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Thermal analysis is widely employed for the characterization of nanomaterials. It encompasses a variety of techniques that allow the evaluation of the physicochemical properties of a material by monitoring its response under controlled temperature. In the case of carbon nanomaterials, such as carbon nanotubes and graphene derivatives, thermogravimetric analysis (TGA) is particularly useful to determine the quality and stability of the sample, the presence of impurities and the degree of functionalization or doping after post-synthesis treatments. Furthermore, TGA is widely used to evaluate the thermal stability against oxidation by air, which can be, for instance, enhanced by the purification of the material and by nitrogen doping, finding application in areas where a retarded combustion of the material is required. Herein, we have evaluated key parameters that play a role in the data obtained from TGA, namely, gas flow rate, sample weight and temperature rate, used during the analysis. We found out that the heating rate played the major role in the process of combustion in the presence of air, inducing an increase in the temperature at which the oxidation of CNTs starts to occur, up to ca. 100 °C (from 1 °C min -1 to 50 °C min -1 ). The same trend was observed for all the evaluated systems, namely N-doped CNTs, graphene produced by mechanical exfoliation and N-doped reduced graphene samples. Other aspects, like the presence of impurities or structural defects in the evaluated samples, were analyzed by TGA, highlighting the versatility and usefulness of the technique to provide information of structural aspects and properties of carbon materials. Finally, a set of TGA parameters are recommended for the analysis of carbon nanomaterials to obtain reliable data.

The new compounds R 2 TaO 4- x N x with R = La, Ce, Nd, and Eu and 1.20 ≤ x ≤ 2.81 have been obtained by a solid-state reaction between metal nitrides and oxides or oxynitrides under N 2 gas at temperatures between 1200 and 1700 °C. They are the first examples of rare earth transition metal oxynitrides with an n = 1 Ruddlesden-Popper structure and show different anion stoichiometries, crystal structures, and magnetic properties. Synchrotron X-ray powder diffraction and electron diffraction indicate that the lanthanum, cerium, and neodymium compounds crystallize in the orthorhombic space group Pccn , with cell parameters a = 5.72949(2), b = 5.73055(5), and c = 12.77917(6) Å for La 2 TaO 1.31 N 2.69 , a = 5.70500(5), b = 5.71182(4), and c = 12.61280(7) Å for Ce 2 TaO 1.19 N 2.81 , and a = 5.70466(3), b = 5.70476(5), and c = 12.32365(5) Å for Nd 2 TaO 1.46 N 2.54 . In contrast, Eu 2 TaO 2.80 N 1.20 shows a tetragonal I 4 1 / acd superstructure doubling the c axis, with parameters a = 5.71867(2) and c = 25.00092(19) Å. Refinement of neutron powder diffraction data of Ce 2 TaO 1.19 N 2.81 indicated the nitrogen order in the two equatorial positions of the tantalum octahedron, with refined N/O occupancies of 0.930(7)/0.070 and 0.876(13)/0.124, and the axial position is occupied by 50% of each anion. This anion ordering agrees with the distribution predicted by Pauling's second crystal rule. Magnetization measurements show that the cerium and europium compounds are ordered magnetically at low temperatures, while the neodymium compound remains paramagnetic down to 2 K, as a consequence of suppression of the effective magnetic moment of the latter when reducing the temperature., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N 2 or N 2 /H 2 gas. The nitrogen stoichiometry was tuned from 0.63 to 1.78 atoms per Eu or Ta atom, starting with appropriate N/O ratios in the mixture of the reactants Eu 2 O 3 , EuN and Ta 3 N 5 , or Eu 2 O 3 and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO 2.37 N 0.63 and Eu 3 Ta 3 O 3.66 N 5.34 , showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO 2.37 N 0.63 is a simple perovskite with cubic Pm 3̅ m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu 3 Ta 3 O 3.66 N 5.34 is the first example of a triple perovskite oxynitride and shows space group P 4/ mmm with crystal parameters a = 3.99610(2), c = 11.96238(9) Å. The tripling of the c-axis in this phase is a consequence of the partial ordering of europium atoms with different charges in two A sites of the perovskite structure with relative ratio 2:1, where the formal oxidation states +3 and +2 are respectively dominant. Magnetic data provide evidence of ferromagnetic ordering developing at low temperatures in both oxynitrides, with saturation magnetization of about 6 μ B and 3 μ B per Eu ion for EuTaO 2.37 N 0.63 and the triple perovskite Eu 3 Ta 3 O 3.66 N 5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu 2+ in the latter compound.

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON 2 that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N 2 at 1500 °C between La 2 O 3 , LaN, and Ta 3 N 5 or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO 1+ x N 2- x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I 2/ m with lattice parameters for LaTaON 2 of a = 5.71458(7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I 2/ m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON 2 ε r ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula.

