BaAl4-type Structure Research Articles

Magnetic properties of single-crystalline CeCuGa3

The magnetic behavior of single-crystalline CeCuGa3 has been investigated. The compound forms in a tetragonal BaAl4-type structure consisting of rare-earth planes separated by Cu-Ga layers. If the Cu-Ga site disorder is reduced, CeCuGa3 adopts the related, likewise tetragonal BaNiSn3-type structure, in which the Ce ion are surrounded by different Cu and Ga layers and the inversion symmetry is lost. In the literature conflicting reports about the magnetic order of CeCuGa3 have been published. Single crystals with the centrosymmetric structure variant exhibit ferromagnetic order below approx. 4 K with a strong planar anisotropy. The magnetic behavior above the transition temperature can be well understood by the crystal-field splitting of the 4f Hund's rule ground-state multiplet of the Ce ions.

BaAl4-type structure derivatives in the Ga-rich part of the Y–Cu–Ga system

The crystal structures of seven Ga-rich compounds that exist in the Y–Cu–Ga system were studied by the X-ray single-crystal and powder diffraction methods. The crystal structures of six phases were shown to be constructed from the orthorhombic distorted BaAl4-like fragments by multiplication of its a and b lattice constants. These compounds are: 1-Y1.02Cu1.35Ga2.63 (modified BaAl4-type structure, 1×1 multiplication), 2-Y1.04Cu1.12Ga2.84 and 3-Y1.05Cu1.00Ga2.95 (own type structure, 3×3), 4-Y1.08Cu0.72Ga3.20 (own, 9×9), 5-Y3Cu4Ga7 and 6-Y3.21Cu3.09Ga7.70 (La3Al11, 3×1), 7-YCuGa2 (own, 3×3).

Novel RZn 2Ga 2 (R = La, Ce, Pr, Nd, Sm) intermetallic compounds with BaAl 4-type structure

Novel RZn 2Ga 2 intermetallics with R = La, Ce, Pr, Nd, Sm have been synthesized and characterized with regards to their crystal structures and magnetic properties. The compounds form with a tetragonal structure of the BaAl 4 type (space group I4/ mmm). Except for LaZn 2Ga 2, they exhibit localised magnetism due to the presence of magnetic moments on the respective trivalent rare earth ions. The Pr-, Nd- and Sm-based compounds order antiferromagnetically at low temperatures with likely contribution of some ferromagnetic components.

The new ternary phases of La3(Zn0.874Mg0.126)11and Ce3(Zn0.863Mg0.137)11

The new ternary intermetallic title compounds, namely trilanthanum undeca(zinc/magnesium), La(3)(Zn(0.874)Mg(0.126))(11), (I), and tricerium undeca(zinc/magnesium), Ce(3)(Zn(0.863)Mg(0.137))(11), (II), are isostructural and crystallize in the orthorhombic La(3)Al(11) structure type. These three phases belong to the same structural family, the representative members of which may be derived from the tetragonal BaAl(4) structure type by a combination of internal deformation and multiple substitution. Compared to the structure of La(3)Al(11), in (I), a significant decrease of 11.9% in the unit-cell b axis and an increase in the other two directions, of 3.6% along a and 5.2% along c, are observed. Such an atypical deformation is caused by the closer packing of atoms in the unit cell due to atom shifts that reflect strengthening of metallic-type bonding. This structural change is also manifested in a significant difference in the coordination around the smaller atoms at the 8l Wyckoff position (site symmetry m). The Al atom in La(3)Al(11) is in a tricapped trigonal prismatic environment (coordination number 9), while the Zn atoms in (I) and (II) are situated in a tetragonal antiprism with two added atoms (coordination number 10).

