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Now showing items 1 - 11 of 11

  • 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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  • The monoclinic superstructure of the M2Pt6Al15 series (M=Ca, Sc, Y, La, Lu)

    Radzieowski, Mathis   Stegemann, Frank   Hoffmann, Rolf-Dieter   Janka, Oliver  

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  • On new ternary equiatomic scandium transition metal aluminum compounds ScTAl with T = Cr, Ru, Ag, Re, Pt, and Au

    Radzieowski, Mathis   Benndorf, Christopher   Haverkamp, Sandra   Eckert, Hellmut   Janka, Oliver  

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  • On new ternary equiatomic scandium transition metal aluminum compounds ScTAl with T =3D Cr,Ru,Ag,Re,Pt,and Au

    Radzieowski, Mathis   Benndorf, Christopher   Haverkamp, Sandra   Eckert, Hellmut   Janka, Oliver  

    The new equiatomic scandium transition metal aluminides ScTAl for T =3D Cr, Ru, Ag, Re, Pt, and Au were obtained by arc-melting of the elements followed by subsequent annealing for crystal growth. The samples were studied by powder and single crystal X-ray diffraction. The structures of three compounds were refined from-single crystal X-ray diffractometer data: ScCrAl, MgZn2 type, P6(3)/mmc, alpha =3D 525.77(3), c =3D 858.68(5) pm, R-1 =3D 0.0188, wR(2) =3D 0.0485, 204 F-2 values, 13 variables, ScPtAl, TiNiSi type, Pnma, alpha =3D 642.83(4), b =3D 428.96(2), c =3D 754.54(5) pm, R-1 =3D 0.0326, wR(2) =3D 0.0458, 448 F-2 values, 20 variables and ScAuAl, HfRhSn type, P (6) over bar 2c, alpha =3D 722.88(4), c =3D 724.15(4) pm, R-1 =3D 0.0316, wR(2) =3D 0.0653, 512 F-2 values, 18 variables. Phase pure samples of all compounds were furthermore investigated by magnetic susceptibility measurements, and Pauli-paramagnetism but no superconductivity was observed down to 2.1 K for all of them. The local structural features and disordering phenomena have been characterized by Al-27 and Sc-45 magic angle spinning (MAS) and static NMR spectroscopic investigations.
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  • ChemInform Abstract: On New Ternary Equiatomic Scandium Transition Metal Aluminum Compounds ScTAl with T: Cr, Ru, Ag, Re, Pt, and Au.

    Radzieowski, Mathis   Benndorf, Christopher   Haverkamp, Sandra   Eckert, Hellmut   Janka, Oliver  

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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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  • Synthesis,crystal and electronic structure,physical properties and Sb-121 and Eu-151 Mossbauer spectroscopy of the Eu(14)AlPn(11) series (Pn =3D As,Sb)

    Radzieowski, Mathis   Block, Theresa   Klenner, Steffen   Zhang, Yuemei   Fokwa, Boniface P. T.   Janka, Oliver  

    The pnictides Eu14AlAs11 and Eu14AlSb11 were synthesized from the elements in sealed niobium ampoules. They crystallize in the tetragonal crystal system (Eu14AlAs11: a =3D 1627.6(2), c =3D 2180.0(4) pm; Eu14AlSb11: a =3D 1725.6(2), c =3D 2289.7(7) pm) with space group I4(1)/acd, isostructural to Ca14AlSb11 and can be described as Zintl phases. The Al atoms are surrounded by four pnictogen atoms, forming [AlPn(4)](9-) tetrahedra. Additionally isolated Pn(3-) anions and linear Pn(3)(7-) trimers can be found in the crystal structure. The compounds can be described according to (Eu2+)(14)(AlPn(9-))(Pn(3-))(4)(Pn(3)(7-)). Eu14AlAs11 and Eu14AlSb11 both exhibit an antiferromagnetic transition at T-N =3D 10.5(1) K and 12.5(1) K, respectively. For the antimonide, the magnetic transition has been confirmed by additional heat capacity measurements. Resistivity investigations indicate that Eu14AlSb11 is a semiconductor with a band gap of E-g =3D 0.28(5) eV close to room temperature. According to the Zintl formalism, the Eu atoms are divalent, which has been confirmed by magnetic susceptibility and additional Eu-151 Mossbauer spectroscopic studies. The measurements conducted at 6 K, below the magnetic ordering temperature, show a full hyperfine field splitting with complex spectra underlining the recorded magnetic data. Furthermore, Sb-121 Mossbauer spectroscopic studies have been conducted to study the different antimonide entities in the title compounds.
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  • Synthesis,crystal and electronic structures,physical properties and Sb-121 and Eu-151 Mossbauer spectroscopy of the alumo-antimonide Zintl-phase Eu5Al2Sb6