Ferroelectric oxides have attracted much attention due to their wide range of applications, particularly in electronic devices such as nonvolatile memories and tunnel junctions. As a result, the monolithic integration of these materials into silicon technology and their nanostructuration to develop alternative cost-effective processes are among the central points in the current technology. In this work, we used a chemical route to obtain nanowire thin films of a novel Sr1+δMn8O16 (SMO) hollandite-type manganese oxide on silicon. Scanning transmission electron microscopy combined with crystallographic computing reveals a crystal structure comprising hollandite and pyrolusite units sharing the edges of their MnO6 octahedra, resulting in three types of tunnels arranged along the c axis, where the ordering of the Sr atoms produces natural symmetry breaking. The novel structure gives rise to ferroelectricity and piezoelectricity, as revealed by local direct piezoelectric force microscopy measurements, which confirmed the ferroelectric nature of the SMO nanowire thin films at room temperature and showed a piezoelectric coefficient d33 value of 22 ± 6 pC N-1. Moreover, we proved that flexible vertical SMO nanowires can be harvested providing an electrical output energy through the piezoelectric effect, showing excellent deformability and high interface recombination. This work indicates the possibility of engineering the integration of 1D manganese oxides on silicon, a step which precedes the production of microelectronic devices.

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON 2 , which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P6 3 mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX 3 layers. A synergetic second-order Jahn-Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W 6+ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity., (© 2020 Wiley-VCH GmbH.)

Magnetic particle hyperthermia, in which colloidal nanostructures are exposed to an alternating magnetic field, is a promising approach to cancer therapy. Unfortunately, the clinical efficacy of hyperthermia has not yet been optimized. Consequently, routes to improve magnetic particle hyperthermia, such as designing hybrid structures comprised of different phase materials, are actively pursued. Here, we demonstrate enhanced hyperthermia efficiency in relatively large spherical Fe/Fe-oxide core-shell nanoparticles through the manipulation of interactions between the core and shell phases. Experimental results on representative samples with diameters in the range 30-80 nm indicate a direct correlation of hysteresis losses to the observed heating with a maximum efficiency of around 0.9 kW/g. The absolute particle size, the core-shell ratio, and the interposition of a thin wüstite interlayer are shown to have powerful effects on the specific absorption rate. By comparing our measurements to micromagnetic calculations, we have unveiled the occurrence of topologically nontrivial magnetization reversal modes under which interparticle interactions become negligible, aggregates formation is minimized and the energy that is converted into heat is increased. This information has been overlooked until date and is in stark contrast to the existing knowledge on homogeneous particles., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Reduced graphene oxide (RGO) decorated with superparamagnetic iron oxide nanoparticles (SPION) is a novel composite nanomaterial with a myriad of promising applications. However, processes such as the fast and simple synthesis of non-agglomerated monodispersed SPION on RGO and the accurate characterization of particle size distributions remain challenging. Here we present how to solve these two problems. Firstly, we introduce a new microwave-assisted synthesis of stabilized SPION on RGO which is fast, simple and up-scalable but at the same time renders well dispersed SPION with narrow size distributions. The coverage of the RGO flakes with SPION is extensively tuned and the results are compared with a non-stabilized microwave-assisted method. Secondly, we implement an accurate method for the determination of particle size distributions from magnetization curves in RGO-SPION composite nanomaterials. This method is applied to the prepared composites with different particle size distributions, degrees of particle agglomeration and coverage of the RGO flakes. The influence of sample characteristics in the size determination method is discussed and the results are compared with the values obtained from transmission electron microscopy (TEM) and X-ray diffraction (XRD), showing that the method is well suited for these and potentially other types of superparamagnetic composite nanomaterials., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of piezoelectric devices as prospective sensors with applications based on the operation in the high-frequency range. However, to date, it has not been possible to make existing quartz manufacturing methods compatible with integration on silicon and structuration by top-down lithographic techniques. Here, we report an unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i) laser interference lithography, (ii) soft nanoimprint lithography on Sr-doped SiO 2 sol-gel thin films, and (iii) self-assembled SrCO 3 nanoparticle reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (from 1 μm down to 50 nm) and heights (up to 2 μm) were obtained. This work demonstrates the complementarity of soft-chemistry and top-down lithographic techniques for the patterning of epitaxial quartz thin films on silicon while preserving its epitaxial crystallinity and piezoelectric properties. These results open up the opportunity to develop a cost-effective on-chip integration of nanostructured piezoelectric α-quartz MEMS with enhanced sensing properties of relevance in different fields of application.

Magnetic resonance imaging (MRI) is a useful tool for disease diagnosis and treatment monitoring. Superparamagnetic iron oxide nanoparticles (SPION) show good performance as transverse relaxation (T 2 ) contrast agents, thus facilitating the interpretation of the acquired images. Attachment of SPION onto nanocarriers prevents their agglomeration, improving the circulation time and efficiency. Graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), are appealing nanocarriers since they have both high surface area and functional moieties that make them ideal substrates for the attachment of nanoparticles. We have employed a fast, simple and environmentally friendly microwave-assisted approach for the synthesis of SPION-RGO hybrids. Different iron precursor/GO ratios were used leading to SPION, with a median diameter of 7.1 nm, homogeneously distributed along the RGO surface. Good relaxivity (r 2 *) values were obtained in MRI studies and no significant toxicity was detected within in vitro tests following GL261 glioma and J774 macrophage-like cells for 24 h with SPION-RGO, demonstrating the applicability of the hybrids as T 2 -weighted MRI contrast agents.