RE(AuAl2)nAl2(AuxSi1−x)2: A New Homologous Series of Quaternary Intermetallics Grown from Aluminum Flux

The combination of early rare earth metals (La- to Gd and Yb), gold, and silicon in molten aluminum results in the formation of intermetallic compounds with four related structures, forming a new homologous series: RE[AuAl2]nAl2(AuxSi(1-x))2, with x approximately 0.5 for most of the compound and n = 0, 1, 2, and 3. Because of the highly reducing nature of the Al flux, rare earth oxides instead of metals can also be used in these reactions. These compounds grow as large plate-like crystals and have tetragonal structure types that can be viewed as intergrowths of the BaAl4 structure and antifluorite-type AuAl2 layers. REAuAl2Si materials form with the BaAl4 structure type in space group I4/mmm (cell parameters for the La analogue are a = 4.322(2) A, c = 10.750(4) A, and Z = 2). REAu2Al4Si forms in a new ordered superstructure of the KCu4S3 structure type, with space group P4/nmm and cell parameters of the La analogue of a = 6.0973(6) A, c = 8.206(1) A, and Z = 2. REAu3Al6Si forms in a new I4/mmm symmetry structure type with cell parameters of a = 4.2733(7) A, c = 22.582(5) A, and Z = 2 for RE = Eu. The end member of the series, REAu4Al8Si, forms in space group P4/mmm with cell parameters for the Yb analogue of a = 4.2294(4) A, c = 14.422(2) A, and Z = 1. New intergrowth structures containing two different kinds of AuAl2 layers were also observed. The magnetic behavior of all these compounds is derived from the RE ions. Comparison of the susceptibility data for the europium compounds indicates a switch from 3-D magnetic interactions to 2-D interactions as the size of the AuAl2 layer increases. The Yb ions in YbAu(2.91)Al(6)Si(1.09) and YbAu(3.86)Al(8)Si(1.14) are divalent at high temperatures.

BaAl4-type derivative structure in superconducting La–Pd–Ge system

There are many ternary intermetallic compounds with BaAl4-type and its derivative structures. LaPd2Ge2 is already known as one of the BaAl4-type superconductors with Tc of 1.12K. We have synthesized a new intermetallic compound La3Pd4Ge4 with higher Tc of 2.75K than that of LaPd2Ge2, which had the structure with the insertion of the AlB2-type layer in LaPd2Ge2. A detailed crystal structure analysis of La3Pd4Ge4 was carried out by neutron powder diffraction because it is useful to the analysis of small interplanar spacing. The neutron diffraction study indicates that La3Pd4Ge4 has U3Ni4Si4-type structure with a lower symmetry than that of LaPd2Ge2, and the typical interatomic distances of La3Pd4Ge4 are shorter than those of LaPd2Ge2. The coordination of Pd to three Ge in the AlB2-type layer of La3Pd4Ge4 is characteristic, which is not stabilized in other lanthanide-substituted intermetallics Ln3Pd4Ge4 (Ln=Y, Gd–Yb).

A New Superstructure for the BaAl4-Structure Type: An Experimental and Theoretical Study of La2NiAl7

A new ternary aluminide, La2NiAl7, has been synthesized from the elements in sealed silica tubes. The crystal structure of La2NiAl7 was determined by single-crystal X-ray diffraction and crystallizes in the tetragonal space group I4mm (No. 123) with Z = 4 and lattice parameters a = 6.221(3) and c = 21.403(2) Å. Refinement based on F02 yielded R1 = 0.0313 and wR2 = 0.0392 [I > 2σ(I)]. It is a new structure type, which belongs to the family of BaAl4-related structures. The La2NiAl7 structure corresponds to a 3D superstructure (√2a × √2b × 2c) of the BaAl4-structure type, whereas La3Al11 is only a 1D superstructure (3a × b × c). Tight-binding band structure calculations using the density functional theory have been carried out to understand the structural stability of La2NiAl7 as well as La3Al11.

