux

The magnetic properties of the NdNi1−xCux compounds (0.4≤ x ≤ 1) with the FeB-type orthorhombic structure have been studied using electrical resistivity and magnetization measurements. For x ≤ 0.7, the compounds are ferromagnetic (TC ≈ 35 K) with strong anisotropy, while the compositions with x = 0.9 and 1 are antiferromagnetic, suggesting the existence of complex magnetic structures. This evolution can be understood as a consequence of the modification of the magnetic interactions in large anisotropy compounds.

INCOMMENSURATE AND LOCK-IN MAGNETIC PHASE TRANSITIONS IN TbSi

INCOMMENSURATE AND LOCK-IN MAGNETIC PHASE TRANSITIONS IN TbSi

CeSi: A non-Kondo trivalent Ce compound

The magnetic properties of CeSi, which crystallizes in the FeB-type orthorhombic structure, have been investigated. CeSi orders antiferromagnetically at 5.6 K. Its ground-state magnetic moment attains a value of 1.47 μB in a field of 57 kOe at 3.0 K, which is far above the highest value ever observed for any Ce compound. The magnetic properties of this compound are like conventional stable moment rare-earth compounds and differ from other nearly trivalent Ce compounds. More specifically, unlike nearly trivalent Ce intermetallic compounds, no evidence for Kondo effect is apparent in this compound.

Observation of the highest ground-state magnetic moment of Ce in CeSi: Possibility of a quartet ground state.

The magnetic properties of CeSi which crystallizes in the FeB-type orthorhombic structure have been investigated. Experimental results indicate that CeSi is an antiferromagnet (${T}_{N}$=5.6 K) with interesting ground-state properties. Its ground-state magnetic moment attains a value of 1.47${\ensuremath{\mu}}_{B}$ in a field of 57 kOe at 3.0 K, which is far above the highest value ever observed for any Ce compound. The ground state surprisingly appears to be a quartet in spite of a noncubic crystal symmetry of this compound. Unlike other nearly trivalent Ce intermetallic compounds, no evidence for the Kondo effect is apparent in this compound.

Phase diagram and magnetovolume effect of pseudo-binary CrBFeB system

The Phase diagram of the pseudo-binary CrBFeB system was studied in the temperature range from 900 to 2200°C by using a piston-cylinder high pressure apparatus as a reaction vessel. The CrB-type phase was found to be unstable relative to the FeB-type phase at higher temperature. The magnetism and thermal expansion of the samples having the FeB-type structure were also studied in connection with their Invar properties.

New alkaline earth equiatomic phases: SrAu and BaAu

The phases SrAu and BaAu were synthesized and characterized structurally. SrAu, mP40, space group P/2 1 m, a = 40.13(2), b = 4.697(1), c = 6.192(4)A˚, β = 94.21(6)°, Z = 20, is a new stacking variant of the FeB CrB type, with Jagodzinski notation hch 2 ch 2 chc. The structure was refined from single crystal diffractometric data with R = 0.090. BaAu, oP8, space group Pnma, a = 8.338(5), b = 4.925(1), c = 6.390(4)A˚, Z = 4, crystallizes in the FeB structure type. A general correlation among the AB equiatomic phases formed by the alkaline earths with VIII to IVB group elements is found by reporting the B B chain angle against the combination R AB σ = |( r A s + r A p) − ( r B s + r B p)| with Zunger's r orbital radii.

Quadrupolar coupling and structural instability in PrAg 17−xCu x

Results are reported for magnetization and neutron diffraction studies of the pseudobinary compounds PrAg 1−xCu x. As x is raised from zero to 0.25, the structure remains cubic CsCl-type and the bilinear exchange is nearly unchanged, whereas the effective quadrupolar coupling deduced from the detailed magnetization-field behavior above the antiferromagnetic Néel point ( ≈ 10 K) increases three-fold in magnitude. The x = 0.5 compound, when cooled below ≈ 150 K, transforms from CsCl-type to an orthorhombic FeB-type structure. Detailed comparison of these structures shows that the instability of the CsCl-type may derive from the softening of zone-boundary phonons. Such phonons, via the dynamic Jahn-Teller mechanism, would account for the quadrupolar interactions of negative (antiferro) sign which grow with increasing x.

Crystal structures of CaAg, CaAu and SrZn

CaAg (space group, Cmcm; a = 4.058 A ̊ , b = 11.457 A ̊ , c = 4.654 A ̊ ) and CaAu (space group, Cmcm; a = 3.961 A ̊ , b = 11.075 A ̊ , c = 4.576 A ̊ ) crystallize with the CrB-type structure. SrZn (space group, Pnma; a = 8.724 A ̊ , b = 4.607 A ̊ , c = 6.417 A ̊ ) crystallizes with the FeB-type structure. The atomic parameters of CaAg and SrZn were refined using single-crystal diffractometric data. Equiatomic compounds of calcium, strontium, barium, europium and ytterbium, which crystallize with structures based on the trigonal prismatic coordination of the partner atom, are examined in terms of the shape of the trigonal prism.