Knowing the interactions controlling aggregation processes in magnetic nanoparticles is of strong interest in preventing or promoting nanoparticles' aggregation at wish for different applications. Dipolar magnetic interactions, proportional to the particle volume, are identified as the key driving force behind the formation of macroscopic aggregates for particle sizes above about 20 nm. However, aggregates' shape and size are also strongly influenced by topological ordering. 1-D macroscopic chains of several micrometer lengths are obtained with cube-shaped magnetic nanoparticles prepared by the gas-aggregation technique. Using an analytical model and molecular dynamics simulations, the energy landscape of interacting cube-shaped magnetic nanoparticles is analysed revealing unintuitive dependence of the force acting on particles with the displacement and explaining pathways leading to their assembly into long linear chains. The mechanical behaviour and magnetic structure of the chains are studied by a combination of atomic and magnetic force measurements, and computer simulation. The results demonstrate that [111] magnetic anisotropy of the cube-shaped nanoparticles strongly influences chain assembly features.

Layered CaTaN 2 and MgTa 2 N 3 and cubic Mg 2 Ta 2 N 4 were prepared by direct solid state reaction from the binary nitrides Ta 3 N 5 and A 3 N 2 (A: Mg, Ca). CaTaN 2 showed a slight Ca deficiency (0.11 moles per formula), and a monoclinic distortion from previously reported R 3̅ m symmetry, with space group C 2/ m and cell parameters a = 5.4011(2), b = 3.1434(1), c = 5.9464(2) Å and β = 107.91(3)°. Ca 2+ and Mg 2+ deintercalation was investigated in the three compounds both chemically and electrochemically. No significant Mg 2+ extraction could be inferred for MgTa 2 N 3 and Mg 2 Ta 2 N 4 , neither after reaction with NO 2 BF 4 nor after electrochemical oxidation at 100 °C in alkyl carbonate electrolytes. Rietveld refinement of the X-ray powder diffraction pattern of chemically oxidized Ca 0.89 TaN 2 indicates a decrease of the Ca content to 0.34 concomitant to the disappearance of the monoclinic distortion and expansion of the interlayer space from 5.658 to 5.762 Å, space group R 3̅ m and cell parameters a = 3.1103(1) and c = 17.287(1) Å. Deintercalation in this compound was also achieved electrochemically at 100 °C. Results of density functional theory calculations seem to indicate that reaction mechanisms for CaTaN 2 oxidation additional and/or alternative to deintercalation are taking place, which is likely related to the loss of crystallinity observed upon oxidation and the irreversibility of the process., Competing Interests: The authors declare no competing financial interest.

The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fm3[combining macron]m double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with TC ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

A simple crossover from two-dimensional to three-dimensional correlated disorder of O and N atoms on a cubic lattice has been discovered within the Ba1-xSrxTaO2N series of perovskite oxynitrides. The crossover is driven by lattice expansion as x decreases, and provides a rapid increase in entropy due to a change from subextensive to extensive configurational entropy regimes.

Topochemical nitridation in ammonia at moderate temperatures of cation ordered Sr 2 FeWO 6 produces new antiferromagnetic double perovskite oxynitrides Sr 2 FeWO 6-x N x with 0 < x ≤ 1. Nitrogen introduction induces the oxidation of Fe 2+ to Fe 3+ and decreases T N from 38 K (x = 0) to 13 K for Sr 2 FeWO 5 N which represents the first example of a double perovskite oxynitride with both high cationic order and nitrogen content. This synthetic approach can be extended to other cation combinations expanding the possibility of new materials in the large group of double perovskites.

Ag 2 Cu 3 Cr 2 O 8 (OH) 4 , a new Ag-Cu-Cr-O layered mixed oxide, prepared by soft hydrothermal heterogeneous reactions, is reported. The new phase is an oxyhydroxide and presents a structure with alternating brucite-like Cu-O and Ag-O layers connected by individual chromate groups. The crystallographic structure has been solved and refined from high resolution powder X-ray diffraction data and is supported by density functional theory calculations, yielding a triclinic, space group P1[combining macron], a = 5.3329(1) Å, b = 5.3871(1) Å, c = 10.0735(1) Å, α = 80.476(1)°, β = 87.020(1)°, γ = 62.383(1)°. Bond valence sums suggest the formulation of Ag + 2 Cu 2+ 3 Cr 6+ 2 O 8 (OH) 4 , an electronic state fully supported by X-ray photoelectron spectroscopy (XPS) and Cr K-edge X-ray absorption near edge structure (XANES) measurements. Ag 2 Cu 3 Cr 2 O 8 (OH) 4 exhibits bidimensional Cu-O-Cu ferromagnetic correlations that are apparent at much higher temperatures than in other similar Cu-O layered structures, without coupling between Cu-O layers, which represents a unique case in the recent family of silver copper oxides. The role of Ag inducing bidimensionality in copper oxides is therefore expanded further with the presence of chromate anions. Ab initio calculations using density functional theory show that the electronic states involved originate mainly from Cu and OH orbitals, with minor contributions from Cr and the O atoms linking the Cr tetrahedra to the brucitic Cu-O layer, and almost no contribution from Ag. Further modeling of the in-plane magnetic interactions between Cu atoms suggests that the coupled magnetized stripes are responsible for the observed behavior. The results are discussed in relation with previous Ag-Cu mixed oxide phases where metallic behavior or ferro-antiferro transitions had been observed. The structure of this new Ag-Cu-O phase as compared with previous silver copper oxides supports the conclusion that the Ag-Cu layered ordering is favored under oxidizing conditions.

