Wilfried Hermes

Synthesis, crystal structure and spin-density-wave anomaly of the iron arsenide-fluoride SrFeAsF

The new quaternary iron arsenide-fluoride SrFeAsF with the tetragonal ZrCuSiAs-type structure was synthesized and the crystal structure was determined by X-ray powder diffraction (P4/nmm, a=399.30(1), c=895.46(1) pm). SrFeAsF undergoes a structural and magnetic phase transition at 175 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (P4/nmm) to orthorhombic (Cmme). 57Fe Mossbauer spectroscopy experiments show a single signal at room temperature at an isomer shift of 0.30(1) mm/s and magnetic hyperfine-field splitting below the phase transition temperature. Our results clearly show that SrFeAsF exhibits a spin density wave (SDW) anomaly at 175 K very similar to LaFeAsO, the parent compound of the iron arsenide-oxide superconductors and thus SrFeAsF may serve as a further parent compound for oxygen-free iron arsenide superconductors.

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-α-Me3Si-benzyl, with two equivalents of Cp(BIG)H. 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 Cp(BIG)⋅⋅⋅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 (151)Eu Mössbauer effect spectroscopic examination of [Eu(Cp(BIG))2] was examined over the temperature range 93-215 K 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 203 K. 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 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln2O3 standards. No XANES temperature dependence was found from room temperature to 100 K. 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 412 nm (24,270 cm(-1)) gives a symmetrical single band in the emission spectrum at 606 nm (νmax =16495 cm(-1), FWHM: 2090 cm(-1), Stokes-shift: 2140 cm(-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.

Physical Properties of Superbulky Lanthanide Metallocenes: Synthesis and Extraordinary Luminescence of [Eu[superscript II](Cp[superscript BIG])[subscript 2]] (Cp[superscript BIG]=(4-nBu-C[subscript 6]H[subscript 4])[subscript 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-α-Me3Si-benzyl, with two equivalents of Cp(BIG)H. 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 Cp(BIG)⋅⋅⋅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 (151)Eu Mössbauer effect spectroscopic examination of [Eu(Cp(BIG))2] was examined over the temperature range 93-215 K 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 203 K. 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 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln2O3 standards. No XANES temperature dependence was found from room temperature to 100 K. 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 412 nm (24,270 cm(-1)) gives a symmetrical single band in the emission spectrum at 606 nm (νmax =16495 cm(-1), FWHM: 2090 cm(-1), Stokes-shift: 2140 cm(-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.

New thermodynamic data for CoTiO3, NiTiO3 and CoCO3 based on low-temperature calorimetric measurements

The low-temperature heat capacities of nickel titanate (NiTiO3), cobalt titanate (CoTiO3), and cobalt carbonate (CoCO3) were measured between 2 and 300 K, and thermochemical functions were derived from the results. Our new data show previously unknown low-temperature lambda-shaped heat capacity anomalies peaking at 37 K for CoTiO3 and 26 K for NiTiO3. From our data we calculate standard molar entropies (298.15 K) for NiTiO3 of 90.9 ± 0.7 J mol-1 K-1 and for CoTiO3 of 94.4 ± 0.8 J mol-1 K-1. For CoCO3, we find only a small broad heat capacity anomaly, peaking at about 31 K. From our data, we suggest a new standard entropy (298.15 K) for CoCO3 of 88.9 ± 0.7 J mol-1 K-1.

Complex charge ordering in CeRuSn

At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4−δ)+. Charge ordering produces a doubling of the unit cell along the c axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, a phase transition with very broad hysteresis has been observed in various bulk properties like electrical resistivity, magnetic susceptibility, and specific heat. The present x-ray-diffraction results show that at low temperatures the doubling of the CeCoAl type structure is replaced by an ill-defined modulated ground state. In this state, at least three different modulation periods compete, with the dominant mode close to a tripling of the basic cell. The transition is accompanied by a significant contraction of the c axis. XANES data suggest that the average Ce valence remains constant; thus the observed c-axis contraction is not due to any valence transition. We propose a qualitative structure model with modified stacking sequences of Ce3+ and Ce(4−δ)+ layers in the various modulated phases. Surprisingly, far below 100 K, the modulated state is sensitive to x-ray irradiation at photon fluxes available at a synchrotron. With photon fluxes of order 1012 s−1, the modulated ground state can be destroyed on a time scale of minutes and the doubling of the CeCoAl cell observed at room temperature is recovered. The final state is metastable at 10 K. Heating the sample above 60 K again leads to a recovery of the modulated state. Thus CeRuSn exhibits both thermally and x-ray induced reversible transformations of the Ce3+/Ce(4−δ)+ charge-ordering pattern. Such a behavior is unique among any known intermetallic compound.

Structure and properties of Eu2Pt3Sn5

Abstract The new stannide Eu2Pt3Sn5 was synthesized by induction melting of the elements in a sealed tantalum tube in a water-cooled silica sample chamber. The structure was refined on the basis of single crystal X-ray diffractometer data: Y2Rh3Sn5 type, Cmc21, a = 453.30(9), b = 2662.9(8), c = 731.8(2) pm, wR2 = 0.0472, 1493 F2 values and 62 variables). The platinum and tin atoms build up a complex three-dimensional [Pt3Sn5] network with a broad range of Pt—Sn (264–293 pm) and Sn—Sn (308–373 pm) distances. The two crystallographically independent europium sites occupy large voids of coordination numbers 20 (Eu1) and 18 (Eu2) within the [Pt3Sn5] network. Eu2Pt3Sn5 shows Curie-Weiss behaviour in the temperature range 50–305 K with an experimental magnetic moment of 7.82(1) μB per Eu atom indicating purely divalent europium. Cp measurements show a broad anomaly with two maxima at 4.9(2) and 6.1(2) K. 151Eu Mössbauer spectra revealed both crystallographically independent europium sites to be divalent. At 4.2 K two rather different sizes of magnetic hyperfine field splittings are observed due to separate ordering of the two sites. The 119Sn Mössbauer spectrum shows complex superposition of the five signals in the paramagnetic range.