New intermetallic compounds in the RE–Zn–Al systems and their crystal structure

Crystal structures of some new compounds have been determined using X-ray powder data. The intermetallic compounds YbZn 1.75Al 2.25, SmZn 1.62Al 2.38, NdZn 2.34Al 1.66 have the BaAl 4-type structure (s.g. I4/ mmm). Other compounds adopt the La 3Al 11-type structure (s.g. Immm): Y 3Zn 3.2Al 7.8, Gd 3Zn 3.4Al 7.6, Tb 3Zn 4.6Al 6.4, Dy 3Zn 3.4Al 7.6, Ho 3Zn 4.4Al 6.6, ∼Er 3Zn 4Al 7, ∼Tm 3Zn 4Al 7.

Single crystal growth, and magnetic and electronic properties of EuGa4

Large single crystals of the title compound were synthesized from the pure elements using an excess of molten Ga as a solvent. EuGa4 crystallizes in the body-centered tetragonal space group I4/mmm (BaAl4-structure type) with unit cell parameters a=4.407(1) Å and c=10.635(3) Å, and c/a ratio of 2.413. Temperature (2–350 K) dependence of the DC magnetization, along with the temperature dependence of the electrical resistivity and electronic heat capacity, is presented. According to these measurements, EuGa4 orders antiferromagnetically near TN=15 K.

Single crystal growth, and magnetic and electronic properties of EuGa 4

Large single crystals of the title compound were synthesized from the pure elements using an excess of molten Ga as a solvent. EuGa 4 crystallizes in the body-centered tetragonal space group I4/ mmm (BaAl 4-structure type) with unit cell parameters a=4.407(1) Å and c=10.635(3) Å, and c/ a ratio of 2.413. Temperature (2–350 K) dependence of the DC magnetization, along with the temperature dependence of the electrical resistivity and electronic heat capacity, is presented. According to these measurements, EuGa 4 orders antiferromagnetically near T N=15 K.

The s-p bonded representatives of the prominent BaAl4 structure type: a case study on structural stability of polar intermetallic network structures.

This work presents a detailed, combined experimental and theoretical study on the structural stability of s-p bonded compounds with the BaAl4 structure type (space group I4/mmm, Z = 2) as part of a broad program to investigate the complex questions of structure formation and atomic arrangements in polar intermetallics. From ab initio calculations employing pseudopotentials and a plane wave basis set, we extracted optimized structural parameters, binding energies, and the electronic structure of the systems AeX(III)4, AeX(II)2X(IV)2, AeX(II)2X(III)2 (Ae = Ca, Sr, Ba; X(II) = Mg, Zn; X(III) = Al, Ga; X(IV) = Si, Ge). For all systems we found a pronounced pseudo-gap in the density of states separating network X42- bonding from antibonding electronic states that coincides with the Fermi level for an electron count of 14 electrons per formula unit, the optimum value for stable BaAl4-type polar intermetallics. However, the synthesis and structural characterization (from X-ray single crystal and powder diffraction data) of the new compounds AeZn2-Al2+, AeZn2-deltaGa2+delta (Ae = Ca, Sr, Ba; delta = 0-0.2) and AeMg0.9Al3.1, AeMg1.7Ga2.3 (Ae = Sr, Ba) manifested that electron deficiency is rather frequent for BaAl4-type polar intermetallics. The site preference for different "X" elements in the ternary systems was quantified by calculating "coloring energies", which, for some systems, was strongly dependent on the size of the electropositive Ae component. The Ae2+ cations decisively influence the nearest neighbor distances in the encapsulating polyanionic networks X4(2-) and the structures of these networks are surprisingly flexible to the size of the Ae component without changing the overall bonding picture. A monoclinically distorted variant of the BaAl4 structure occurs when the cations become too small for matching the size of encapsulating X4(2-) cages. An even larger size mismatch leads to the formation of the EuIn4 structure type.