The fabrication procedure of hollow iron oxide nanoparticles with a large surface to volume ratio by a single-step gas condensation process at ambient temperature is presented. Fe clusters formed during the sputtering process are progressively transformed into hollow cuboids with oxide shells by the Kirkendall mechanism at the expense of oxygen captured inside the deposition chamber. TEM and Raman spectroscopy techniques point to magnetite as the main component of the nanocuboids; however, the magnetic behavior exhibited by the samples suggests the presence of FeO as well. In addition, these particles showed strong stability after several months of exposure to ambient conditions, making them of potential interest in diverse technological applications. In particular, these hierarchical hollow particles turned out to be very efficient for both As(III) and As(V) absorption (326 and 190 mg/g, respectively), thus making them of strong interest for drinking water remediation.

LnCrO(3-x)N(x) perovskites with Ln = La, Pr and Nd and nitrogen contents up to x = 0.59 have been synthesised through ammonolysis of LnCrO4 precursors. These new materials represent one of the few examples of chromium oxynitrides. Hole-doping through O(2-)/N(3-) anion substitution suppresses the magnetic transition far less drastically than Ln(3+)/M(2+) (M = Ca, Sr) cation substitutions because of the greater covalency of metal-nitride bonds. Hence, nitride-doping is a more benign method for doping metal oxides without suppressing electronic transitions.

The new compounds LaSrSiO3N and LaBaSiO3N activated with Eu(2+) are orange-red light-emitting luminescent materials under excitation in the UV-blue range. They represent the first examples of stoichiometric alkaline earth oxynitridosilicates with a β-K2SO4 structure. The isostructural compound LaEuSiO3N is ferromagnetic with a Curie temperature of 3 K and also shows red luminescence (λmax = 705 nm) under excitation at 405 nm.

The template assisted growth of single-layered inorganic nanotubes is reported. Single-crystalline lead iodide single-layered nanotubes have been prepared using the inner cavities of carbon nanotubes as hosting templates. The diameter of the resulting inorganic nanotubes is merely dependent on the diameter of the host. This facile method is highly versatile opening up new horizons in the preparation of single-layered nanostructures., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

NaMnMoO3F3·H2O was precipitated at low temperature from aqueous dissolutions of Na2MoO4·2H2O in aqueous HF (or NaHF2) using either Mn(CH3COO)2·4H2O or MnF2 as manganese precursors. Chemical analysis, IR spectra, and effective paramagnetic moment are in agreement with the proposed formula. Electron microscopy studies indicate that the sample is constituted of very thin plate-like microcrystals. The average crystal structure (a = 3.5947(1), b = 21.246(1), and c = 7.3893(2) Å and Cmcm (No. 63) SG) has been elucidated through powder diffraction methods (synchrotron and neutron). Tiny superstructure peaks are observed that can be indexed with the space group Pbca and cell parameters a = 7.1894(2), b = 21.246(1), and c = 7.3893(2) Å. Electron diffraction confirms the doubling of a parameter. Ordering of O and F is proposed in agreement with Pauling's second crystal rule and bond strength sums for each anionic position. The structure that is formed has a layered stacking arrangement along the b-axis of the MnO4F2 octahedra layers that share corners along a- and c-directions. The large interlayer space contains MoO4F2 octahedra (connected to the layer) together with sodium ions and water molecules, tentatively forming hydrogen bonding with fluorine anions. The origin of the superstructure could not be fully elucidated, but it is presumably related to slight distortions within the MnO4F2 octahedra.

The correlated anion order in the oxynitride perovskite NdVO(2)N, where disordered zig-zag VN chains segregate into planes within a pseudo-cubic lattice, is similar to that in materials such as SrTaO(2)N containing d(0) transition metal cations. However, NdVO(2)N has 3d(1) V(4+) cations and the 3d-electrons are itinerant, showing that the anion chain order in oxynitride perovskites is robust to electron-doping.