Phase relationship of ternary rare earth-transition metal alloys with CrB and FeB structures or stacking variants

The structures of the FeB-CrB stacking variants and their homogeneity ranges in the pseudobinary systems R 1− x R′ x Ni (Gd 1− x R′ x Ni (R′ ≡ Tb, Dy, Ho, Er, Tm, Lu, Y) and Sm 1− x R′ x Ni (R 1 ≡ Tb, Dy, Ho, Er, Y)) and RT 1− x T′ x (GdNi 1− x Pt x , DyPd 1− x Pt x and DyPd 1− x Ni x ) were investigated. The structures of four new monoclinic (space group P2 1/ m) intermediate phases were determined from single-crystal diffractometer data. Gd 0.5Er 0.5Ni and DyPd 0.65Ni 0.35 are new examples of h 2c base structure stacking (Gd 0.7Y 0.3 Ni structure type), GdNi 0.9Pt 0.1 is a new representative of h 2c 2 base structure stacking (Gd 0.55Dy 0.45Ni-type structure) and Gd 0.7Er 0.3Ni is a new example of h 2c 3 base structure stacking (Gd 0.75Y 0.25Ni-type structure). The low temperature form of DyPd was established to have an FeB-type structure with α = 6.921(3) A ̊ , b = 4.561(2) A ̊ and c = 5.532(3) A ̊ . The sequence of the different FeB-CrB stacking variants in a given pseudobinary R 1− x R' x Ni ( r R > r Tb ⩾ r R' ) with increasing x is accompanied by a systematic increase in the percentage of hexagonal stacking of their base structures. Since the formation of mixed stacking variants requires rather subtle geometric conditions, their range of existence rapidly decreases when the difference between the radii of the rare earth elements is increased until no intermediate stacking occurs.

Abstract: Quadrupolar interactions and structural instabilities in PrAg1−xCux

In order to elucidate the unusually strong quadrupole-quadrupole interactions discovered earlier in PrAg, magnetic and neutron diffraction studies are being made of the pseudobinary compounds PrAg1−xCux. For PrAg0.75Cu0.25, which, like PrAg, has a cubic CsCl-type structure, analysis of high-field magnetization data above the antiferromagnetic Néel point (TN∼9 K) shows that the effective biquadratic (quadrupolar) coupling is about twice as strong as in PrAg, whereas the average bilinear exchange coupling is slightly weaker. PrAg0.5Cu0.5, which has a CsCl-type structure at room temperature, was found to transform to an orthorhombic FeB-type structure (similar to that of PrCu) when it is cooled below ∼150 K. Detailed comparison of the two structures of PrAg0.5Cu0.5 indicates that the instability of the CsCl-type structure probably stems from the softening of certain zone-boundary phonons, which presumably grows as x increases towards 0.5. Such phonons would account for strong effective quadrupolar interactions of negative (antiferro) sign, precisely of the type seen in PrAg and PrAg0.75 Cu0.25. a) Work supported by NSF Grant No. DMR 78-12777. b) Work supported by the U.S Department of Energy. c) Work sponsored by the U. S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corp. 1 T. O. Brun, J. S. Kouvel, and G. H. Lander, Phys. Rev. B 13, 5007 (1976).

Crystal field effects on the magnetic structures of the rare earth compounds with the FeB structure

In order to confirm the role of the crystalline electric potential on the stability of non collinear magnetic structures of the rare earth compounds with the FeB-type structure, the magnetic properties of the (Gd 0.5Y 0.5)Ni compound, where the rare earth orbital moment is nul, are studied. Below its Curie temperature (57 K) the compound is ferromagnetic. The spontaneous magnetization at 0 K reaches 7.05 μ B per gadolinium atom. Yttrium and nickel atoms being not magnetic the gadolinium moments are parallel and the exchange interactions are positive. Then the non collinear magnetic structures observed when the alloyed rare earths have an orbital moment result from the competition between a multiaxial anisotropy due to the crystal field effects and isotropic exchange interactions of the Heisenberg type.

Magnetic properties and structures of the rare earth compounds RNi 1−xCu x with the FeB-type structure

Replacing nickel atoms by cooper atoms in the ferromagnetic RNi compounds with the FeB-type structure introduces negative interactions opposed to the positive ones. The strong magnetocrystalline anisotropy leads to transverse wave magnetic structures which are observed with terbium, holmium and erbium. While this structure is stable only just below the Néel temperature for the erbium compounds, it remains stable until very low temperatures for the terbium and holmium ones. This behaviour, which depends on the rare earth being a Kramers or non Kramers ion, is interpreted by studying the effect of the low symmetry crystal field in these compounds. The stability of a transverse wave magnetic structure at 10 K is an example of magnetism induced by the exchange field.

Constitution diagram of the system cerium-germanium in the 50–100 at.% germanium range

Using the results of metallographic, differential thermal, dilatometric, and x-ray phase analyses, a constitution diagram of the system lanthanum-germanium has been constructed covering the range of compositions from 50 to 100 at.% Ge. The existence has been confirmed of the following germanide phases: LaGe with the FeB type structure and LaGe2 with α-GdSi2 and α-ThSi2 type structures. It has been demonstrated that the LaGe phase undergoes no polymorphic transformation.

Gadolinium and dysprosium intermetallic phases. I. The crystal structures of DyGa and GdPt and their related compounds

The intermetallic phnses DyGa, DyGe, GdGa, and GdGe are isostructural with the CrB structure type. The phases GdPt, DyPt, GdNi, and DyNi have structures which are only slight distortions of the FeB structure type. The structures were determined by a combinatibn of single crystal and powder methods. (auth)