Synthesis, Structure, and Bonding of Hypoelectronic SrIn4: Direct Example of a Dominant Size Effect in Structure Selection

The structure of the new title compound, the same type as EuIn4, was refined in the monoclinic space group C2/m, with Z = 4. The compound exhibits a complex three-dimensional network built of four- and five-bonded indium atoms in fused and interbonded pentagons that sandwich the strontium atoms. Both electronic band-structure calculations and property measurements show that the compound is metallic. A detailed band-structure analysis indicates that the compound is hypoelectronic with a one-electron deficiency, but the In−In bonding is effectively optimized in the structure. The important role of cation size in the structure choice is noted in a comparative study of BaIn4, which has the closely related BaAl4-type structure in which barium atoms are sandwiched by six-membered rings.

Transport and thermodynamic properties of

We present measurements of the specific heat, the electrical resistivity, the Hall effect, and the magnetic susceptibility of CeAuAl3, a new heavy-electron compound that crystallizes in an ordered derivative of the tetragonal BaAl4-type structure. For comparison we have also done some of these measurements on the isostructural non-magnetic reference compound LaAuAl3, which appears to be a simple metal. Below T N = 1.32K, CeAuAl3 orders antiferromagnetically and below 1K, we encounter Fermi liquid behaviour with considerably enhanced effective masses, i.e., a quadratic temperature dependence of the resistivity with a large prefactor and a sizable linear-in-T contribution to the specific heat. This linear-in-T contribution increases by more than a factor 50 from its value at T ≫ T N to its value at T ≫ T N. Consequently CeAuAl3 develops a heavy-electron ground state, coexisting with antiferromagnetic order. The small energy scales involved in the problem make CeAuAl3 a good candidate for tuning it, by varying external parameters, towards a quantum critical point. At high temperatures we observe local moment behaviour. From the temperature dependence of the magnetic susceptibility and the specific heat we have derived the crystalline-electric-field-split level scheme of the Ce3+ J = 5/2 multiplet. Distinct features in the electrical resistivity provide additional evidence for this level splitting.

Crystal Structure of CaCu0.15Ga3.85—A New Variant of the BaAl4 Structure Type: Structure Analysis from X-Ray Powder Diffraction Data

The new intermetallic compound CaCu0.15Ga3.85 was found during the investigation of the stability of the BaAl4 structure type in ternary gallium-containing systems. The crystal structure has been investigated by means of a powder diffraction technique. The experimental diffraction data set has been obtained by the use of a Ge(111) primary monochromator and a linear position sensitive detector. The sample was measured in transmission mode using CuKαI radiation in the range from 10° to 130° in 2θ with a step width of 0.02°. Unit cell parameters have been determined by means of automatic indexing and refined from the diffraction angles of 45 reflections; monoclinic system, a = 11.5741(2) Å, b = 4.2264(1) Å, c = 4.3573(1) Å, β = 110.499(1)°, V = 199.65(1) Å3, space group C 2/m. Full profile refinement of coordinate and anisotropic thermal displacement parameters led to the residual values R(I) = 0.058 and R(P) = 0.155 for all 193 reflections possible in the range measured. The final values of the atomic coordinates are as follows: 2Ca in 2(a) 0 0 0, Beq = 1.0(2) Å2 4(Ga0.962Cu0.038)1 in 4(i) x0z, x = 0.3858(1), z = 0.4099(5), Beq = 0.50(7) Å2; 4(Ga0.962Cu0.038)2 in 4(i), x = 0.2524(2), z = 0.7764(4), Beq = 0.66(7) Å2. Atomic coordination numbers are 20 for Ca, 9 for (Ga0.962Cu0.038)1, and 12 for (Ga0.962Cu0.038)2. The shortest interatomic distances are observed between gallium (copper) atoms: 2.485, 2.568, 2.605, and 2.655 Å. The structure of CaCu0.15Ga3.85 is the first representative of a new structure type and is described as a distorted variant of the BaAl4 structure type; (a, b, c )CaCu0.15Ga3.85 = (a, b, c )BaAl4 × (1, 0, 1; 1, 0, 0; 0, 1, 0). Isotypic compounds were found in the ternary systems {Ca, Yb}-{Ni, Pd, Pt, Cu, Ag, Au}-Ga.