Porous mixed-valent manganese oxides are a group of multifunctional materials that can be used as molecular sieves, catalysts, battery materials, and gas sensors. However, material properties and thus activity can vary significantly with different synthesis methods or process conditions, such as temperature and time. Here, we report on a new synthesis route for MnO(2) and LaSr-doped molecular sieve single crystalline nanowires based on a solution chemistry methodology combined with the use of nanoporous polymer templates supported on top of single crystalline substrates. Because of the confined nucleation in high aspect ratio nanopores and of the high temperatures attained, new structures with novel physical properties have been produced. During the calcination process, the nucleation and crystallization of ε-MnO(2) nanoparticles with a new hexagonal structure is promoted. These nanoparticles generated up to 30 μm long and flexible hexagonal nanowires at mild growth temperatures (T(g) = 700 °C) as a consequence of the large crystallographic anisotropy of ε-MnO(2). The nanocrystallites of MnO(2) formed at low temperatures serve as seeds for the growth of La(0.7)Sr(0.3)MnO(3) nanowires at growth temperatures above 800 °C, through the diffusion of La and Sr into the empty 1D-channels of ε-MnO(2). Our particular growth method has allowed the synthesis of single crystalline molecular sieve (LaSr-2 × 4) monoclinic nanowires with composition La(0.7)Sr(0.3)MnO(3) and with ordered arrangement of La(3+) and Sr(2+) cations inside the 1D-channels. These nanowires exhibit ferromagnetic ordering with strongly enhanced Curie temperature (T(c) > 500 K) that probably results from the new crystallographic order and from the mixed valence of manganese.

Transition-metal oxynitrides with perovskite-type structures are an emerging class of materials with optical, photocatalytic, dielectric and magnetoresistive properties that may be sensitive to oxide-nitride order, but the anion-ordering principles were unclear. Here we report an investigation of the representative compounds SrMO(2)N (M = Nb, Ta) using neutron and electron diffraction. This revealed a robust 1O/2(O(0.5)N(0.5)) partial anion order (up to at least 750 °C in the apparently cubic high-temperature phases) that directs the rotations of MO(4)N(2) octahedra in the room-temperature superstructure. The anion distribution is consistent with local cis-ordering of the two nitrides in each octahedron driven by covalency, which results in disordered zigzag M-N chains in planes within the perovskite lattice. Local structures for the full range of oxynitride perovskites are predicted and a future challenge is to tune properties by controlling the order and dimensionality of the anion chains and networks.

A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO(1+x)N(2-x). Ammonolysis of Eu(2)W(2)O(9) yields scheelite-type intermediates EuWO(4-y)N(y) with a very small degree of nitride substitution (y = 0.04) and then EuWO(1+x)N(2-x) perovskites that show a wide range of compositions -0.16 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO(1+x)N(2-x) phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.

New oxynitrides of RZrO(2)N (R = Pr, Nd, and Sm) have been synthesized via a direct solid-state reaction of R(2)O(3) with Zr(2)ON(2) at 1200-1500 degrees C under 2-3 GPa pressure. Powder X-ray diffraction shows that all three phases adopt an orthorhombic Pnma perovskite superstructure [a = 5.8537(1) A, b = 8.1707(1) A, and c = 5.7093(1) A for NdZrO(2)N] and the structural distortion increases with decreasing R(3+) ionic radius. This method may enable new mixed-metal oxynitrides to be synthesized without the use of nitriding gas atmospheres.

The hydrothermal reduction of commercial CuO by Py yields Cu(2)O@PPy nanowires, branched nanostructures and bigger anisotropic crystals, which form spontaneously through oriented attachment in the absence of any capping agent or template.

We have explored a new strategy to discover materials with large resistive or capacitive responses to magnetic fields by synthesizing EuMO2N (M = Nb, Ta) perovskites that combine ferromagnetic order of S = 7/2 Eu2+ spins with possible off-center distortions of the d0 M5+ cations enhanced by covalent bonding to N. EuNbO2N shows colossal magnetoresistances at low temperatures and a giant magnetocapacitance. However, the latter response originates from a microstructural effect rather than an intrinsic multiferroism.

Here we report on the synthesis of ultrasmall gamma-Fe2O3 nanoparticles (5 nm) presenting a very narrow particle size distribution and an exceptionally high saturation magnetization. The synthesis has been carried out by decomposition of an iron organometallic precursor in an organic medium. The particles were subsequently stabilized in an aqueous solution at physiological pH, and the colloidal dispersions have been thoroughly characterized by complementary techniques. Particular attention has been given to the assessment of the mean particle size by transmission electron microscopy, X-ray diffraction, dynamic light scattering, magnetic, and relaxometric measurements. The good agreement found between the different techniques points to a very narrow particle size distribution. Regarding the magnetic properties, the particles are superparamagnetic at room temperature and present an unusually high saturation magnetization value. In addition, we describe the potential of these particles as specific positive contrast agents for magnetic resonance molecular imaging.

