Dirk Johrendt

Publications of Dirk Johrendt

• The role of different negatively charged layers in Ca10(Fe1-xPtxAs)10(Pt3+yAs8) and superconductivity at 30 K in electron-doped (Ca0.8La0.2)10(FeAs)10(Pt3As8)

  Authors: Tobias Stürzer, Gerald Derondeau, Dirk Johrendt

  03/2012;

  The recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of CaThe recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of Ca atoms. We show that high Tc's above 15 K only emerge if the iron-arsenide layers are at most free of platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by charge doping of pure FeAs layers. We point out, that two different negatively charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the electrons provided by the Ca2+ ions, which is unique in the field of iron-based superconductors. In the parent compound Ca10(FeAs)10(Pt3As8), no excess charge dopes the FeAs-layer, and superconductivity has to be induced by Pt-substitution, albeit below 15 K. In contrast, the additional Pt-atom in the Pt4As8layer shifts the charge balance between the layers equivalent to charge doping by 0.2 electrons per FeAs. Only in this case Tc raises to 38 K, but decreases again if additionally platinum is substituted for iron. This charge doping scenario is supported by our discovery of superconductivity at 30 K in the electron-doped La-1038 compound (Ca0.8La0.2)10(FeAs)10(Pt3As8) without significant Pt-substitution.

• The interplay of electron doping and chemical pressure in Ba(Fe(1-y)Coy)2(As(1-x)Px)2

  Authors: Veronika Zinth, Dirk Johrendt

  03/2012;

  The effects of internal chemical pressure on electron doped iron arsenide superconductors are studied in the series Ba(Fe(1-y)Coy)2(As(1-x)Px)2. Combinations of both dopants induce superconductivityThe effects of internal chemical pressure on electron doped iron arsenide superconductors are studied in the series Ba(Fe(1-y)Coy)2(As(1-x)Px)2. Combinations of both dopants induce superconductivity also in such areas where only one would not suffice, and can likewise move the system into an overdoped state, while no higher critical temperature than 31 K in BaFe2(As(1-x)Px)2 was found. The phase diagram gives no evidence of holes in BaFe2(As(1-x)Px)2 as suggested by recent photoemission experiments. Chemical and physical pressure act similarly in Ba(Fe(1-y)Coy)2(As(1-x)Px)2, but our data reveal that the most important control parameter is the length of the Fe-As bond and not the unit cell volume. This emphasizes that differences between chemical and physical pressure which manifest oneself as the non-linear reduction of the Fe-As distance in BaFe2(As(1-x)Px)2 are strongly linked to the superconducting properties also in the Co doped compounds.

• Microscopic Coexistence of Superconductivity and Magnetism in Ba_{1-x}K_{x}Fe_{2}As_{2}.

  Authors: Erwin Wiesenmayer, Hubertus Luetkens, Gwendolyne Pascua, Rustem Khasanov, Alex Amato, Heidi Potts, Benjamin Banusch, Hans-Henning Klauss, Dirk Johrendt

  Physical review letters. 12/2011; 107(23):237001.

  It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba_{1-x}K_{x}Fe_{2}As_{2} exhibits a mesoscopic phase separation of magnetism and superconductivityIt is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba_{1-x}K_{x}Fe_{2}As_{2} exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba_{1-x}K_{x}Fe_{2}As_{2} showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.

• Superconductivity up to 35 K in the iron platinum arsenides (CaFe(1-x)Pt(x)As)10Pt(4-y)As8 with layered structures.

  Authors: Catrin Löhnert, Tobias Stürzer, Marcus Tegel, Rainer Frankovsky, Gina Friederichs, Dirk Johrendt

  Angewandte Chemie (International ed. in English). 09/2011; 50(39):9195-9.

• Microscopic co-existence of superconductivity and magnetism in Ba1-xKxFe2As2

  Authors: Erwin Wiesenmayer, Hubertus Luetkens, Gwendolyne Pascua, Rustem Khasanov, Alex Amato, Heidi Potts, Benjamin Banusch, Hans-Henning Klauss, Dirk Johrendt

  08/2011;

  It is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in theIt is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq 0.25) showing that this paradigm is wrong. Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magneto-elastically coupled orthorhombic lattice distortion below the superconducting phase transition.