NEW INTERMETALLIC COMPOUNDS IN THE TERNARY SYSTEM Yb-T-Al (T=Ag, Au, Pd, Pt)

Structural characteristics of a number of ternary rare earth/transition metal/aluminium compounds are hexagonal nets of the AlB 2-type, Kagomé nets and tetragonal pyramides. Crystal structures of the new compounds in the ternary Yb-T-Al systems contain units and sections of them. Kagomé nets stacked in an ..ABAB.. sequence are observed in YbAg1.5Al0.5 ( MgZn 2-type). The crystal structure of Yb8Ag21Al45 is related to the BaHg 11-type. YbAu0.8Al3.2 ( CaZn 2 Al 2-type) contains tetragonal pyramides which are connected by common edges. Fragments of the AlB 2- and BaAl 4-type structure are present in Yb5Au15Al16. YbPdAl2 and YbPtAl2 ( YbPdAl 2-type) contain pentagonal prisms as the characteristic structural elements.

Ternary gallides REAg xGa 4− x (RE ≡ La, Ce, Pr, Nd, Sm)

Ternary gallides (La, Ce, Pr, Nd, Sm)Ag x Ga 4− x have been prepared from the elements by arc melting under argon. From X-ray powder diffraction analysis the new compounds were all found to crystallize with the BaAl 4-type structure. The homogeneity range of each of the REAg x Ga 4− x phases at 600 °C was established. Magnetic susceptibilities were determined over a temperature range extending from 4.2 to 1100 K. Above liquid nitrogen temperatures the paramagnetic behaviour of the (Ce, Pr, Nd)-Ag x Ga 4− x compounds is characterized by magnetic moments close to the ideal trivalent rare earth values. SmAgGa 3 alloy is characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets; LaAg x Ga 4− x is diamagnetic. At very low temperatures onset of magnetic ordering is observed for CeAg 0.65Ga 3.35 (3 K), CeAg 0.75Ga 3.75 (5 K) and PrAg 0.78Ga 3.22 (8 K). No superconductivity was observed down to 4.2 K.

Magnetic properties of pseudo-ternaries of the composition Nd(Mn1−xCrx)2Si2

Structural and magnetic properties of a number of pseudo-ternaries of the composition Nd(Mn1−xCrx)2Si2 have been examined. In the composition range 0<x<0.6 they crystallize in tetragonal BaAl4-type structure. Magnetic properties were examined between 4.2 K and 400 K in applied fields up to 20 kOe. NdMn2Si2 exhibits anomalies in the magnetization versus temperature plots at about 40 K and 380 K—the former due to the ordering of the rare earth moments and the latter due to the ordering of the transition metal sublattice. Addition of chromium to this compound increases the crystal cell volume significantly and modifies the magnetic behavior. The transition metal sublattice orders antiferromagnetically at 380 K, 260 K and 200 K for compositions of X=0, 0.2 and 0.3, respectively. Compounds of the composition X=0.2, 0.3 and 0.4 exhibit unusual thermomagnetic hystereses. The thermomagnetic curves for these compositions in low external fields (<10 kOe) exhibit an increase in the magnetization upon warming the sample from 4.2 K to about 30 K. Above this temperature the rare earth sublattice begins to disorder. These results are interpreted on the basis of a model considering the competition of exchange interactions of the rare earth and transition metal sublattices. The possibility of modifying the nature of magnetic interaction between rare earth and Mn atoms through the incorporation of a second transition metal atom (in this instance Cr) into the lattice will be discussed. Experimental data obtained for the compositions of X=0.2, 0.3 and 0.4 seem to indicate that the rare earth ion changes its easy direction of magnetization from the easy c-axis to the basal plane (or cone) as the material is cooled below 50 K.