Electrochemical polymerization of N-vinyl carbazole (VC) on carbon nanotube (CNT) films was studied by cyclic voltammetry (CV) in LiClO4/acetonitrile solutions. Comparing the cyclic voltammograms recorded on a blank Pt electrode with those obtained when single- or multi-walled carbon nanotube (SWNT or MWNT) films were previously deposited onto the Pt electrode, a downshift of the VC reduction peak potential is observed in the latter case. The influences of monomer concentration, type of solvent, and supporting electrolyte on the polymerization conditions and electrochemical properties of these composite materials are also investigated. The morphological aspects of poly(N-vinyl carbazole) (PVC)-functionalized CNTs are revealed by transmission electron microscopy (TEM) studies. A covalent functionalization of carbon nanotubes with PVC is invoked as a result of attenuated total reflection infrared (ATR-IR) spectroscopic studies. Using PVC-functionalized CNTs as a positive electrode and an electrolytic solution containing LiPF6, the charge-discharge characteristics of the rechargeable lithium cells are determined. High specific discharge capacity (approximately 45 and 115 mA h g(-1)) is reported for PVC-functionalized single- and multi-walled carbon nanotubes, respectively.

A new phase has been observed during the sodium intercalation of hafnium nitride chloride as intermediate between the host beta-HfNCl and the already reported Na(0.29)HfNCl with Tc of 24 K; the new intermediate shows interlayer spacings ranging from 9.48 to 9.67 A, corresponds to a second stage intercalate of HfNCl and is superconducting with a critical temperature of 20 K.

Antifluorite-type lithium chromium oxide nitrides were prepared by solid-state reaction of Li(3)N, Li(2)O, and Cr(2)N. Depending on the reaction time and starting Li/Cr and O/Cr ratios, either an ordered or a disordered phase (or mixtures of both) is obtained. The formation of the former is favored by short reaction times and low Cr/O ratios whereas the formation of the latter is favored by higher Cr/O ratios and longer reaction times. The two phases were characterized, and the first one was confirmed to be the already reported Li(14)Cr(2)N(8)O phase, whereas the stoichiometry of the second is Li(10)CrN(4)O(2). Interestingly, even if both contain cationic vacancies in the structure, electrochemical lithium intercalation could only be achieved for Li(10)CrN(4)O(2). This phase exhibits a reversible capacity of 160 mAh/g very stable upon cycling. Bond valence and first-principles DFT calculations were carried out to understand the absence of lithium insertion in Li(14)Cr(2)N(8)O. Li-Li repulsion and destabilization of the tetrahedral CrN(4) units induced by occupation of the potential sites, as well as the absence of energetically favorable pathways for transport of the ions to these sites, are suggested to be the reasons.

The layered nitrides beta-MNX (M = Zr, Hf; X = Cl, Br) crystallize in the space group R&thremacr;m with a hexagonal cell of dimensions a = 3.6031(6) Å, c = 27.672(2) Å for beta-ZrNCl, a = 3.5744(3) Å, c = 27.7075(9) Å for beta-HfNCl, and a = 3.6379(5) Å, c = 29.263(2) Å for beta-ZrNBr. Lithium intercalation using n-buthyllithium in hexane solutions leads to solvent free superconductors of formula Li(0.20)ZrNCl, Li(0.42)HfNCl, Li(0.67)HfNCl, and Li(0.17)ZrNBr showing critical temperatures of 12, 18, 24, and 13.5 K, respectively. Whereas several samples of beta-ZrNBr and beta-ZrNCl showed reproducibility in the lithium uptake and in the corresponding critical temperatures, different samples of beta-HfNCl subjected to the same treatment in n-buthyllithium showed lithium uptakes ranging from 0.07 to 0.67, and corresponding critical temperatures between 0 and 24 K. A linear dependence of T(c) versus the lithium content is observed when all the superconducting samples are considered. The results obtained from electrochemical lithiation are consistent with those obtained with chemical methods, as samples with larger capacity on discharge are also those found to have larger lithium contents after chemical lithiation. Most samples present a reduction step around 1.8 V vs Li(0)-Li(+) whose origin is still unclear. The electrochemical capacity on discharge for beta-HfNCl and beta-ZrNBr depends on the milling time spent in the preparation of the electrodes, with long milling times resulting in lower intercalation degree. Possible causes for this effect are either the creation of structural defects (e.g., stacking faults) or some sample decomposition induced by local heating. The same phenomena are proposed to account for the different behavior of beta-HfNCl samples, although additional aspects such as the presence of hydrogen, oxygen, or extra hafnium atoms in the structure have to be considered. Tight-binding band structure calculations for beta-MNX (M = Zr, X = Cl, Br; M = Hf, X = Cl), ZrCl, and Y(2)C(2)Br(2) are reported. The density of states and Fermi surfaces of the beta-MNX phases as well as the relationship between the electronic structure of the beta-ZrNCl and ZrCl are discussed. Despite the structural relationships, the electronic structures near the Fermi level of the beta-MNX and Y(2)Br(2)C(2) phases are found to be very different.