• Superconductivity up to 35 K in the iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 with layered structures

  Authors: Catrin Löhnert, Tobias Stürzer, Marcus Tegel, Rainer Frankovsky, Gina Friederichs, Dirk Johrendt

  07/2011;

  We report the synthesis and crystal structures of three new superconducting iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 (x = 0-0.15, y = 0-0.4). The structures are stacking variants of FeAs- andWe report the synthesis and crystal structures of three new superconducting iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 (x = 0-0.15, y = 0-0.4). The structures are stacking variants of FeAs- and slightly puckered Pt4-yAs8-layers with square coordinated platinum separated by calcium-layers, respectively. Arsenic atoms in the Pt4-yAs8-layers form (As2)4- dumbbells according to Zintl's concept, providing charge balance in (Ca2+Fe2+As3-)10(Pt2+)3[(As2)4-]4. Superconductivity was observed at 13-35 K. We suggest that the highest Tc above 30 K occurs in the 1048 phase with clean FeAs-layers that are indirectly electron-doped according to (Ca2+Fe2+As3-)10(Pt2+)4[(As2)4-]4*2e-. We also suggest that the lower critical temperatures occur in the 1038- and a-1048 phases due to Pt-doping at the Fe-site. DFT band structure calculations indicate that the contribution of the Pt4-yAs8-layers to the Fermi surface is small and that the Fermi energy is slightly either below or above a quasi-gap in the Pt-states. The new platinum-iron compounds represent the first iron-based superconductors with so far unknown structure types and can serve as a new platform for further studies that go beyond the known systems.

• Recovery of a parentlike state in Ba(1-x)K(x)Fe(1.86)Co(0.14)As2.

  Authors: Veronika Zinth, Til Dellmann, Hans-Henning Klauss, Dirk Johrendt

  Angewandte Chemie (International ed. in English). 06/2011; 50(34):7919-23.

• P2(2-) and P(3-) units in the [Rh8P9]δ- polyanion of La4Rh8P9.

  Authors: Ulrike Pfannenschmidt, Dirk Johrendt, Frederik Behrends, Hellmut Eckert, Matthias Eul, Rainer Pöttgen

  Inorganic chemistry. 02/2011; 50(7):3044-51.

  The phosphide La(4)Rh(8)P(9) was synthesized from the elements in a bismuth flux. The structure was refined from single crystal diffractometer data: space group Cmcm, a = 1303.1(2), b = 1893.2(2), cThe phosphide La(4)Rh(8)P(9) was synthesized from the elements in a bismuth flux. The structure was refined from single crystal diffractometer data: space group Cmcm, a = 1303.1(2), b = 1893.2(2), c = 576.70(6) pm, wR2 = 0.0277, 1380 F(2) values, 65 variables. The rhodium and phosphorus atoms build up a three-dimensional [Rh(8)P(9)] polyanion which leaves larger cages for the three crystallographically independent lanthanum sites. The rhodium atoms have between four and six phosphorus neighbors at Rh-P distance ranging from 229 to 254 pm. Three of the four crystallographically independent phosphorus atoms are isolated (P(3-) units), while the P4 atoms form dimers with double bond character (208 pm P-P). The P(2)(2-) diphosphenide units bond side-on to a Rh3 and end-on to four Rh5 atoms. (31)P magic angle spinning (MAS) NMR spectroscopy is able to resolve three of the four crystallographically distinct phosphorus sites. The doubly bonded phosphorus site P4 is characterized by an axially symmetric shielding tensor of moderate anisotropy Δσ = σ(33) - σ(iso) = 257 ppm. Electronic band structure calculations prove the metallic character and reveal the significant difference between the isolated P(3-) and the phosphorus atoms of the P(2)(2-) units. Magnetic susceptibility measurement reveals Pauli paramagnetism.

• Suppression of superconductivity by V-doping and possible magnetic order in Sr2VO3FeAs

  Authors: Marcus Tegel, Tanja Schmid, Tobias Stürzer, Masamitsu Egawa, Yixi Su, Anatoliy Senyshyn, Dirk Johrendt

  08/2010;

  Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries areSuperconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs. Comment: text revised, magnetic q-vector added, one reference added 4 pages, 4 figures

• Different response of the crystal structure to isoelectronic doping in BaFe2[As(1-x)P(x)]2 and [Ba(1-x)Sr(x)]Fe2As2

  Authors: Marianne Rotter, Christine Hieke, Dirk Johrendt

  05/2010;

