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Now showing items 1 - 16 of 28

  • An Unusual Valence State: Trivalent Europium in Intermetallic Eu2Ir3Al9

    Stegemann, Frank   Block, Theresa   Klenner, Steffen   Janke, Oliver  

    Eu2Ir3Al9, synthesized from the elements in tantalum tubes, is one of the rare examples for trivalent europium in the field of intermetallic compounds. The compound crystallizes in the Y2Co3Ga9-type structure (space group Cmcm), with lattice parameters fitting in between the isostructural samarium and gadolinium compounds. In the crystal structure, the Eu atoms form Al-3-triangle-centered honeycomb layers and exhibit a coordination number of 17 in the shape of a fivefold-capped hexagonal prism (Eu@Ir6Al6+Al-5). Magnetic measurements indicate an overall low susceptibility, in line with van Vleck paramagnetism caused by the Eu3+ cations. Fits of the susceptibility yield a coupling constant of lambda=3D290(10) K and an effective magnetic moment of mu(eff)=3D4.56(1) mu(B), in line with a slight hybridization of the F-7(0) and F-7(1) state. Eu-151 Mossbauer spectroscopic investigations unambiguously prove the presence of solely Eu3+ in the bulk material.
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  • Unusual Valence State - Trivalent Europium in Intermetallic Eu2Ir3Al9

    Stegemann, Frank   Block, Theresa   Klenner, Steffen   Janka, Oliver  

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  • Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K

    Radzieowski, Mathis   Stegemann, Frank   Block, Theresa   Stahl, Juliane   Johrendt, Dirk   Janka, Oliver  

    Eu2Pt6Al15 has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (Sc2Pt6Al15 type) with space group Cmcm(alpha,0,0)0s0 (alpha =3D 2/3). Full ordering of the Pt and Al atoms within the [Pt6Al15](delta-) polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T =3D 41.6(1) K, which was also observed as lambda-type anomaly in heat capacity measurements (T =3D 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu2+ at higher temperatures (>55 K) to Eu3+ at low temperatures. This valence change was proven by temperature dependent Eu-151 Mossbauer spectroscopic investigations. Isostructural Eu2Pt6Ga15 was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at T-N =3D 13.1(1) K.
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  • On Ternary Intermetallic Aurides:CaAu2Al2,SrAu2-xAl2+x and Ba3Au5+xAl6-x

    Stegemann, Frank   Benndorf, Christopher   Zhang, Yuemei   Bartsch, Manfred   Zacharias, Helmut   Fokwa, Boniface P. T.   Eckert, Hellmut   Janka, Oliver  

    The intermetallic compound CaAu2Al2, and the members of the solid solutions SrAu2-xAl2+x (0 <=3D x <=3D 0.33) and Ba3Au5+xAl6-x (x =3D 0, 0.14, 0.49) were synthesized from the elements in sealed tantalum ampoules. The Ca compound crystallizes with the orthorhombic ThRu2P2 type structure, whereas the targeted SrAu2Al2 was found to form a solid solution according to SrAu2-xAl2+x. For the Ba system no "BaAu2Al2" was found, however, Ba3Au5+xAl6-x was discovered to crystallize in the monoclinic space group C2/c with its own structure type. The samples were investigated by powder X-ray diffraction and their crystal structures were refined on the basis of single-crystal X-ray diffraction data. All compounds were characterized furthermore by susceptibility measurements. The crystallographic aluminum sites of CaAu2Al2 and Ba3Au5Al6 can be differentiated by Al-27 solid state NMR spectra on the basis of their different electric field gradients, in agreement with theoretical calculations. The electron transfer from the alkaline earth metals and the aluminum atoms onto the gold atoms was investigated by X-ray photoelectron spectroscopy (XPS) classifying these intermetallics as aurides, in full agreement with the calculated Bader charges.
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  • SrPt2Al2 - A(3+2)D-incommensurately modulated variant of the CaBe2Ge2 type structure