The synthesis of adenine from the polymerization of concentrated ammonium cyanide solutions is well known. We show here that guanine is also produced by this reaction but at yields ranging from 10 to 40 times less than that of adenine. This synthesis is effective at both +80 and -20 degrees C. Since high concentrations of NH(4)CN are obtainable only by freezing, this prebiotic synthesis would be applicable to frozen regions of the primitive Earth, the Jovian satellite Europa and other icy satellites, and the parent body of the Murchison meteorite.

The most frequently invoked explanation for the origin of metabolic pathways is the retrograde evolution hypothesis. In contrast, according to the so-called "patchwork" theory, metabolism evolved by the recruitment of relatively inefficient small enzymes of broad specificity that could react with a wide range of chemically related substrates. In this paper it is argued that both sequence comparisons and experimental results on enzyme substrate specificity support the patchwork assembly theory. The available evidence supports previous suggestions that gene duplication events followed by a gradual neoDarwinian accumulation of mutations and other minute genetic changes lead to the narrowing and modification of enzyme function in at least some primordial metabolic pathways.

The notion that comets supplied the primitive Earth with the requisite chemical species for the process of chemical evolution, which is widely held to have led to the origin of life on Earth, has now gained considerable intellectual momentum since its first formulation in 1961. In fact, in the fall of 1991, the University of Wisconsin-Eau Claire hosted a well attended scientific meeting devoted solely to the discussion of this topic, entitled Comets and the Origin and Evolution of Life [see Special Issue of Origins of Life, P.J. Thomas (ed), vol. 21(5-6)]. As a result of the above meeting, the recently completed COSPAR/World Space Congress Symposium on Extraterrestrial Organic Chemistry and the Origins of Life, and numerous independent reports, the role of comets in the Earth's biogenesis has been thoroughly addressed in the literature. At this time, in light of a few recent findings, we present here a concise review of this topic together with a brief discussion of the possible role of cometary material in the origin of life elsewhere in the Universe.

Reverse transcriptase (RT) was first discovered as an essential catalyst in the biological cycle of retroviruses. However, in the past years evidence has accumulated showing that RTs are involved in a surprisingly large number of RNA-mediated transpositional events that include both viral and nonviral genetic entities. Although it is probable that some RT-bearing genetic elements like the different types of AIDS viruses and the mammalian LINE family have arisen in recent geological times, the possibility that reverse transcription first took place in the early Archean is supported by (1) the hypothesis that RNA preceded DNA as cellular genetic material; (2) the existence of homologous regions of the subunit tau of the E. coli DNA polymerase III with the simian immunodeficiency virus RT, the hepatitis B virus RT, and the beta' subunit of the E. coli RNA polymerase (McHenry et al. 1988); (3) the presence of several conserved motifs, including a 14-amino-acid segment that consists of an Asp-Asp pair flanked by hydrophobic amino acids, which are found in all RTs and in most cellular and viral RNA polymerases. However, whether extant RTs descend from the primitive polymerase involved in the RNA-to-DNA transition remains unproven. Substrate specificity of the AMV and HIV-1 RTs can be modified in the presence of Mn2+, a cation which allows them to add ribonucleotides to an oligo (dG) primer in a template-dependent reaction. This change in specificity is comparable to that observed under similar conditions in other nucleic acid polymerases. This experimentally induced change in RT substrate specificity may explain previous observations on the misincorporation of ribonucleotides by the Maloney murine sarcoma virus RT in the minus and plus DNA of this retrovirus (Chen and Temin 1980). Our results also suggest that HIV-infected macrophages and T-cell cells may contain mixed polynucleotides containing both ribo- and deoxyribonucleotides. The evolutionary significance of these changes in substrate specificities of nucleic acid polymerases is also discussed.

This paper discusses the already 60 year old ideas of Oparin and Haldane, who suggested that the first forms of life were anaerobic, heterotrophic bacteria coming from chemical abiotic synthesis of organic compounds.

Different estimates based on dynamical considerations, lunar cratering rates, Solar System chemical abundances, and the single-impact theory on the origin of the Earth-Moon system suggest that comets and other related small, volatile-rich primitive minor bodies captured by the Earth during the early Archean must have been a major source of volatiles on our planet. It is likely that a substantial fraction of the organic molecules present in the colliding cometary nuclei, which may have included nitrogen bases and the precursors of amino acids, were destroyed due to the high temperatures and shock wave energy associated with the collision. However, the presence of H2O, CN, CH, CO, CO2 and other carbon-bearing molecules and radicals in the atmosphere of the Sun and in circumstellar shells around carbon-rich stars suggests that at least simple carbon species could have survived the cometary collisions. Under the anoxic conditions thought to prevail in the prebiotic terrestrial paleoatmosphere, the post-collisional formation of a large number of excited molecules and radicals, and the rapid quenching of the expanding gaseous ball may have led, upon rapid cooling, to the formation of molecules of biogenic elements and to their eventual deposition in localized environments where complex organic compounds of biochemical significance may have been produced and accumulated.