  Superconductivity up to 30 K in charge neutrally doped BaFe2[As(1-x)P(x)]2 has been ascribed to chemical pressure, caused by the shrinking unit cell. But the latter induces no superconductivity inSuperconductivity up to 30 K in charge neutrally doped BaFe2[As(1-x)P(x)]2 has been ascribed to chemical pressure, caused by the shrinking unit cell. But the latter induces no superconductivity in [Ba(1-x)Sr(x)]Fe2As2 in spite of the same volume range. We show that the spin-density-wave (SDW) state of BaFe2As2 becomes suppressed in BaFe2[As(1-x)P(x)]2 by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates z(As), z(P). High-resolution X-ray diffraction experiments with BaFe2[As(1-x)P(x)]2 single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in [Ba(1-x)Sr(x)]Fe2As2. Since the Fe-As bond length is a gauge for the magnetic moment, our results show why the SDW is suppressed by P-, but not by Sr-doping. Only the Fe-P interaction increases the width of the iron 3d bands, which destabilizes the magnetic SDW ground state. The simultaneous contraction of the Fe{As bonds is rather a consequence of the vanishing magnetism. Ordered structure models of BaFe2[As(1-x)P(x)]2 obtained by DFT calculations agree perfectly with the single-crystal X-ray structure determinations. The contraction of the Fe-As bonds saturates at doping levels above x = 0.3, which corrects the unreasonable linear decrease of the so-called pnictide height. Comment: 7 Pages, 8 Figures, citations added, text revised

• The crystal structure of FeSe0.44Te0.56

  Authors: Marcus Tegel, Catrin Loehnert, Dirk Johrendt

  12/2009;

  The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type, P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022,The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type, P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2). Significantly different z-coordinates of tellurium and selenium at the 2c site are clearly discernible and were refined to z_Te=0.2868(3) and z_Se=0.2468(7). Thus the chalcogen heights differ by 0.24 A and the Fe-Se bonds are by 0.154 A shorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond angles occur. An elevated U33 displacement parameter of the iron atom is suggestive of a slightly puckered Fe layer resulting from different combinations of Se or Te neighbors. Such strong disorder underlines the robustness of superconductivity against structural randomness and has not yet been considered in theoretical studies of this system. Comment: 9 pages, 2 figures, 3 tables

• Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

  Authors: Marcus Tegel, Franziska Hummel, Yixi Su, Tapan Chatterji, Michela Brunelli, Dirk Johrendt

  11/2009;

  The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-rayThe iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined. Comment: 6 pages, 6 figures

• Complex Interrupted Tetrahedral Frameworks in the Nitridosilicates M(7)Si(6)N(15) (M=La, Ce, Pr).

  Authors: Christian Schmolke, Oliver Oeckler, Daniel Bichler, Dirk Johrendt, Wolfgang Schnick

  Chemistry (Weinheim an der Bergstrasse, Germany). 09/2009;

  A new structure type of nitridosilicates with an interrupted framework has been identified for M(7)Si(6)N(15) with M=La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnaceA new structure type of nitridosilicates with an interrupted framework has been identified for M(7)Si(6)N(15) with M=La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnace at temperatures between 1550-1625 degrees C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce(7)Si(6)N(15) has been determined by using single-crystal X-ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P1 , no. 2; a=13.009(3), b=25.483(5), c=25.508(10) A; alpha=117.35(3), beta=99.59(3), gamma=99.63(3) degrees ; V=7114(2) A(3); Z=18; R1=0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R3 , no. 148) have also been observed (a=43.420(6), c=6.506(2) A; V=10 623(3) A(3); Z=27; R1=0.0309). Pr(7)Si(6)N(15) (3) and La(7)Si(6)N(15) (4) are isostructural with 1 as evidenced by twinned single-crystal data for 3 (P1 , no. 2; a=12.966(3), b=25.449(10), c=25.459(10) A; alpha=117.28(3), beta=99.70(4), gamma=99.60(4) degrees ; V=7068(4) A(3); Z=18; R1=0.0526) and powder diffraction data for 4 (P1 , no. 2; a=13.109(9), b=25.606(18), c=25.609(18) A; V=7223(12) A(3); Z=18; R(P)=0.0194; R(F) ( (2) )=0.0936). The crystal structure of M(7)Si(6)N(15) (M=La, Ce, Pr) is built up exclusively of corner-sharing tetrahedrons that appear as Q(2)-, Q(3)-, and Q(4)-type tetrahedrons forming different ring sizes within a less condensed three-dimensional network. Among the characteristic structural motifs are saw-blade-shaped 12-rings and finite chains consisting of four corner-sharing SiN(4) tetrahedrons. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce(7)Si(6)N(15) show paramagnetic behavior with an experimental magnetic moment of 2.29 mu(B) per Ce, thereby corroborating the existence of Ce(3+).

