Zinc oxide is considered as a very promising material for optoelectronics. However, to date, the difficulty in producing stable p-type ZnO is a bottleneck, which hinders the advent of ZnO-based devices. In that context, nitrogen-doped zinc oxide receives much attention. However, numerous reviews report the controversial character of p-type conductivity in N-doped ZnO, and recent theoretical contributions explain that N-doping alone cannot lead to p-typeness in Zn-rich ZnO. We report here that the ammonolysis at low temperature of ZnO(2) yields pure wurtzite-type N-doped ZnO nanoparticles with an extraordinarily large amount of Zn vacancies (up to 20%). Electrochemical and transient spectroscopy studies demonstrate that these Zn-poor nanoparticles exhibit a p-type conductivity that is stable over more than 2 years under ambient conditions.

International audienceTiOxNy and VOxNy powders have been synthesized using oxide precursors and a conventional nitridation method. It enables to control of oxygen content and surface area. The electrochemical performances of the different powders have been investigated. A strong dependence on the surface area as well as on the nature of the oxynitride has been found. A typical value of 300 μF cm−2 has been determined for VOxNy powders, while TiOxNy powders only show 50 μF cm−2. In this last case it is believed that only double layer capacitance or weak redox reactions participate in charge storage mechanism while for vanadium based oxynitrides, a thin layer below the surface (≈4 Å) is involved in charge storage via faradic reactions. VOxNy electrodes can be operated in different aqueous electrolytes, but only double layer capacitance is measured in neutral electrolytes. The highest capacitance values (≈80 F g−1) are measured in KOH and fair cycling ability is achieved when the electrochemical window is limited, thus avoiding oxidative potentials

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Reaction between rare-earth tantalates RTaO4 and ammonia flow at 900−950 °C forms oxynitrides belonging to different structure types, perovskites RTaON2, pyrochlores R2Ta2O5N2, and defect fluorites RTa(O,N,□)4, depending on the size of the R element. The nature of the oxide precursor is a crucial parameter affecting the ammonolysis reaction. A comparative study has been carried out between oxide powders prepared by a ceramic route and a chimie douce process. The pyrochlore structure is restricted to large rare-earths (R = Nd→Gd). For smaller R elements, thermal ammonolysis of reactive precursors elaborated from the citrate combustion route results in a fluorite-type oxynitride solid solution. X-ray and neutron diffraction studies evidence a totally disordered cubic fluorite unit cell, which, however, appears to be not suitable for the structure refinement, thus revealing a more complex atom arrangement.

A comprehensive study on the thermoelectric effect of Ag substitution in GeTe solid solutions, a congenital base for high efficient TAGS-m [(GeTe) m (AgSbTe 2 ) 100-m ] thermoelectric materials, was performed. First-principles calculations were carried out to probe the changes arising from doping on the electronic band structure of GeTe, which exhibits a rhombohedral (r) structure at temperatures lower than 700 K. Aliovalent Ag substitution in GeTe increases the hole concentration and decreases the thermoelectric figure of merit (zT) due to the reduction of the Seebeck coefficient, which is ascribed mainly to the lowering of the Fermi level together with the loss of band degeneracy. Band structure and effective mass calculations of these doped materials also point to a soaring contribution from several hole pockets in the valence band. First-principles calculations carried out with two other group-11 transition metals (Cu, Au) reveal that silver substitution has the lowest impact on the thermopower of r- Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 GeTe. A mean zT of ∼0.85 at 773 K is achieved for Ge 1-x Ag x Te (x ≤ 0.04) solid solutions. The study highlights the limits in doping just a coinage metal to GeTe and recapitulates the need for pair substitution to enhance the thermoelectric properties of GeTe-based solid-state solutions.

International audienceA series of N 2 -containing perovskite phases was prepared in the La-(Ba)-Ti-O system in order to study the dinitrogen retention phenomenon from a thermochemical viewpoint. High-temperature oxide melt solution calorimetry was undertaken to determine the energetics of the corresponding startingoxynitrides, intermediate phases, and oxides. Calorimetric results show that nitrogen is weakly bound within the oxide matrix and most of the enthalpy of oxidation of the intermediate phase is devoted to its structure change between the starting perovskite structure and the formation of a layered-perovskiteLa2Ti2O7 oxide

Converting photon energy to chemical energy using inorganic materials requires successful capturing of photons, excitons dissociation and charge carriers diffusions. The present study reports thorough analysis of optoelectronic properties of visible--light--responsive SrTaO 2 N perovskites to quantify its absorption coefficient, the generated charge carriers' effective masses, dielectric constants, and electronic structures. The measurements on such intrinsic properties were attempted using both epitaxial and polycrystalline SrTaO 2 N films deposited by radio frequency (RF) magnetron sputtering under N 2 reactive plasma. Density functional theory calculations using the HSE06 functional provided reliable values of these optoelectronic properties. Such quantities obtained both by the measurements and the calculations gave excellent correspondences, but also provide possible variations that account for the small discrepancies observed. One of the significant factors determining the optical properties was found to be the anion ordering in the perovskite structure imposed by the cations. As a result, the different anion ordering has noticeable influence in the optical properties and the large sensitivity of hole effective mass. Determination of relative band positions to the water redox properties was also attempted by Mott--Schottky plots. All these results offer the opportunity to understand why SrTaO 2 N material possesses intrinsically all the ingredients to lead to a highly efficient water splitting device.

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (latt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed tube melt processing, including: (i) slow cooling of the melt; (ii) quenching and annealing; (iii) consolidation by spark plasma sintering (SPS); and also by the state-of-the-art (iv) Flash-SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (tot) conductivities of CuPb18SbTe20 samples were akin to that of LAST alloys. Both the flash-SPSed and the slow cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultra-low latt  0.58 W/mK at 723 K, comparable with that its Ag counterpart, regardless of differences in their size of the precipitates, type of precipitate-matrix interfaces and other nanoscopic architectures.The sample processed by flash sintering manifested higher figure of merit (zT  0.9 at 723 K), due to better optimization and trade-off between the transport properties by decreasing the carrier concentration and latt without degrading the carrier mobility. In spite of their comparable σ and tot, the zT of the Cu samples were low compared to the Ag samples due to their contrasting thermopower values. First-principles calculations attribute this variation in Seebeck to the dwindling of the energy gap (from 0.1 eV to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag), when Cu replaces Ag. Materials and Methods ReagentsPb (Strem Chemicals, 99.999%), Sb (Alfa Aesar, 99.999%), Cu (Alfa Aesar, 99.999%) and Te (JGI, 99.999%) were used for synthesis without any further purification. SynthesisIn this work, several different processing routes were investigated, however, the first step (synthesis) was common to all of the processing routes. Samples of CuPb18SbTe20 were synthesized using the vacuumsealed tube melt processing. Stoichiometric amounts of the starting elements of Cu, Pb, Sb and Te were introduced into a fused silica tube. The tube was prepared by cleaning with hydrofluoric (HF) acid and distilled water, then dried under vacuum. The ampoules were sealed under a vacuum of 10 -6 Torr, then placed in a rocking furnace and slowly heated to 1223 K over a period of 12 hours, then held at that temperature for 15 hours. Four different batches of samples were prepared, the first two were produced directly from the molten material, followed by: (i) cooling the melt to room temperature over a period of 18 hours (samples denoted as 'SS'); (ii) rapidly quenching the tube in water, followed by annealing at 973 K for 8 hours (denoted as 'MQ'). The other two batches used the material produced by method (i), which was crushed and milled. The powders were then ...