Thirty years ago it was suggested that comets impacting on the primitive Earth may have represented a significant source of terrestrial volatiles, including some important precursors for prebiotic synthesis (Oró, 1961, Nature 190: 389). This possibility is strongly supported not only by models of the collisional history of the early Earth, but also by astronomical evidence that suggests that frequent collisions of comet-like bodies from the circumstellar disk around the star beta Pictoris are taking place. Although a significant fraction of the complex organic compounds that appear to be present in cometary nuclei were probably destroyed during impact, it is argued that cometary collisions with the primitive Earth represented an important source of both free-energy and volatiles, and may have created transient, gaseous environments in which prebiotic synthesis may have taken place.

The hypothesis that vestiges of the ancestral RNA-dependent RNA polymerase involved in the replication of RNA genomes of Archean cells are present in the eubacterial RNA polymerase beta' subunit and its homologues is discussed. We show that in the DNA-dependent RNA polymerases from the three cellular lineages a very conserved sequence of eight amino acids also found in a small RNA-binding site previously described for the E. coli polynucleotide phosphorylase and the S1 ribosomal protein is present. The optimal conditions for the replicase activity of the avian myeloblastosis virus reverse transcriptase are presented. The evolutionary significance of the in vitro modifications of substrate and template specificities of RNA polymerases and reverse transcriptases is also discussed.

The nonenzymatic synthesis of the coenzymes adenosine diphosphate glucose (ADPG), guanosine diphosphate glucose (GDPG), and cytidine diphosphoethanolamine (CDP-ethanolamine) has been carried out under conditions considered to have been prevalent on the early Earth. The production of these compounds was performed by allowing simple precursor molecules to react under aqueous solutions, at moderate temperatures and short periods of time, with mediation by cyanamide or urea. These two condensing agents are considered to have been present in significant amounts on the primitive Earth and have been previously used in the nonenzymatic synthesis of several other important biochemical compounds. In our experiments, ADPG was obtained by heating glucose-1-phosphate (G1P) and ATP in the presence of cyanamide for 24 h at 70 degrees C. The reaction of G1P and GTP under the same conditions yielded GDPG. The cyanamide-mediated production of CDP-ethanolamine was carried out by reacting a mixture of ethanolamine phosphate and CTP for 24 h at 70 degrees C. The separation and identification of the reaction products was carried out by paper chromatography, thin-layer chromatography, high performance thin-layer chromatography, high performance liquid chromatography, both normal and reverse-phase, UV spectroscopy, enzymatic assays, and acid hydrolysis. Due to the mild conditions employed, and to the relative ease of these reactions, these studies offer a simple attractive system for the nonenzymatic synthesis of phosphorylated high-energy metabolic intermediates under conditions considered to have been prevalent on the ancient Earth.

The synthesis of putrescine was accomplished by decarboxylation of L-ornithine when this amino acid was heated in aqueous solution and in the absence of oxygen. Chromatographic, radioisotopic, and enzymatic techniques were used to demonstrate that one mole of non-radioactive putrescine and one mole of 14CO2 was formed during the heating of L-(l-14C)-ornithine. This work indicates that the synthesis of putrescine can occur starting with ornithine and in conditions that are presumed could have existed on the primitive Earth. The possible significance of these results in the prebiotic molecular evolution is briefly discussed.

Some properties of DNA condensed with spermidine have been compared with the properties of DNA condensed with Co3+(NH3)6 to determine whether condensation of DNA with these trivalent cations protects DNA against the action of DNase I and increases transcription and encapsulation of DNA into liposomes. It was shown that DNA condensed with Co3+(NH3)6 was resistant to the action of the endonuclease DNase I such as DNA condensed with spermidine was. However, DNA condensed with Co3+(NH3)6 was significantly less active in transcription with the E. coli RNA polymerase than DNA-spermidine condensed forms. In addition, it was demonstrated that both compacted forms of DNA were more efficiently encapsulated into neutral liposomes; however, negatively, charged liposomes were scarcely formed in the presence of DNA condensed with Co3+(NH3)6. These experiments and the well documented properties of polyamines increasing the resistance to radiations and hydrolysis of nucleic acids, as well as their biological activities, such as replication, transcription, and translation, together with the low concentration of Co3+ in the environment, lead us to propose spermidine as a plausible prebiotic DNA condensing agent rather than Co3+ and the basic proteins proposed by other authors. Then, we consider the possible role and relevance of the polyamine-nucleic acids complexes in the evolution of life.

Transbilayer diffusion of Mn2+ ions occurred in liposomes formed from dipalmitoyl-phosphatidylcholine or egg-yolk phosphatidylcholine and egg-yolk phosphatidate (molar ratio 2:1) containing DNA and DNase I within their aqueous compartments. Cation diffusion was demonstrated by the hydrolytic activity of DNase I, activated by the Mn2+ ions that diffused into the vesicles, and this was confirmed by light scattering. Phosphatidate, a cone-shaped lipid which has been synthesized under simulated prebiotic conditions, was necessary for cation diffusion across the liposome membranes. Such liposomes represent a simple precellular system that interchanges cations with the surroundings and provides a microenvironment for enzymatic reactions, as evidenced by the hydrolysis of DNA by DNase I inside these closed lipid compartments.