• Mixed Valence Europium Nitridosilicate Eu(2)SiN(3).

  Authors: Martin Zeuner, Sandro Pagano, Philipp Matthes, Daniel Bichler, Dirk Johrendt, Thomas Harmening, Rainer Pöttgen, Wolfgang Schnick

  Journal of the American Chemical Society. 08/2009;

  The mixed valence europium nitridosilicate Eu(2)SiN(3) has been synthesized at 900 degrees C in welded tantalum ampules starting from europium and silicon diimide Si(NH)(2) in a lithium flux. TheThe mixed valence europium nitridosilicate Eu(2)SiN(3) has been synthesized at 900 degrees C in welded tantalum ampules starting from europium and silicon diimide Si(NH)(2) in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a = 542.3(11) pm, b = 1061.0(2) pm, c = 1162.9(2) pm, Z = 8, 767 independent reflections, 37 parameters, R1 = 0.017, wR2 = 0.032). Eu(2)SiN(3) is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN(4) tetrahedra running parallel [100] with a maximum stretching factor f(s) = 1.0. The compound is isostructural with Ca(2)PN(3) and Rb(2)TiO(3), and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu(2+) and Eu(3+), respectively. (151)Eu Mossbauer spectroscopy of Eu(2)SiN(3) differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu(2+) and Eu(3+) signals over the whole temperature range. The Eu(2+) site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 mu(B)/f.u. and a small contribution of Eu(3+), in accordance with Eu(2+) and Eu(3+) in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T(C) = 24 K. DFT calculations of Eu(2)SiN(3) reveal a band gap of approximately 0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu(2+) and Eu(3+).

• Low Temperature Crystal Structures and Magnetic Properties of the V4-Cluster Compounds Ga1-xGexV4S8

  Authors: Daniel Bichler, Herta Slavik, Dirk Johrendt

  06/2009;

  The solid solution Ga1-xGexV4S8 (x = 0 - 1) was synthesized by solid state reactions and characterized by temperature-dependent x-ray powder diffraction and static magnetic susceptibilityThe solid solution Ga1-xGexV4S8 (x = 0 - 1) was synthesized by solid state reactions and characterized by temperature-dependent x-ray powder diffraction and static magnetic susceptibility measurements. The compounds crystallize in the cubic GaMo4S8-type structure (space group F-43m), built up by heterocubane-like [V4S4](5-x)+ cubes and [Ga1-xGexS4](5-x)- tetrahedra arranged in a NaCl-like manner. The successive substitution of Ga3+ by Ge4+ increases the electron count in the molecular orbital (MO) of the V4-cluster gradually from seven to eight. We observe an almost linear increase of the magnetic moments, connected with a transition from ferromagnetic to antiferromagnetic ordering around x <= 0.5. Remarkably, the low temperature structural phase transitions as known from the ternary compounds were also detected in the solid solution. The gallium-rich compounds (0 < x < 0.5) undergo rhombohedral distortions like GaV4S8 (space group R3m), whereas distortions to orthorhombic symmetry (space group Imm2) as known from GeV4S8 occur in the germanium-rich part of the solid solution (0.5 <= x < 1). Comment: 14 pages, 9 figures

• Low Temperature Crystal Structure and 57Fe Moessbauer Spectroscopy of Sr3Sc2O5Fe2As2

  Authors: Marcus Tegel, Inga Schellenberg, Franziska Hummel, Rainer Poettgen, Dirk Johrendt

  05/2009;

  The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c :The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to delta(c)/c : delta(a)/a = 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Moessbauer spectroscopic data show a single signal at 4.2, 77, and 298 K, respectively, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mm/s at 298 K to 0.49(1) mm/s at 4.2 K. No indication for magnetic ordering was found. Comment: 12 pages, 4 figures

• The layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs

  Authors: Marcus Tegel, Franziska Hummel, Sebastian Lackner, Inga Schellenberg, Rainer Poettgen, Dirk Johrendt

  04/2009;

  Polycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-rayPolycrystalline samples of the layered iron arsenides Sr2CrO3FeAs and Ba2ScO3FeAs were synthesized by high temperature solid state reactions and their crystal structures determined by the X-ray powder diffraction. Their structures are tetragonal (P4/nmm; Sr2CrO3FeAs: a = 391.12(1) pm, c = 1579.05(3) pm; Ba2ScO3FeAs: a = 412.66(5) pm, c = 1680.0(2) pm, Z = 2) and isotypic to Sr2ScO3CuS. Iron arsenide layers are sandwiched between perowskite-like oxide blocks and separated by ~1600 pm, which is much larger compared to the 1111 iron arsenide superconductors. The bond length and angles within the FeAs layers are adapted to the space requirements of the oxide blocks. Measurements of the magnetic susceptibility and electrical resistivity show no hint for a SDW-like anomaly in both compounds. Sr2CrO3FeAs shows Curie-Weiss paramagnetism above 160 K with an effective magnetic moment of 3.83(3) muB in good agreement with the theoretical value of 3.87 muB for Cr3+. Antiferromagnetic ordering was detected below TN ~ 31 K. 57Fe Moessbauer spectra of Sr2CrO3FeAs show a single signal that broadens below the magnetic ordering temperature due to a small transferred hyperfine field induced by the magnetic ordering of the chromium atoms. 57Fe-Moessbauer spectra of Ba2ScO3FeAs show single signals at 298, 77, and 4.2 K which are only subject to weak quadrupole splitting. Comment: substantial addititions and corrections, references updated 19 pages, 7 figures

• Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2

  Authors: Marianne Rotter, Marcus Tegel, Inga Schellenberg, Falko M. Schappacher, Rainer Poettgen, Joachim Deisenhofer, Axel Guenther, Florian Schrettle, Alois Loidl, Dirk Johrendt

  01/2009;

  Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. ThePolycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad SDW phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Moessbauer-spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF magnetic ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single crystal (Ba1-xKx)Fe2As2.

• Nitridogermanate Nitrides Sr7[GeN4]N2 and Ca7[GeN4]N2: Synthesis Employing Sodium Melts, Crystal Structure, and Density-Functional Theory Calculations.

  Authors: Sebastian Junggeburth, Oliver Oeckler, Dirk Johrendt, Wolfgang Schnick

  Inorganic chemistry. 12/2008;

  The alkaline earth nitridogermanate nitrides AE 7[GeN 4]N 2 ( AE = Ca, Sr) have been synthesized using a Na flux technique in sealed Ta tubes. According to single-crystal X-ray diffraction theThe alkaline earth nitridogermanate nitrides AE 7[GeN 4]N 2 ( AE = Ca, Sr) have been synthesized using a Na flux technique in sealed Ta tubes. According to single-crystal X-ray diffraction the isotypic compounds crystallize in space group Pbcn (No. 60) with Z = 4, (Sr 7[GeN 4]N 2: a = 1152.6(2), b = 658.66(13), c = 1383.6(3) pm, V = 1050.5(4) x 10 (6) pm (3), R1 = 0.049; Ca 7[GeN 4]N 2: a = 1082.6(2), b = 619.40(12), c = 1312.1(3) pm, V = 879.8(3) x 10 (6) pm (3), R1 = 0.016). Owing to the high N/Ge ratio, the compounds contain discrete N (3-) ions coordinated by six AE (2+) besides discrete [GeN 4] (8-) tetrahedrons. One of the AE (2+) ion is coordinated by only four N (3-) ions, which is rather an unusual low coordination number for Sr (2+). Together with the isolated [GeN 4] (8-) tetrahedrons, these Sr (2+) ions form chains of alternating cation centered edge sharing tetrahedrons. The electronic structure and chemical bonding in Sr 7[GeN 4]N 2 has been analyzed employing linear muffin-tin orbital (LMTO) band structure calculations.

• Superconductivity at 38 K in the Iron Arsenide (Ba1-xKx)Fe2As2.

  Authors: Marianne Rotter, Marcus Tegel, Dirk Johrendt

  Physical review letters. 10/2008; 101(10):107006.

  The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity atThe ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at T_{c}=38 K in (Ba1-xKx)Fe2As2 with x approximately 0.4. The parent compound BaFe2As2 crystallizes in the tetragonal ThCr2Si2-type structure, which consists of (FeAs);{delta-} iron arsenide layers separated by Ba2+ ions. BaFe2As2 is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs);{-} layers, which suppress the anomaly and induce superconductivity. The T_{c} of 38 K in (Ba0.6K0.4)Fe2As2 is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure.