Study of superconducting Ba–Ge–Co compounds (original) (raw)
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Cage-Forming Compounds in the Ba–Rh–Ge System: From Thermoelectrics to Superconductivity
Inorganic Chemistry, 2013
Phase relations and solidification behavior in the Ge-rich part of the phase diagram have been determined in two isothermal sections at 700 and 750°C and in a liquidus projection. A reaction scheme has been derived in the form of a Schulz−Scheil diagram. Phase equilibria are characterized by three ternary compounds: τ 1-BaRhGe 3 (BaNiSn 3-type) and two novel phases, τ 2-Ba 3 Rh 4 Ge 16 and τ 3-Ba 5 Rh 15 Ge 36-x , both forming in peritectic reactions. The crystal structures of τ 2 and τ 3 have been elucidated from singlecrystal X-ray intensity data and were found to crystallize in unique structure types: Ba 3 Rh 4 Ge 16 is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and R F = 0.0273), whereas atoms in Ba 5 Rh 15 Ge 36−x (x = 0.25) arrange in a large orthorhombic unit cell (Fddd, a = 0.84570(2) nm, b = 1.4725(2) nm, c = 6.644(3) nm, and R F = 0.034). The body-centeredcubic superstructure of binary Ba 8 Ge 43 □ 3 was observed to extend at 800°C to Ba 8 Rh 0.6 Ge 43 □ 2.4 , while the clathrate type I phase, κ I-Ba 8 Rh x Ge 46−x−y □ y , reveals a maximum solubility of x = 1.2 Rh atoms in the structure at a vacancy level of y = 2.0. The cubic lattice parameter increases with increasing Rh content. Clathrate I decomposes eutectoidally at 740°C: κ I ⇔ (Ge) + κ IX + τ 2. A very small solubility range is observed at 750°C for the clathrate IX, κ IX-Ba 6 Rh x Ge 25−x (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba 8 Rh x Ge 46−x (x = 0−6). The physical properties have been investigated for the phases κ I , τ 1 , τ 2 , and τ 3 , documenting a change from thermoelectric (κ I) to superconducting behavior (τ 2). The electrical resistivity of κ I-Ba 8 Rh 1.2 Ge 42.8 □ 2.0 increases almost linearly with the temperature from room temperature to 730 K, and the Seebeck coefficient is negative throughout the same temperature range. τ 1-BaRhGe 3 has a typical metallic electrical resistivity. A superconducting transition at T C = 6.5 K was observed for τ 2-Ba 3 Rh 4 Ge 16 , whereas τ 3-Ba 5 Rh 15 Ge 35.75 showed metallic-like behavior down to 4 K.
Crystal structure and superconductivity in (Cu,Hg)Ba2Ca4Cu5Oy
Physica C: Superconductivity, 1997
A new solid-solution superconductor, (Cu,Hg)Ba2Ca4Cu50 r, with the tetragonal lattice parameters a = 3.849(2) A and c = 21.496(3) ,~, has been synthesized by the high-temperature and high-pressure technique. The structure refinement of the as-prepared (Cu,Hg)-1245 by the single-crystal X-ray diffraction method confirmed the substitutional composition (Cuo.76Hgo.24)Ba2CaaCU5Olz45, whose cationic composition was close to that obtained by chemical analysis. The additional Cu atoms at the mercury site construct the chain-like oxygen coordination. The superstructure observed in Cu-1245 disappeared by the partial substitution of mercury for copper in the present (Cu,Hg)-1245. The structure refinement of the as-prepared Hg-1245 revealed the mercury-site deficiency of about 20%.
Superconductivity and electronic structure of Bi-based compounds
The superconductor W7Re13B has been studied by the magnetic measurements and microwave absorption. The crystal structure of W 7 Re 13 B is cubic (β-Mn type). This compound exhibits a sharp superconducting transition at a temperature of T c = 7.2 K. The electronic structure of W 7 Re 13 B has been studied by X-ray photoelectron spectroscopy and the band structure has been calculated by the full-potential local-orbital minimum-basis method using the scalar-relativistic mode. The main contribution to the density of states at the Fermi level is from 5d electrons of W and Re. The W and Re bands are similar and substituting W by Re does not change the total density of states.
Superconductivity in Novel Ge-Based Skutterudites: {Sr,Ba}Pt4Ge12
Physical Review Letters, 2007
Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below Tc = 5.35 K, and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.
