Structural, Superconducting and Magnetic Properties of La(3-x)R(x)Ni2B2N3 (R = Ce, Pr, Nd)

We report on structural and superconducting properties of La(3-x)R(x)Ni2B2N3 where La is substituted by the magnetic rare-earth elements Ce, Pr, Nd. The compounds Pr3Ni2B2N3 and Nd3Ni2B2N3 are characterized for the first time. Powder X-ray diffraction confirmed all samples R3Ni2B2N3 with R = La, Ce, Pr, Nd and their solid solutions to crystallize in the body centered tetragonal La3Ni2B2N3 structure type. Superconducting and magnetic properties of La(3-x)R(x)Ni2B2N3 were studied by resistivity, specific heat and susceptibility measurements. While La3Ni2B2N3 has a superconducting transition temperature Tc ~ 14 K, substitution of La by Ce, Pr, and Nd leads to magnetic pair breaking and, thus, to a gradual suppression of superconductivity. Pr3Ni2B2N3 exibits no long range magnetic order down to 2 K, Nd3Ni2B2N3 shows ferrimagnetic ordering below T_C = 17 K and a spin reorientation transition to a nearly antiferromagnetic state at 10 K.Comment: 5 pages, 4 figures, presented at 17. International Conference on Solid Compounds of Transition Elements, Annecy, France; 05.09.2010 - 10.09.201

Filled skutterudite superconductor CaOs4P12 prepared by high-pressure synthesis

In this paper, we report the transport, thermodynamic, and superconducting properties of a new filled skutterudite CaOs4P12 synthesized under high pressure and high temperature. The electrical resistivity of 3.4–4.8 mΩcm, carrier concentration of 3.8–6.1 × 1020cm−3 , and positive Hall coefficient suggest that CaOs4P12 is a semimetal with hole carriers. An anomaly due to low-energy optical modes corresponding to an Einstein temperature of 150 K was observed in the specific heat. Resistivity, dc magnetic susceptibility, and specific heat measurements indicate bulk superconductivity below 2.5 K. The specific heat anomaly at Tc, ∆C/γTc ≈ 1.4, is in agreement with the BardeenCooper-Schrieffer (BCS) value of 1.43. The electron-phonon coupling constant λep is estimated to be 0.47. CaOs4P12 is classified as a BCS-type, weakly coupled type-II superconductor with an upper critical field of Hc2 ≈ 22 kOe and Ginzburg-Landau coherence length of ξ ≈ 12 nm

On the constitution and thermodynamic modeling of the phase diagrams Nb-Mn and Ta-Mn