Intermediate-valent Ce23Ru7Mg4 and RE23Ru7Mg4 (RE = La, Pr, Nd) with Pr23Ir7Mg4-type Structure

The rare earth-rich magnesium compounds RE23Ru7Mg4 (RE = La, Ce, Pr, Nd) were synthesized from the elements in sealed tantalum ampoules in an induction furnace. They crystallize with the hexagonal non-centrosymmetric Pr23Ir7Mg4-type structure, space group P63mc. The structures of La23Ru6.88(1)Mg4 (a = 1017.7(4), c = 2286.5(5) pm, wR2 = 0.0277, 2708 F2, 71 variables), Ce23Ru7Mg4 (a = 993.5(3), c = 2243.9(8) pm, wR2 = 0.0573, 2268 F2, 70 variables), and Pr23Ru7Mg4 (a = 996.8(3), c = 2241.5(6) pm, wR2 = 0.0492, 2565 F2, 70 variables) have been refined from single-crystal diffractometer data. The structures are built up from complex threedimensional networks of edge- and corner-sharing RE6Ru trigonal prisms. Cavities within these networks are filled by slightly elongated Mg4 tetrahedra (311 - 315 pm in Pr23Ru7Mg4) and RE6 octahedra. The cerium compound has an a parameter which is even smaller than that of Nd23 Ru7Mg4, indicating intermediate-valent cerium. This was confirmed by magnetic susceptibility measurements. Ce23Ru7Mg4 shows an average, reduced magnetic moment of 2.01 μB/Ce atom. Pr23Ru7Mg4 contains stable trivalent praseodymium (3.64 μB/Pr atom) Graphical Abstract Intermediate-valent Ce23Ru7Mg4 and RE23Ru7Mg4 (RE = La, Pr, Nd) with Pr23Ir7Mg4-type Structure

Structure and Magnetic Properties of GdPt2In and GdPt2Sn

The platinum-rich intermetallic compounds GdPt2In and GdPt2Sn were synthesized by arc-melting of the elements and subsequent annealing. The structures were refined from single crystal X-ray diffractometer data: ZrPt2Al type, space group P63/mmc, a = 455.1(1), c = 899.3(3) pm, wR2 = 0.0361, 166 F2 values, 9 variables for GdPt2In, and a = 453.2(1), c = 906.5(2) pm, wR2 = 0.0915, 166 F2 values, 9 variables for GdPt2Sn. The platinum and indium (tin) atoms build up threedimensional [Pt2In] and [Pt2Sn] networks with short Pt-In (Pt-Sn) distances and Pt2 dumb-bells (290 and 297 pm in GdPt2In and GdPt2Sn). The gadolinium atoms have coordination number 14 with 8 Pt and 6 In (Sn) neighbors. Magnetic susceptibility measurements on GdPt2In show Curie-Weiss behavior with an experimental magnetic moment of 8.06(2) μB/Gd atom. GdPt2In orders ferromagnetically at 27.7(2) K Graphical Abstract Structure and Magnetic Properties of GdPt2In and GdPt2Sn

Unusually Short Ce-Ru Distances in CeRuAl and Related Compounds

The aluminide CeRuAl with orthorhombic LaNiAl-type structure contains two crystallographically independent cerium sites which both exhibit relatively short Ce-Ru distances, i. e. 280 - 302 pm for Ce1 and 286 - 310 pm for Ce2. Susceptibility measurements show intermediate valence behavior of the cerium atoms (1.19(1) μB per formula unit) and no magnetic ordering down to 2 K. Chemical bonding analysis reveals a non-magnetic ground state and strong Ce-Ru bonding. The Ce-Ru bonding peculiarities of CeRuAl are discussed in line with those of other binary and ternary cerium- ruthenium compounds Graphical Abstract Unusually Short Ce–Ru Distances in CeRuAl and Related Compounds

Structure and properties of the 5.5 K antiferromagnet EuAu2Ge2

Abstract A well crystallized sample of EuAu2Ge2 was synthesized by arc-melting of the elements. The structure was refined on the basis of single crystal X-ray diffracto meter data: ThCr2Si2 type, I4/mmm, a = 447.3(1), c = 1034.0(4) pm, wR2 = 0.0575, 148 F2 values and 9 variables. The gold atoms have tetrahedral germanium coordination (261 pm Au—Ge). These AuGe4/4 tetrahedra share four common edges, leading to layers which are connected via Ge—Ge bonds (247 pm). The europium atoms fill cages of coordination number 16 within the [Au2Ge2]δ– polyanion. Temperature dependent susceptibility measurements show a magnetic moment of 7.79(1)μB/formula unit. Together with the 151 Eu Mössbauer spectroscopic data (isomer shift of –11.43 mm/s at 77 K), the magnetic data point to stable divalent europium. EuAu2Ge2 orders antiferromagnetically at TN1 = 5.5(1) K, followed by a spin-reorientation at TN2 = 3.5 K. A metamagnetic step occurs around 20 kOe in the 4 K magnetization isotherm.