    Hoffmann, Rolf-Dieter   Stegemann, Frank   Janka, Oliver  

    The first ternary compound in the Sr-Pt-Al system, SrPt2Al2, was synthesized by melting of the elements in a sealed niobium tube in a high frequency furnace. The structure of SrPt2Al2 was determined by single-crystal X-ray diffraction on the basis of data sets recorded at 90, 150, 210, 270 and 300 K. The compound crystallizes in an incommensurately modulated (3+2)D structure being related to the tetragonal primitive CaBe2Ge2 type structure. The structure was refined in the orthorhombic superspace group Pmmn(alpha, 0,0) 0s0(0,beta,0) s00 (alpha = 0.222(1) a*, beta = 0.224(1) b*, a = b = 436.89(5), c = 1010.16(13) pm at 90 K) with R-M = 0.0403, R-S,R-1 = 0.0597 and R-S,R-2 = 0.2046. The structural relation with CaBe2Ge2 along with a refinement in the tetragonal superspace group P4/nmm(alpha, 0,0)-0000(0,alpha,0)00s0 is discussed with respect to the temperature dependence of the q-vectors and by group-subgroup relations. The modulation, from the standpoint of the crystal chemistry, is mainly caused by the distortion of the Pt arrangements within the pseudo-tetragonal ab plane.
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  • Unusually strong heteroatomic bonding in the complex polyanion of intermetallic Ba6Pt22Al53

    Stegemann, Frank   Touzani, Rachid Stefan   Janka, Oliver  

    The new intermetallic compound Ba6Pt22Al53 was obtained from reaction of the elements. The title compound crystallizes in the hexagonal crystal system with lattice parameters of a =3D 1427.5(3) and c =3D 1614.3(4) pm and space group P6(3)/mcm (Z =3D 2) and adopts a new structure type (Pearson code hP162, Wyckoff sequence l(2)k(3)j(2)i(2)hg(2)ecb). In the structure, the Pt and Al atoms form a complex polyanionic [Pt22Al53](delta-) network, while the Ba cations reside in cavities. The structure can be described as a packing of layers, formed by the Pt@Al-n polyhedra parallel to (001). The layers are stacked in an ABCB fashion. Within the [Pt22Al53] framework, one of the shortest Pt-Al distances (239 pm) reported is found, suggesting extremely strong covalent bonding interactions. Due to slightly enhanced displacement parameters of the Al9 site, temperature dependent single-crystal X-ray diffraction (285 and 90 K) and low-temperature heat capacity experiments were conducted. However, no structural phase transition was observed down to 2 K. With the help of DFT calculations, the bonding situation was elucidated in detail, verifying the strong heteroatomic bonding and the destabilization of the structure upon in silico distortions. Mulliken charge calculations confirm the picture of the polyanionic framework and cationic Ba atoms within the cavities.
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  • Experimental and theoretical investigations of the polar intermetallics SrPt3Al2 and Sr2Pd2Al

    Stegemann, Frank   Benndorf, Christopher   Touzani, Rachid St.   Fokwa, Boniface P.T.   Janka, Oliver  

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  • Rare-earth solid-state NMR spectroscopy of intermetallic compounds:The case of the Lu-175 isotope

    Benndorf, Christopher   de Oliveira Junior, Marcos   Bradtmueller, Henrik   Stegemann, Frank   Poettgen, Rainer   Eckert, Hellmut  

    The feasibility of high-resolution Lu-175 solid-state NMR spectroscopy in intermetallic compounds crystallizing with cubic crystal structures is explored by magic-angle spinning NMR at different magnetic flux densities. The large quadrupole moment of this isotope (3.49 x 10(-28) m(2)) restricts observation of the NMR signal to nearly perfectly ordered crystalline samples. Signals are successfully detected and analyzed in the binary pnictides LuPn (NaCl-type structure; Pn =3D P, As, Sb) and the intermetallic compounds LuPtSb and LuAuSn, both crystallizing with the MgAgAs-type structure. Sources of line broadening are discussed based on field-dependent static and MAS-NMR spectra, providing guidance with respect to measurement conditions resulting in reliable results. The results highlight the importance of ionic/ covalent bonding effects for the detectability of the signal, which reduce the probability of real structure effects commonly observed in intermetallic compounds. No Lu-175 NMR signals can be observed in various cubic Heusler compounds. This is attributed to mixed site occupancies and other structural defects producing electric field gradients whose interaction with the Lu-175 quadrupole moments broadens the signal beyond detection.
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  • Gallium-indium ordering in REPt2Ga3In (RE=3DY,Gd-Yb) phases with NdRh2Sn4-type structure