Superconductivity of various borides and the role of carbon in their high performance
Superconductor Science and Technology, 2009
The superconductivity of MgB 2 , Mg 1−x Al x B 2 and NbB 2+x is compared. The stretched c-lattice parameter (c = 3.52Å) of MgB 2 in comparison to NbB 2.8 (c = 3.32Å) and AlB 2 (c = 3.25Å) decides empirically the population of their π and σ bands and, as a result, their T c values at 39 and 11 K, respectively, for the first two and no superconductivity for the latter. Besides stretching of the c-lattice parameter not only the density of the carriers but also their signs change in these isostructural di-borides. The thermoelectric power of these compounds clearly demonstrates their changing π and σ band contributions and the ensuing appearance/disappearance of superconductivity. An increased c parameter increases the boron plane constructed hole type σ band population and decreases the contribution from the Mg or Al plane electron type π band. This turns the hole type (mainly σ band conduction) MgB 2 superconductor (39 K) into the electron type (mainly π band conduction) non-superconducting AlB 2 . The importance of hole type σ band conduction dominating the superconductivity of the various borides is further established by the high performance of intrinsically pinned MgB 2−x C x . Our results on MgB 2 added with nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when compared with MgB 2−x C x , clearly demonstrate that the main role is played by C substitution at the B site in the host MgB 2 and the ensuing σ plane disorder and vortex pinning. The best strategy could be to add (<10 nm) nanoparticles to MgB 1.8 C 0.2 to ensure both extrinsic pinning by the former and intrinsic pinning by the latter.
Superconductivity above 130 K in the Hg-Pb-Ba-Ca-Cu-O system
Physical Review B, 1994
High volume fractions of the n =2 and n =3 members of a cuprate system of general formula Hg& "Pb"Ba&Ca"&Cu"O&"2+q (with an average value of x at 0.33) have been synthesized via a relatively facile process using low oxygen pressures. For as-synthesized n =3 (Hg, Pb-1:2:2:3)bulk superconducting critical temperatures (T,) of up to 133 K have been observed. For n =2 (Hg, Pb-1:2:1:2),which is obtained with some rare-earth-ion substitution at the calcium site, T, is 125 K after annealing in a reducing atmosphere. High-resolution lattice imaging coupled with structural simulations show that the Pb ions go into the Hg layer, leading to a chemical pressure-induced compositional modulation in this plane. Strong fiux pinning and an irreversibility field of 1 T at 80 K are observed for Hg, Pb-1:2:2:3.
Solubility and superconductivity of Gd1+xBa2−xCu3O7−δ compounds (x=−0.1,0,0.05,0.1,0.15 and 0.2)
Physica C: Superconductivity, 2005
The RE/Ba substitution has a significant effect on the growth, microstructure and flux pinning properties of meltprocessed Gd 1+x Ba 2Àx Cu 3 O 7Àd superconductors. We prepared a series of Gd 1+x Ba 2Àx Cu 3 O 7Àd , (Gd123 solid solutions) with x = À0.1, 0, 0.05, 0.1, 0.15 and 0.2. The single phase of Gd123 solid solutions was confirmed with both X-ray diffraction and DTA analysis for x = 0 to 0.1 as-grown samples, quenched in air. DTA/TG analysis in air showed that the thermal stability decreases with increasing x. After the post-annealing in flowing argon gas at 940°C for 10 h (ArPA), the Gd123 solid solutions with x = 0.1, 0.15 and 0.2 were decomposed into Gd 2 BaCuO 5 and Gd123 solid solutions with low substitution levels and minor liquid phases. The critical transition temperatures (T c Õs) of Gd123 solid solutions with the ArPA show dramatical increase compared to the samples without ArPA. The Gd123 solid solutions annealed in O 2 at 400°C show a narrower superconducting transition width than those treated at 350°C. These results contribute to improve the properties of the melt-processed bulk superconductors.
Journal of Advanced Ceramics, 2016
The paper describes the sol-gel process applied to synthesize a mercury-free Ba 2 Ca 2 Cu 3 O x precursor and the physicochemical properties of an Hg 0.8 Pb 0.2 Ba 2 Ca 2 Cu 3 O 8+ high-temperature superconductor (HTS) polycrystalline sample. The Ba 2 Ca 2 Cu 3 O x precursor with desired chemical and phase compositions was obtained using EDTA gel process followed by decomposition and calcination under optimized conditions. An Hg 0.8 Pb 0.2 Ba 2 Ca 2 Cu 3 O 8+ superconductor with a fine-grained microstructure, composed predominantly of the (Hg,Pb)-1223 phase in the form of plate-like crystallites or oval grains and with advantageous magnetic properties (T c(on) = 129.2 K, T c = 6.5 K), was synthesized using the high-pressure crystallization method. At 20 K and 1 T, the critical current density of the studied sample was approximately 26 A/mm 2 , while at 4 K and 1 T it increased to 155 A/mm 2. The high volume fraction of the (Hg,Pb)-1223 phase (89.1%) in the high-T c sample was associated with the low value of the average copper valence (2.11) in the calcinated Ba 2 Ca 2 Cu 3 O x precursor. Small amounts of non-superconducting secondary phases-BaCuO 2 , CaO, CaHgO 2 , CuO, Ca 2 CuO, BaPbO 3-were also identified within the microstructure of Hg 0.8 Pb 0.2 Ba 2 Ca 2 Cu 3 O 8+ .