Discovery of superconductivity in Nb4SiSb2 with a V4SiSb2-type structure and implications of interstitial doping on its physical properties.

We report on the discovery, structural analysis, and the physical properties of Nb4SiSb2 - a hitherto unknown compound crystallizing in the V4SiSb2-type structure with the tetragonal space group I4/mcm and unit cell parameters a = 10.3638(2) Å and c = 4.9151(2) Å. We find Nb4SiSb2 to be a metal undergoing a transition to a superconducting state at a critical temperature of T c ≈ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of ΔC(T c)/γT c = 1.33 mJ mol-1 K-2. We find that for Nb4SiSb2, the unoccupied sites on the 4b Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at T c ≈ 1.2 K for Nb4Cu0.2SiSb2, and T c ≈ 0.8 K for Nb4Pd0.2SiSb2 as well as for Nb4Pt0.14SiSb2. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.

Crystal orientation-dependent etching and trapping in thermally-oxidised Cu2O photocathodes for water splitting.

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu2O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. -8.6 mA cm-2 and -7 mA cm-2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (VRHE), respectively, in the champion sample with an onset potential of 0.92 VRHE and a fill factor of 44%. An applied bias photon-to-current efficiency of 3.6% at 0.56 VRHE was obtained, which represents a new record for Cu2O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu2O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu2O/Ga2O3 interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu2O sample during the ALD process, while the reduction from Cu2O to Cu is not favourable.

Synthetic control over polymorph formation in the d-band semiconductor system FeS2.

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS2. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS2 polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H2S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 × 1018 cm-3. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS2 phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite - that is now synthetically available in a straightforward fashion - is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.

Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric η-Carbide Nb4Rh2C1-δ.

The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Cooper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as µ0 H Pauli ≈ 1.86[T/K]·T c for a weak-coupling Bardeen-Schrieffer-Cooper (BCS) superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy Fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbide-type compound Nb4Rh2C1-δ, with a normalized specific heat jump of ΔC/γT c = 1.64. We find that this material has a remarkably high upper critical field of µ0 H c2(0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of µ0 H Pauli = 18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi.

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100-xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100-xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm-1 K-1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.

Substitution of indium for chromium in TlIn5-x Cr x Se8: crystal structure of TlIn4.811(5)Cr0.189(5)Se8.

The new thallium penta-(indium/chromium) octa-selenide, TlIn4.811(5)Cr0.189(5)Se8, has been synthesized by solid-state reaction. It crystallizes isotypically with TlIn5Se8 in the space group C2/m. Although the two Tl positions are disordered and only partially occupied, no Tl deficiency was observed. The insertion of chromium in the structure has been confirmed by EDS analysis. Chromium substitutes indium exclusively at one of three In sites, viz. at one of the positions with site symmetry 2/m (Wyckoff position 2a). In the crystal structure, edge-sharing InSe6 octa-hedra, and (In,Cr)Se6 octa-hedra and InSe4 tetra-hedra make up two types of columns that are linked into a framework in which two different types of channels parallel to [010] are present. The Tl atoms are located in the larger of the channels, whereas the other, smaller channel remains unoccupied.

Magnetic and thermoelectric properties of the ternary pseudo-hollandite BaxCr5Se8 (0.5 < x < 0.55) solid solution.

The structure of Ba0.5Cr5Se8 has been recently resolved, and its thermoelectric and magnetic properties have been studied. A ZT of 0.12 was found at around 800 K. Here, we report a study on the pseudo-hollandite BaxCr5Se8 solid-solution with 0.5 ≤ x ≤ 0.55 and its thermoelectric and magnetic properties. There is no significant impact either on the cell parameters depending on the cation content or on the magnetic properties. However, thermoelectric properties are radically changed depending on x content. While the low thermal conductivity, around 0.8 W m(-1) K(-1), remains similar for all samples, a respective increase and decrease of the resistivity and the Seebeck coefficient are observed with increasing Ba content. The maximum Seebeck coefficient is found with Ba0.5Cr5Se8 at around 635 K with 315 µV K(-1), and the Seebeck coefficient then decreases and is correlated with an activation of minority charge carriers confirmed by Hall measurements. A similar but steeper behavior is observed for the Ba0.55Cr5Se8 temperature dependence plot at around 573 K. Finally, the best thermoelectric performances are found using the lowest content of Ba, unlike when x tends to 0.55, ZT approaches a tenth of the initial best value. BaxCr5Se8 compounds are antiferromagnetic with TN = 58 K. A large peak in thermal conductivity is observed around the antiferromagnetic transition for all stoichiometry.