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Abstract Mixed anion oxides are emerging materials showing a variety of physical and chemical properties. Among them oxynitrides are widely investigated because of notable photocatalytic, dielectric, luminescent and electronic properties. Nitrides show more positive free energies of formation than oxides because of the higher stability of N 2 molecule with respect to O 2 and the unfavourable electron affinity of nitrogen compared to oxygen. However the stability of oxynitrides is higher than for nitrides, and they easily form from oxides in presence of reactive gases as NH 3. In addition to ammonolysis several synthetic strategies have been developed in the last years leading to the stabilization of relevant materials with a variety of structures. In this review we will discuss recent progress in the synthesis of oxynitrides emphasizing the importance of kinetic factors and the influence of preparative parameters on the structure types and oxidation states of the cations, and the consequences on physical properties. [ABSTRACT FROM AUTHOR]

The thermodynamic stability of a series of nitrogen - doped reduced graphene oxides prepared by ammonolysis of graphene oxide has been investigated by high temperature oxidation calorimetry. In terms of enthalpy and depending on the concentration of nitrogen, the nitrogen - doped reduced graphene oxides can be up to 73 kJ·mol−1 more stable than graphite plus nitrogen. There is a linear relationship between the nitrogen content and the formation enthalpy, which indicates a decrease in stability with increasing nitrogen content. [ABSTRACT FROM AUTHOR]

Physical and chemical properties of graphene can be tailored by nitrogen doping. As a consequence of the continuous race to achieve the highest possible amount of doping there is a growing tendency to assume that all N species are incorporated within the graphene lattice (doping). Here we show that this is not always the case and employ several complementary techniques that allow a proper assessment of the type of nitrogen present. The nature of the nitrogen atoms has been tuned by ammonolysis of graphene oxide in the range of 100 °C–800 °C. This allows us to expand the capabilities of the synthetic approach to afford not only N-doping (at high temperature) but also to introduce amine and amide moieties at 100 °C; the latter presenting a much higher dispersability in aqueous media than graphene oxide. Interestingly, the sample with the highest amount of nitrogen (14.7 wt. %) cannot be referred to as N-doped graphene since it also contains N-bearing aliphatic moieties. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism of the chemical and electrochemical alkali metal intercalation reactions in β-HfNCl has been investigated through electrochemical potential spectroscopy (EPS), in-situ powder X-ray diffraction during electrochemical intercalation and room temperature chemical intercalation experiments. EPS experiments in lithium cells reveal the presence of a plateau, at 1.8V vs. Li+/Li0 accounting for ca. 0.14mol Li, that indicates the formation of a new intermediate phase, and then a gradual decrease of potential with composition that extends up to very high lithium contents (ca. 1.1 per formula), consistent with the formation of a solid solution. Sodium electrochemical intercalation experiments showed a relatively similar behaviour with a plateau at 1.4V vs. Na+/Na0, corresponding to ca. 1.7V vs. Li+/Li0. In-situ monitored powder X-ray diffraction electrochemical intercalation experiments showed that the electrolyte solvent (ethylene carbonate/dimethyl carbonate, EC/DMC or propylene carbonate, PC) co-intercalated with the alkaline atom. This leads to a large expansion of the interlayer spacing that reaches a value of 21.06Å in the lithium co-intercalated phase with EC/DMC, Li x (EC/DMC) y HfNCl, and 22.01Å in the sodium co-intercalated phase with PC, Na x (PC) y HfNCl. Chemical intercalation using naphthyl-sodium solutions in tetrahydrofuran (THF) leads to solvent-free, multiple-phase samples showing in different proportions the pristine and the superconducting stage 2 and stage 1 phases. The composition of the intercalated samples depends on the pristine sample, the concentration of the naphthyl-sodium solution, the ratio Na:HfNCl and the reaction time. Pristine samples exhibiting low lithium intercalation degree upon electrochemical reduction gave the second stage as the major phase when treated with short reaction times or using low Na:HfNCl ratios, coexisting either with the host or with the first stage phase, whereas stage 1 is obtained as the major phase from pristine samples showing high electrochemical capacities. The staging behaviour and the multiphase nature of these samples account for the wide superconducting transitions and the different critical temperatures observed in these superconductors. [Copyright &y& Elsevier]

α-HfNBr has been prepared at 760 °C in a sealed evacuated fused silica tube by reaction between NH4Br and Hf followed by purification through chemical vapour transport under a temperature gradient. The crystal structure of this compound at room temperature has been determined for the first time by Rietveld refinement of X-ray powder diffraction data, electron diffraction and high resolution electron microscopy. It crystallises in the orthorhombic space group Pmmn with the unit cell parameters a=4.1165(2), b=3.5609(2), c=8.6440(3) A˚. α-HfNBr is isotypic to FeOCl and is built from layers of composition Br&z.sbnd;Hf&z.sbnd;N&z.sbnd;N&z.sbnd;Hf&z.sbnd;Br stacked along c that are separated by a Van der Waals gap. The hafnium atoms are six-coordinated by four nitrogen atoms and two bromine atoms. The resulting coordination polyhedron may be described as a highly distorted octahedron. Electron diffraction and high resolution electron microscopy studies reveal that the material is almost free of defects in the inner part of the crystals and confirm the structural model refined by the Rietveld method. [ABSTRACT FROM AUTHOR]