The constitution of the two phase diagrams Nb-Mn and Ta-Mn has been determined from light optical and transmission and scanning electron microscopy (LOM, TEM and SEM) with energy dispersive (EDX) as well as wavelength dispersive (WDX) X-ray spectroscopy, X-ray powder (XPD) and single crystal diffraction (XSCD), differential thermal analysis (DTA) and/or differential scanning calorimetry (DSC). The Laves phases NbMn2 and TaMn2 are the only binary compounds in these systems. High-temperature differential thermal analyses revealed congruent melting for NbMn2 with T,(NbMn2) = 1515 +/- 15 degrees C, whereas TaMn2 melts incongruently with T-m(TaMn2)= 1797 +/- 40 degrees C close to a depleted peritectic reaction. Both Laves phases engage in eutectic reactions l (Mn) + Nb(Ta)Mn-2 (T-eut = 1220 +/- 10 degrees C at 4.9 at% Nb and T-eut = 1234 +/- 10 degrees C at 0.7 at% Ta, respectively). NbMn2 also forms a eutectic with (Nb): l (Nb) + NbMn2 at T-eut = 1493 +/- 15 degrees C and 53.2 at% Nb. Mn shows remarkably large maximum solid solubilities of 19.4 at% Mn in (Nb) as well as of 21.3 at% Mn in (Ta). Detailed atom site distribution has been established for the Laves phases by means of temperature dependent X-ray single crystal data (both C14 - MgZn2-type). Combined data from XPD, EDX/WDX and SEM microstructure indicate that for both Laves phases extended homogeneity regions exist: Nb1+xMn2+x (62.5-73.0 at% Mn at 950 degrees C: -0.19 <= x <= 1.125) and Ta1+xMn2-x (59.5-68.5 at % Mn: -0.055 <= x <= 1.215). Density functional theory (DFT) calculations favor Nb(Ta)/Mn antisite occupation rather than defects. The phases, "NbMn" and "TaMn", adopted earlier in the literature as binary system inherent compounds, were shown (TEM, WDX electron microprobe data and X-ray Rietveld refinements) to be oxygen stabilized phases of the Ti4Ni2O type (so-called eta(eta)-phases) with modified Nb(Ta)/Mn site substitution to comply with the formula Nb(Ta)(3-x)Mn3+xO1-y (defect eta-W3Fe3C-type). From magnetic susceptibility and magnetization measurements, both oxide stabilized eta phases eta-Nb3Mn3O1-y and eta-Ta3Mn3O1-y were found to order ferromagnetically below T-c similar to 77 K, but the Laves phases NbMn2, TaMn2 reveal weakly temperature dependent paramagnetism. No trace of the rhombohedral kyphase (W6Fe7-type) has been encountered in our investigation of the two binary phase diagrams. Thermodynamic and transport properties (specific heat, electrical resistivity and magnetic susceptibility/magnetization) classify the Laves phases with metallic behavior whilst mechanical properties (elastic moduli from DFT and nanoindentation as well as hardness and thermal expansion) group both Laves phases among rather hard and brittle intermetallics. Based on (i) the experimentally derived constitution of the Nb-Mn and Ta-Mn systems, and (ii) on new own DFT data of the energy of formation of the Laves phases, a CALPHAD (CALculation of PHAse Diagrams) calculation of both systems was made providing a complete set of optimized thermodynamic data. Furthermore, the DFT calculations provided information on the instability of the eta-Ta3Mn3 structure and the atom-site specific stabilization effect of oxygen.Web of Science865art. no. 15871

Report on my research at the SSPC Laboratory, Division of Chemistry, Graduate School of Science, Kyoto University

This report reviews my research conducted as Visiting Fellow at the Solid State Physics and Chemistry (SSPC) Laboratory in the Division of Chemistry financed by the International Research Unit of Advanced Future Studies at Kyoto University. The subject of my research are electronic ground state instabilities and quantum critical phenomena in strongly correlated electron systems, especially those among inter-metallic rare earth compounds. Thereby, two classes of materials are in the focus of my present work. Studies of materials near an itinerant magnetic instability: nearly and weakly ferromagnetic compounds and the characterization of their spinfluctuation features and magneto-volume effects. The second focus is on cerium and ytterbium based metals with electronic ground state instabilities being related to the mechanisms of Kondo- and/or magnetic frustration effects. Future collaboration plans of the SSPC Laboratory with me and my collaborators at TU Wien are finally outlined, also targeting interdisciplinary research with additional partners

Neutron diffraction study of superconducting La3Ni2B2N3−δ

The final publication is available via https://doi.org/10.1016/j.jallcom.2017.05.017.We have studied structural properties of La3Ni2B2N3−δ samples with distinctly different values of the superconducting transition temperature by means of powder neutron diffractometry and specific heat measurements. The refinement of lattice site occupations reveals full occupations for all sites in the La3Ni2B2N3 structure with space group I4/mmm except for nitrogen site N(2). For samples with Tc = 13.0 K and 13.7 K we obtain for the N(2) site distinctly different occupation factors of 0.90 and 0.93, respectively. The latter confirms a direct relation between the nitrogen stoichiometry and the superconducting transition temperature. Based on the analysis of temperature dependent lattice parameters, atomic displacement factors, lattice heat capacity data and ab initio phonon density of states calculations we discuss the thermal expansion and vibrational properties of superconducting La3Ni2B2N3−δ

Incorporation of platinum atoms in a silicon-free boride of the YB50-type structure

The final publication is available via https://doi.org/10.1016/j.jallcom.2016.03.015.A new Pt-doped yttrium boride of the YB50 family, YB45-xPty, x=2.12, y=0.21 (space group Pbam, a=16.6246(4) Å, b=17.6453(4) Å, c=9.4167(2) Å), has been synthesized by arc-melting pure elements and subsequent annealing at 1123 K. A single crystal has been studied in order to assess the Pt-doping effect on the crystal structure. Insertion of Pt in two 4h interstitial sites of the boron atom framework leads to the transformation of -[B12]-[B12]- icosahedral chain into -[B11]-Pt-[B11]- for 38.6% of them as well as, to a lesser extent, induces the disorder into the [B15] polyhedron and neighboring interstitial B site (97.3% vs 2.7%).Austrian Science Funds (FWF

Pd6CuB3 - a new structure type of borides. Th7Fe3-type derivative structures in Pd(Pt)-Cu-B systems

The final publication is available via https://doi.org/10.1002/chem.201602767.A missing member of the series of Th7Fe3-type derivative structures, h-(Pd0.864Cu0.136)7B3 (unique structure type Pd6CuB3, space group P63cm, a=12.9426(9) Å, c=4.8697(4) Å) was obtained from as cast alloys and alloys annealed at 600 °C - 650 °C. Further substitution of Cu by Pd led to formation of a Mn7C3-type structure, o-(Pd0.93Cu0.07)7B3 (space group Pnma, a=4.8971(2) Å, b=7.5353(3) Å, c=12.9743(6) Å). Isotypic h-(Pt0.70Cu0.30)7B3 was observed in the Pt-Cu-B system as a low temperature phase (T≤600 °C), whereas the B-filled Th7Fe3-type (HT h-(Pt0.717Cu0.283)7B3+x, space group P63mc, a=7.4424(12) Å, c=4.8549(8) Å) proved to be stable at high temperature. The three structures are built of columns of face connected metal octahedra and columns of metal tetrahedra alternatingly fused by common faces and vertices. Boron atoms are found in trigonal prisms formed by metal atoms; additionally octahedral boron coordination was encountered in HT h-(Pt0.717Cu0.283)7B3+x. A superconductivity was discovered for Pt4.9Cu2.1B3 (Pd6CuB3-type) and Pt5.04Cu1.96B3.3 (B-filled Th7Fe3-type) below 0.67 and 0.66 K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0 K differ as 0.37 T and 0.27 T for the two compounds.Austrian Science Funds (FWF

Elucidating the lack of magnetic order in the heavy fermion CeCu2Mg

The final publication is available via https://doi.org/10.1103/PhysRevB.95.115146.Magnetic, transport, and thermal properties of CeCu2Mg are investigated to elucidate the lack of magnetic order in this heavy-fermion compound with a specific heat value, Cmag/T|T0 ≈ 1.2 J/mol K² and robust effective magnetic moments (μeff ≈ 2.46 μB). The lack of magnetic order is attributed to magnetic frustration favored by the hexagonal configuration of the Ce sublattice. In fact, the effect of magnetic field on Cmag/T and residual resistivity ρ0 does not correspond to that of a Fermi liquid (FL) because a broad anomaly appears at Tmax ≈ 1.2 K in Cmag(T)/T, without changing its position up to μoH=7.5 T. However, the flattening of Cmag/T|T0 and its magnetic susceptibility χ(T0), together with the T² dependence of ρ(T), reveal a FL behavior for T ≤ 2 K which is also supported by Wilson and Kadowaki-Woods ratios. The unusual coexistence of FL and frustration phenomena can be understood by placing paramagnetic CeCu2Mg in an intermediate section of a frustration-Kondo model. The entropy, Smag, reaches 0.87 Rln6 at T ≃ 100 K, with a tendency to approach the expected value Smag = Rln6 of the J=5/2 ground state of Ce3+