    Horiacha, Myroslava   Zaremba, Vasyl I.   Stegemann, Frank   Poettgen, Rainer  

    The gallium-rich quaternary intermetallic compounds REPt2Ga3In (RE=3DY, Gd-Yb) were synthesized by arc melting of the elements. Small single crystals were grown by induction heating the arc-melted buttons in sealed tantalum ampoules. The REPt2Ga3In phases were characterized through X-ray powder patterns. The structures of GdPt2Ga2.95In1.05 [a=3D1760.28(4) pm, b=3D429.09(5) pm, c=3D675.37(14) pm, wR2=3D0.0618, 1104 F-2 values, and 45 parameters] and TbPt2Ga3.14In0.86 [a=3D1746.56(3) pm, b=3D427.05(5) pm, c=3D672.69(13) pm, wR2=3D0.0554, 1086 F-2 values, and 45 parameters] were refined from X-ray single-crystal diffractometer data. These gallide indides are the first ternary ordered representatives of the orthorhombic NdRh2Sn4-type structure, space group Pnma. The striking basic building units are platinum-centered trigonal prisms Pt1@Gd2Ga4 and Pt2@Gd2Ga2In2 with larger edge lengths for the indium-substituted prism. Refinements of the occupancy parameters indicate the formation of small homogeneity ranges. The indium atoms are positioned at the edges of adjacent prisms and form zig-zag chains (330pm In-In) extending along the y-axis. Temperature-dependent magnetic susceptibility data of GdPt2Ga3In show Curie-Weiss paramagnetism with an anomaly observed at T=3D12.7(1) K, proven as intrinsic by zero-field heat capacity measurements, pointing towards a stable canted antiferromagnetic ground state. [GRAPHICS] .
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  • New members of the tetragonal RE14T3Al3 (RE=3DSc,Y,Dy-Tm,Lu; T=3DCu,Rh,Pd) series

    Stegemann, Frank   Janka, Oliver  

    The rare-earth metal-rich RE14T3Al3 series (RE=3DY, Gd-Tm, Lu) was extended to the transition metals T=3DCu, Rh, and Pd. All compounds have been prepared by arc-melting the rare-earth elements with appropriate amounts of TAl precursors. The compounds crystallize in the tetragonal Gd14Co3In2.7-type structure with space group P4(2)/nmc. Two structures (Y14Rh2.97(1)Al3.03(1): a=3D952.99(4) pm, c=3D2292.98(10) pm, wR2=3D0.0423, 2225 F-2 values, 63 variables; Y14Pd3.16(1)Al2.84(1): a=3D955.06(5) pm, c=3D2298.77(10) pm, wR2=3D0.0416, 2225 F-2 values, 63 variables) have been refined from single-crystal data, indicating T/Al mixing on two crystallographic sites. The lattice parameters of all synthesized representatives have been refined from Guinier powder X-ray diffraction data. The Y and Lu containing compounds show Pauli paramagnetic behavior, indicating that the transition metal atoms have filled d bands and exhibit no localized magnetic moment. From the other compounds only Er14Pd3Al3 was obtained as an X-ray pure sample. It shows paramagnetism, in line with the rare-earth atoms in the trivalent oxidation state and antiferromagnetic ordering below T-N=3D8.0(1)K. Detailed magnetic investigations on the other members were impossible due to the presence of, e.g., RE3T trace impurities. [GRAPHICS] .
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  • Platinum Triangles in the Pt/Al Framework of the Intermetallic REPt6Al3 (RE =3D Ce-Nd,Sm,Gd,Tb) Series

    Eustermann, Fabian   Stegemann, Frank   Renner, Konstantin   Janka, Oliver  

    The compounds of the REPt6Al3 series (RE =3D Ce-Nd, Sm, Gd, Tb) were obtained by reaction of the elements via arc-melting. They were characterized by powder and single-crystal X-ray diffraction (NdPt6Al3: wR =3D 0.0432, 759 F-2 values, 33 variables) as well as by magnetic susceptibility measurements. The isostructural compounds crystallize with a new structure type in the trigonal crystal system with space group R (3) over barc, twelve formula units in the unit cell, and lattice parameters of a =3D 752-755 and c =3D 3882-3945 pm. The crystal structure can be described by different slabs stacked along [001]. One layer features Pt-3 triangles, centering the cavities of a flat honeycomb RE layer that are arranged in a ABCABC sequence. The other layer consists of condensed hexagonal [Pt6Al6] prisms, centered by Pt atoms, separating the before mentioned slabs. Magnetic measurements revealed that all rare-earth atoms are in the trivalent oxidation state, however, due to the low lanthanoide content magnetic ordering phenomena were observed only at low temperatures [SmPt6Al3: T-C =3D 5.0(1) K; GdPt6Al3: T-C =3D 7.3(1) K; TbPt6Al3: T-N =3D 3.6(1) K].
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  • Reactivity of a P-H Functionalized Al/P-Based Frustrated Lewis Pair - Hydrophosphination versus Classic Adduct Formation

    Uhl, Werner   Keweloh, Lukas   Hepp, Alexander   Stegemann, Frank   Layh, Marcus   Bergander, Klaus  

    The P-H functionalized Al/P-based frustrated Lewis pair (FLP) Mes(H)P-C[Al{CH(SiMe3)(2)}(2)]=3DC(H)-tBu (3) has the typical functionality of FLPs with Lewis acidic and basic centers in a single molecule and is able to transfer the P bound H atom to activated substrates. Nevertheless, 3 reacted with NH3 only by adduct formation with the N atom coordinated to Al (5). The relatively low basicity of the P atom prevented the formation of a hydrogen bridge between P and Al. Similarly, benzil and 3 afforded a simple adduct (6) with only one C=3DO group bonded to P and Al. The second C=3DO group and the P-H moiety were unaffected. 6 has two stereogenic centers, and the resulting diastereomers were identified by crystal structure determinations. Migration of the P bound H atom was only observed for substrates containing basic N atoms. Nitriles gave heterocyclic iminophosphine adducts (7) in which the N-H groups were coordinated to the Al atoms. An isocyanate and two isothiocyanates afforded adducts via Al-X and P-C bond formation (X =3D O, S), and phosphaurea derivatives (8a to 8c) resulted from H shift to the exocyclic N atoms. A carbodiimide reacted with 3 by the formation of a phosphaguanidine (8d) with the imine N atom coordinated to Al. The latter reactions proceed under mild conditions and represent formally the hydrophosphination of unsaturated substrates.
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  • Equiatomic AEAuX (AE =3D Ca-Ba,X =3D Al-In) Intermetallics:A Systematic Study of their Electronic Structure and Spectroscopic Properties

    Benndorf, Christopher   Stegemann, Frank   Seidel, Stefan   Schubert, Lea   Bartsch, Manfred   Zacharias, Helmut   Mausolf, Bernhard   Haarmann, Frank   Eckert, Hellmut   Poettgen, Rainer   Janka, Oliver  

    The three intermetallic compounds SrAuGa, BaAuAl and BaAuGa were synthesised from the elements in niobium ampoules. The Sr compound crystallises in the orthorhombic KHg2-type structure (Imma, a=3D465.6(1), b=3D771.8(2), c=3D792.6(2) pm, wR(2)=3D0.0740, 324 F-2 values, 13 variables), whereas the Ba compounds were both found to crystallise in the cubic non-centrosymmetric LaIrSi-type structure (P2(1)3, BaAuAl: a=3D696.5(1) pm; wR(2)=3D0.0427, 446 F-2 values, 12 variables; BaAuGa: a=3D693.49(4) pm, wR(2)=3D0.0717, 447 F-2 values, 12 variables). The samples were investigated by powder X-ray diffraction and their structures refined on the basis of single-crystal X-ray diffraction data. The title compounds, along with references from the literature (CaAuAl, CaAuGa, CaAuIn, and SrAuIn), were characterised further by susceptibility measurements and Al-27 and Ga-71 solid-state NMR spectroscopy. Theoretical calculations of the density of states (DOS) and the NMR parameters were used for the interpretation of the spectroscopic data. The electron transfer from the alkaline-earth metals and the group 13 elements onto the gold atoms was investigated through X-ray photoelectron spectroscopy (XPS), classifying these intermetallics as aurides.
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  • On the divalent character of the Eu atoms in the ternary Zintl phases Eu(5)In(2)Pn(6) and Eu3MAs3 (Pn =3D As-Bi; M =3D Al,Ga)

