The solid solution Gd2NixCu2-xMg: Large reversible magnetocaloric effect and a drastic change of the magnetism by substitution

Various samples of the solid solution Gd2NixCu2-xMg were synthesized from the elements in sealed tantalum ampoules in an induction furnace. All members crystallize with the tetragonal Mo2FeB2 type structure, space group P4/mbm, and they were characterized on the basis of Guinier powder patterns and energy dispersive X-rays analyses. The lattice parameters decrease with increasing nickel content in a Vegard-like manner. The Gd2NixCu2-xMg samples show Curie Weiss behavior with slightly higher magnetic moment values than the theoretical one for a free Gd3+ ion. The substitution of copper by nickel has a drastic influence on the magnetism and magnetic ordering temperature. For Gd2Ni0.5Cu1.5Mg a temperature induced FM -> AFM order-to-order transition was observed, whereas Gd2Ni1.0Cu1.0Mg is a metamagnet with H-Cr of about 8 kOe at 5 K. For both compounds, a large reversible magnetocaloric effect (MCE) near their ordering temperatures occurs. The values of the maximum magnetic entropy change -Delta S-M(max) reach 9.5 and 11.4 J kg(-1) K-1 for the field change of 5 T with no obvious hysteresis loss around 65 K for Gd2Ni0.5Cu1.5Mg and Gd2Ni1.0Cu1.0Mg, respectively. The corresponding relative cooling power with 688 and 630 J kg(-1) is relatively high as compared to other MCE materials in that temperature range. These results indicate that Gd2NixCu2-xMg could be a promising system for magnetic refrigeration at temperatures below liquid N-2. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3466775

Drastic change of the ferromagnetic properties of the ternary germanide GdTiGe through hydrogen insertion

Hydrogen absorption of the CeFeSi- and CeScSi-type forms of GdTiGe was performed. Before hydrogenation they show an antiferromagnetic transition at around 412 K and a ferromagnetic transition at 376 K, respectively. Hydrogenation of both forms leads to formation of the same hydride GdTiGeH which crystallizes with a filled CeScSi-type structure where all the [Gd(4)] tetrahedra are filled by hydrogen. This hydride is paramagnetic in the temperature range 4-300 K. The slightly negative value of the paramagnetic Curie temperature θ(p) confirms that all ferromagnetic interactions were destroyed in the case of the CeScSi-type form. From first-principles calculations with the PAW GGA methodology, the localization of hydrogen within the [Gd(4)] tetrahedra was confirmed through energetic stabilization. It was also seen that the energy changes significantly with volume, indicating the itinerant (delocalized) role of the electrons in the magnetism

Ferromagnetic Ordering in CeZnSn

The stannide CeZnSn was obtained in X-ray-pure form by induction-melting of the elements in a sealed tantalum ampoule. CeZnSn crystallizes with the YPtAs-type structure, space group P63/mmc, ..

Transition from antiferromagnetic ordering to cluster-glass behaviour in the solid solution Gd₄NiMg_1−<i>x</i>Al<i>_x</i> (0 ≤ <i>x</i> ≤ 0.9)

Various samples of the solid solution Gd4NiMg1−xAlx were synthesized from the elements in sealed tantalum ampoules in an induction furnace. These gadolinium-rich compounds crystallize with the cubic Gd4RhIn type structure, space group the F4-; 3m. All samples were characterized on the basis of Guinier powder patterns. The lattice parameter decreases with increasing aluminium content in a Vegard-like manner. The structures of two crystals were refined on the basis of diffractometer data: a = 1359.3(1), wR2 = 0.0711, 481 F2 values, 20 variables for Gd4NiMg0.52(1)Al0.48(1) and a = 1351.40(6), wR2 = 0.0665, 478 F2 values, 21 variables for Gd4NiMg0.10(1)Al0.90(1). The main structural motif is a rigid three-dimensional network of edge- and corner-sharing Gd6Ni trigonal prisms and isolated (Mg/Al)4 tetrahedra in fcc packing. The edge length of the tetrahedra drastically decreases from Gd4NiMg (309 pm) to Gd4NiMg0.10(1)Al0.90(1) (294 pm). The Gd4NiMg1−xAlx samples show Curie–Weiss behaviour with slightly higher magnetic moment values than the theoretical one for a free Gd3+ ion. By substitution of Gd4NiMg with aluminium the cell volume decreases and the atomic disorder (Mg/Al) induces a random distribution of the magnetic interactions. This leads to a shift of the Néel temperatures to lower values when increasing the aluminium content. Already for Gd4NiMg0.4Al0.6 the long-range antiferromagnetic ordering no longer exists and the system comes into a cluster-glass state

Phosphide Oxides RE2AuP2O (RE = La, Ce, Pr, Nd): synthesis, structure, chemical bonding, magnetism, and 31P and 139La solid state NMR