    Radzieowski, Mathis   Block, Theresa   Fickenscher, Thomas   Zhang, Yuemei   Fokwa, Boniface P. T.   Janka, Oliver  

    Eu5Al2Sb6 was synthesized from the elements in niobium ampoules. It crystallizes in the orthorhombic crystal system (a =3D 1027.55(7), b =3D 1200.28(6), c =3D 1324.22(7) pm) with space group Pnma (no. 62), isostructural to Sr5Al2Sb6, and can be described as a Zintl phase. The Al atoms are tetrahedrally surrounded by Sb atoms, forming branched strands. Besides Sb3- anions, antimony Sb-2(-4) dumbbells can also be found in the crystal structure. (EuAlSb6)-Al-5-Sb-2 [equivalent to(Eu2+)(5)(Al3+)(2)(Sb3 -)(4)(Sb-2(4-))] exhibits three magnetic ordering phenomena at T-1 =3D 12.3(1), T-2 =3D 10.6(1) and T-3 =3D 3.0(1) K, obtained by heat capacity measurements. While T-3 is of antiferromagnetic nature, T1 and T2 correspond to canted antiferromagnetic transitions. Resistivity investigations indicate that the title compound is a semiconductor, in line with the band structure calculations. Spin exchange parameters, calculated using mapping analysis, confirm that the magnetic phase transition at T-N =3D 3.5(1) is associated with the ordering of Eu1 atoms. The divalent character has been confirmed by Eu-151 Moussbauer spectroscopic studies. Measurements conducted at 5 K show a hyperfine field splitting for two of the three Eu sites, underlining the magnetic data. Additionally, Sb-121 Moussbauer spectroscopic studies have been conducted on Eu5Al2Sb6 and isostructural Sr5Al2Sb6.
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  • Correlations of Crystal and Electronic Structure via NMR and X-ray Photoelectron Spectroscopies in the RETMAI(2) (RE =3D Sc,Y,La-Nd,Sm,Gd-Tm,Lu; TM =3D Ni,Pd,Pt) Series

    Radzieowski, Mathis   Stegemann, Frank   Doerenkamp, Carsten   Matar, Samir F.   Eckert, Hellmut   Dosche, Carsten   Wittstock, Gunther   Janka, Oliver  

    A total of 35 intermetallic aluminum compounds have been synthesized from the elements via arc melting and characterized by powder X-ray diffraction. A total of 15 of them have been previously reported; however, detailed property investigations were missing. Compounds of the RETMAl2 (rare earth metal RE =3D Sc, Y, La-Nd, Sm, Gd-Tm, Lu) series with transition metal TM =3D Ni, Pd, and Pt crystallize isostructurally in the orthorhombic MgCuAl2 type structure (Cmcm, oC16, fc(2)). Single-crystal X-ray diffraction investigations were conducted on YNiAl2, LaNiAl2, YPdAl2, ScPtAl2, and YPtAl2. The TM and Al atoms form a [TMAl2](delta-) polyanion, the RE atoms reside in cavities within the framework. While the Sc, Y, La, and Lu compounds exhibit Pauli-paramagnetic behavior, consistent with all atoms being closed shell, the other RETMAl2 compounds show paramagnetism along with magnetic ordering at low temperatures, in line with an open-shell trivalent oxidation state for the RE atoms. Solid-state Al-27 NMR investigations were carried out on the Pauli-paramagnetic samples, all showing only a single central transition, in line with one crystallographic site for the respective atoms. The observed quadrupolar coupling constants and electric-field-gradient asymmetry parameters were found to be in good agreement with the density-functional-theory-calculated values. Isotropic resonance shifts are dominated by the Fermi-contact interactions with s-conduction electron densities at the Fermi edge (Knight shifts). The bonding characteristics mirror the electronic density of states and crystal chemistry of the family of intermetallic compounds under consideration. Both the Knight shifts and quadrupolar coupling constants can be predicted based on element-specific increments.
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  • Synthesis, crystal and electronic structure, physical properties and 121 Sb and 151 Eu Mössbauer spectroscopy of the Eu 14 AlPn 11 series (Pn = As, Sb)

    Radzieowski, Mathis   Block, Theresa   Klenner, Steffen   Zhang, Yuemei   Fokwa, Boniface P. T.   Janka, Oliver  

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  • Synthesis, crystal and electronic structures, physical properties and\r 121\r Sb and\r 151\r Eu M?ssbauer spectroscopy of the alumo-antimonide Zintl-phase Eu\r 5\r Al\r 2\r Sb\r 6

    Radzieowski, Mathis   Block, Theresa   Fickenscher, Thomas   Zhang, Yuemei   Fokwa, Boniface P. T.   Janka, Oliver  

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