    Radzieowski, Mathis   Stegemann, Frank   Klenner, Steffen   Zhang, Yuemei   Fokwa, Boniface P. T.   Janka, Oliver  

    Five Zintl phases in the ternary system Eu-M-Pn (M =3D Al, In; Pn =3D As, Sb, Bi) were prepared from the elements in tantalum containers. Eu5In2As6 and Eu5In2Sb6 crystallize in the orthorhombic Ca5Ga2As6 type structure (Pbam, oP26), while Eu5In2Bi6 is isostructural to orthorhombic Ca5Al2Bi6 (Pbam, oP26). Eu3AlAs3 adopts a monoclinic structure type (P2(1)/c, mP28), which is isopointal to Rb3TlO3, and Eu3GaAs3 (Cmce, oS56), finally, crystallizes in the orthorhombic Ba3AlSb3 type structure. All structures have been refined from single crystal X-ray diffraction experiments and can be considered to be Zintl phases with a valence precise sum formula according to (Eu2+)(5)(In3+)(2)(Pn(3-))(4)(Pn(2-))(2) and (Eu2+)(3)(M3+)(As3-)(3). They all feature [MPn(4)] tetrahedra, which are connected in different ways. While in the Ca5Ga2As6 and Ca5Al2Bi6 type representatives double strands via Pn-Pn bonds are formed, in Eu3AlAs3 and Eu3GaAs3, [M2As6](6-) tetrahedral dimers exist. The divalent europium atoms are located in between the chains, providing electroneutrality. The magnetic properties of four compounds have been investigated and complex (antiferro)magnetic ordering has been observed at T-N =3D 16.1(1) (Eu5In2As6), 17.8(1) K (Eu5In2Sb6), 10.0(1) K (Eu3AlAs3) and 10.7(1) K (Eu3GaAs3). The effective magnetic moment and Eu-151 Mossbauer spectroscopic investigations unambiguously proved the divalent character of the Eu atoms. The spectra recorded below the magnetic ordering showed a (full) hyperfine field splitting. Additionally, Sb-121 Mossbauer spectroscopic studies have been conducted on the antimonide Eu5In2Sb6. Finally, computational studies of Eu3AlAs3 and Eu5In2Sb6 indicate semiconducting behavior for the arsenide with a bandgap of ca. 1 eV, while an increased metallicity, manifested in a pseudo gap for the antimonide, is visible at the Fermi level.
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  • Crystal structure and magnetic properties of the ternary rare earth metal-rich transition metallides RE14T3Al3 (RE = Y, Gd–Tm, Lu; T = Co, Ni)

    Stegemann, Frank   Janka, Oliver  

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  • Crystal structure and magnetic properties of the ternary rare earth metal-rich transition metallides RE14T3Al3 (RE =3DY,Gd-Tm,Lu; T=3D Co,Ni)

    Stegemann, Frank   Janka, Oliver  

    The rare earth metal-rich RE14T3Al3 series (RE =3D Y, Gd-Tm, Lu; T =3D Co, Ni) have been prepared by arc-melting the rare earth metals with appropriate amounts of TAl precursors. All compounds crystallize in the tetragonal crystal system with space group P4(2)/nmc in the Gd14Co3In2.7 -type structure. Two structures (Y14Co2.78(1)Al3.22(1): a =3D 952.99(4), c =3D 2292.98(10) pm, wR2 =3D 0.0423, 2225 F-2 values, 63 variables) have been refined from single-crystal data, indicating T/Al mixing on one crystallographic site. The lattice parameters of all members have been refined from powder X-ray diffraction experiments. The Y and Lu containing compounds for T=3D Co and Ni exhibit Pauli paramagnetic behavior, indicating that the Co and Ni atoms exhibit no localized magnetic moment in line with a filled 3d band. The other compounds show paramagnetism, in line with the rare earth atoms in the trivalent oxidation state. Detailed magnetic investigations, however, were impossible due to the presence of e.g. RE3T trace impurities.
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