Polycrystalline samples of the phosphide oxides RE2AuP2O (RE = La, Ce, Pr, Nd) were obtained from mixtures of the rare earth elements, binary rare earth oxides, gold powder, and red phosphorus in sealed silica tubes. Small single crystals were grown in NaCl/KCl fluxes. The samples were studied by powder X-ray diffraction, and the structures were refined from single crystal diffractometer data: La2AuP2O type, space group C2/m, a = 1515.2(4), b = 424.63(8), c = 999.2(2) pm, β = 130.90(2)°, wR2 = 0.0410, 1050 F2 values for Ce2AuP2O, and a = 1503.6(4), b = 422.77(8), c = 993.0(2) pm, β = 130.88(2)°, wR2 = 0.0401, 1037 F2 values for Pr2AuP2O, and a = 1501.87(5), b = 420.85(5), c = 990.3(3) pm, β = 131.12(1)°, wR2 = 0.0944, 1143 F2 values for Nd2AuP2O with 38 variables per refinement. The structures are composed of [RE2O]4+ polycationic chains of cis-edgesharing ORE4/2 tetrahedra and polyanionic strands [AuP2]4−, which contain gold in almost trigonal−planar phosphorus coordination by P3− and P2 4− entities. The isolated phosphorus atoms and the P2 pairs in La2AuP2O could clearly be distinguished by 31P solid state NMR spectroscopy and assigned on the basis of a double quantum NMR technique. Also, the two crystallographically inequivalent La sites could be distinguished by static 139La NMR in conjunction with theoretical electric field gradient calculations. Temperature-dependent magnetic susceptibility measurements show diamagnetic behavior for La2AuP2O. Ce2AuP2O and Pr2AuP2O are Curie−Weiss paramagnets with experimental magnetic moments of 2.35 and 3.48 μB per rare earth atom, respectively. Their solid state 31P MAS NMR spectra are strongly influenced by paramagnetic interactions. Ce2AuP2O orders antiferromagnetically at 13.1(5) K and shows a metamagnetic transition at 11.5 kOe. Pr2AuP2O orders ferromagnetically at 7.0 K.Deutsche ForschungsgemeinschafNRW Forschungsschule Molecules and Material

Plasmonic wavelength-dependent optical switch

Kilbane D, Prinz E, Eul T, et al. Plasmonic wavelength-dependent optical switch. Optics Express. 2023;31(6):9579-9590.We design and experimentally demonstrate an optical switch based on the interference of plasmonic modes in whispering gallery mode (WGM) antennas. Simultaneous excitation of even and odd WGM modes, enabled by a small symmetry breaking via non-normal illumination, allows switching the plasmonic near field between opposite sides of the antenna, depending on the excitation wavelength used in a wavelength range of 60 nm centered around 790 nm. This proposed switching mechanism is experimentally demonstrated by combining photoemission electron microscopy (PEEM) with a tunable wavelength femtosecond laser source in the visible and infrared

Synthesis and Crystal Structure Determination of Ag₉FeS_4.1Te_1.9, the First Example of an Iron Containing Argyrodite

Ag₉FeS_4.1Te_1.9 was prepared by solid state synthesis from stoichiometric amounts of the elements at 873 K. The compound forms gray crystals which are stable against air and moisture. The crystal structure was determined by X-ray diffraction from selected single crystals. Ag₉FeS_4.1Te_1.9 crystallizes in the space group <i>F</i>4̅3<i>m</i>, <i>a</i> = 11.0415(7) Å, <i>V</i> = 1346.1(1) ų, and <i>Z</i> = 4 (powder data at 293 K). The compound shows a reversible phase transition upon cooling to the space group <i>P</i>2₁3, <i>a</i> = 11.0213(1) Å, <i>V</i> = 1338.75(2) ų, and <i>Z</i> = 4 (single crystal data at 200 K). The title compound is the first example of an iron containing argyrodite-type material with Fe³⁺ located in tetrahedral sites. Silver atoms are disordered at room temperature which was taken into account by nonharmonic refinement of the silver positions. The refinement converged to <i>R</i>₁ = 3.51% and <i>wR</i>₂ = 10.66% for the room temperature measurement and to <i>R</i>₁ = 1.55% and <i>wR</i>₂= 5.23% for the 200 K data set (all data). Impedance measurements were performed in the temperature range from 323 to 473 K. Ionic conductivity values are 1.81 × 10^–2 S cm^–1 at 323 K and 1.41 × 10^–1 S cm^–1 at 468 K. The activation energy is 0.19 eV from 323 to 423 K and 0.06 eV from 393 to 473 K. DTA measurements reveal congruent melting at 907 K. A phase transition temperature of 232 K with an enthalpy of 7.9 kJ/mol was determined by DSC measurements. ⁵⁷Fe Mössbauer spectra show one signal at 298 K and a doublet at 78 K, indicating Fe³⁺ and structural distortions upon cooling the samples. Hyperfine field splitting of iron is observed at 5 K. Measurements of the molar susceptibility revealed that the compound is paramagnetic down to a Néel temperature of <i>T</i>_N = 22.1(5) K. Antiferromagnetic ordering is observed at lower temperatures

Structure, homogeneity ranges, magnetic, and electrical properties of the ordered Laves phases <i>RE</i>Ni₄Mg with MgCu₄Sn type structure

New ordered Laves phases RENi4Mg (RE = Sc, Sm, Tb-Lu) were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Six of the structures were refined on the basis of X-ray single crystal data. The diffraction experiments gave hint for small homogeneity ranges RE1+xNi4Mg1−x. Magnetic susceptibility measurements show Curie-Weiss behavior for RE = Gd, Dy, Ho, Tm, Yb and the resulting effective magnetic moments suggest both stable trivalent states for all RE and a non-magnetic state for Ni. Gd1+xNi4Mg1−x (x ≈ 0.12) orders antiferromagnetically at a Néel temperature of TN = 4.6(5) K. Resistivity measurements reflect the metallic nature of these compounds

Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: x-ray, spectroscopic, and density functional theory studies

Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown- 6){Cp*Fe(η4-C10H8)}] (K1), [K(18-crown-6){Cp*Fe(η4-C14H10)}] (K2), [Cp*Fe(η4-C10H8)] (1), and [Cp*Fe(η4-C14H10)] (2) were synthesized and characterized by NMR, UV−vis, and 57Fe Mössbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(η4-C10H8)]− (1−) and [Cp*Fe(η4-C14H10)]− (2−) and reversibly oxidized to the cations [Cp*Fe(η6-C10H8)]+ (1+) and [Cp*Fe(η6-C14H10)]+ (2+). Reduced orbital charges and spin densities of the naphthalene complexes 1−/0/+ and the anthracene derivatives 2−/0/+ were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1− and 2− are best represented by low-spin FeII ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin FeI ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin FeII ion coordinated to a ligand radical L•−. Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state

Physical Properties of Superbulky Lanthanide Metallocenes:Synthesis and Extraordinary Luminescence of [Eu-II(Cp-BIG)(2)] (Cp-BIG=(4-nBu-C6H4)(5)-Cyclopentadienyl)

The superbulky deca-aryleuropocene [Eu(Cp-BIG)(2)], Cp-BIG=(4-nBu-C6H4)(5)-cyclopentadienyl, was prepared by reaction of [Eu(dmat)(2)(thf)(2)], DMAT=2-Me2N--Me3Si-benzyl, with two equivalents of (CpH)-H-BIG. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45% quantum yield). The crystal structure is isomorphic to those of [M(Cp-BIG)(2)] (M=Sm, Yb, Ca, Ba) and shows the typical distortions that arise from (CpCpBIG)-Cp-BIG attraction as well as excessively large displacement parameter for the heavy Eu atom (U-eq=0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(Cp-BIG)(2)] (M=Sm, Eu, Yb), several physical analyses have been applied. Temperature-dependent magnetic susceptibility data of [Yb(Cp-BIG)(2)] show diamagnetism, indicating stable divalent ytterbium. Temperature-dependent Eu-151 Mossbauer effect spectroscopic examination of [Eu(Cp-BIG)(2)] was examined over the temperature range 93-215K and the hyperfine and dynamical properties of the Eu-II species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single-crystal X-ray data at 203K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low-frequency torsional and librational modes in [Eu(Cp-BIG)(2)]. X-ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(Cp-BIG)(2)] (Ln=Sm, Eu, Yb) compounds are divalent. The XANES white-line spectra are at 8.3, 7.3, and 7.8eV, for Sm, Eu, and Yb, respectively, lower than the Ln(2)O(3) standards. No XANES temperature dependence was found from room temperature to 100K. XANES also showed that the [Ln(Cp-BIG)(2)] complexes had less trivalent impurity than a [EuI2(thf)(x)] standard. The complex [Eu(Cp-BIG)(2)] shows already at room temperature strong orange photoluminescence (quantum yield: 45%): excitation at 412nm (24270cm(-1)) gives a symmetrical single band in the emission spectrum at 606nm ((max)=16495cm(-1), FWHM: 2090cm(-1), Stokes-shift: 2140cm(-1)), which is assigned to a 4f(6)5d(1)4f(7) transition of Eu-II. These remarkable values compare well to those for Eu-II-doped ionic host lattices and are likely caused by the rigidity of the [Eu(Cp-BIG)(2)] complex. Sharp emission signals, typical for Eu-III, are not visible