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We report on the precise fabrication of low-cost high-performance electrochemical supercapacitors using reduced graphene oxide/polyaniline nanofiber composite electrodes. An infrared laser has been used to reduce the graphene oxide, converting the initial graphene oxide compact layer into a three dimensional open network of exfoliated graphene flakes. This highly conducting porous structure is very well suited for electrodepositing pseudocapacitive materials owing to its large surface area. Polyaniline nanofibers have been controllably electrodeposited on the graphene flake network, not only extending further the electrode surface area and providing it with a strong pseudocapacitance but also preventing the restacking of the graphene sheets during the subsequent device processing and charge-discharge cycling. The composite electrode presents a specific capacitance of 442 F g−1, as compared to 81 F g−1 for the bare reduced graphene oxide counterpart, and a capacitance retention of 84% over 2000 cycles.

Chemical vapor deposition (CVD) is one of the best ways to scalably grow low cost, high quality graphene on metal substrates; unfortunately, it not ideal for producing graphene on dielectric substrates. Here, we demontrate production of a high quality graphene layer on Sapphire using CVD with a copper catalyst. The catalyst consists of a thin copper film grown epitaxially on α- Al 2 O 3 (0001). After CVD growth of Graphene, the copper can be removed by simple evaporation in the presence of a carbon source ( C 2 H 4 ). We characterized the resulting graphene layer using Raman spectroscopy, atomic force microscopy (AFM), optical transmission and helium atom scattering (HAS). The sample exhibited a reduced Raman D peak and an excellent 2D to G ratio. AFM and HAS show large graphene domains over a macroscopic region. We measured > 86 % transparency over the visible spectrum.

The modulation of electrical and Raman properties of ultrafine (∼4 nm) Ag nanoparticle/graphene/SiO hybrid material at low coverage is evaluated with gradual nanoparticle incorporation by the gas aggregation deposition technique. The different contributing factors, such as doping, impurity scattering or strain, are assessed. Incorporation of Ag nanoparticles produce a very efficient n-type doping of graphene (∼7.5 e per particle) maintaining a constant mobility for particle coverage below ∼0.3 monolayers. Doping efficiency is determined by the probability for nanoparticles to be deposited in contact with graphene. The Fermi level upshift is modeled within the charged impurity scattering mechanism in the whole coverage range. A crossover to the regime where impurity scattering dominates is evidenced at large particle concentration. Surface-enhanced Raman scattering is detected in graphene phonons in the limit of vanishing plasmon resonance and very low coverage (∼0.08) that correspond to ∼1500 nanoparticles at the laser spot. Small distortions of the graphene lattice (±0.012%) induced by the nanoparticles overcome the predicted changes in Raman phonons related to carrier doping and originate I/I damping. This evolution of physical properties with gradual incorporation of Ag nanoparticles is anticipated to provide valuable hints to tune the optical and electronic performance of these graphene-based hybrid systems., This work is supported by MINECO (MAT2012-37276-C03-01) and from Comunidad de Madrid, project S2013/MIT-2740 (PHAMA_2.0-CM).

A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20 V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40 V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10 μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18 nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to 1 μm width and mm length, very promising for terahertz graphene plasmonic applications., X.D. acknowledges a FPI scholarship from Spanish MINECO and C.M. thanks financial support from the “Juan de la Cierva” postdoctoral program JCI-2011-08815. The research leading to these results has received funding from Spanish MINECO under projects MAT2012-37276-C03-01, MAT2012-37276-C03-03 and MAT2011-27470-C02-02, from Comunidad de Madrid under Project S2013/MIT-2740 (PHAMA_2.0-CM) as well as from the European Union Seventh Framework Program under grant agreement No. 604391 “Graphene Flagship.”

The detection, identification, and quantification of different types of molecules and the optical imaging of, for example, cellular processes are important challenges. Here, we present how interference-enhanced Raman scattering (IERS) in adequately designed heterostructures can provide amplification factors relevant for both detection and imaging. Calculations demonstrate that the key factor is maximizing the absolute value of the refractive indices' difference between dielectric and metal layers. Accordingly, Si/Al/AlO/graphene heterostructures have been fabricated by optimizing the thickness and roughness and reaching enhancement values up to 700 for 488 nm excitation. The deviation from the calculated enhancement, 1200, is mainly due to reflectivity losses and roughness of the Al layer. The IERS platforms are also demonstrated to improve significantly the quality of white light images of graphene and are foreseen to be adequate to reveal the morphology of 2D and biological materials. A graphene top layer is adequate for most organic molecule deposition and often quenches possible fluorescence, permitting Raman signal detection, which, for a rhodamine 6G (R6G) monolayer, presents a gain of 400. Without graphene, the nonquenched R6G fluorescence is similarly amplified. The wavelength dependence of the involved refractive indices predicts much higher amplification (around 10) for NIR excitation. These interference platforms can therefore be used to gain contrast and intensity in white light, Raman, and fluorescence imaging. We also demonstrate that surface-enhanced Raman scattering and IERS amplifications can be efficiently combined, leading to a gain of >10 (at 488 nm) by depositing a Ag nanostructured transparent film on the IERS platform. When the plasmonic structures deposited on the IERS platforms are optimized, singlemolecule detection can be actively envisaged., L.A.-F. acknowledges a FPI grant (BES-2013-062759) from Spanish Ministerio de Economía y Competitividad (MINECO). The research leading to these results has received funding from MINECO (MAT2012-37276-C03-01 and MAT2015-65356-C3-1-R) and Comunidad de Madrid (S2013/MIT-2740, PHAMA2.0-CM).

The oxidation and corrosion of copper are fundamental issues studied for many decades due to their ubiquitous and transversal impact. However, the oxidation of copper used as catalyst for graphene synthesis has opened a singular problem not yet solved. Contradictory results are reported about the protecting or enhancing role of graphene in copper oxidation. We study short- and long-term oxidation of copper with different characteristics, such as oxygen content and morphology, with and without graphene, and in polycrystalline copper foils and almost totally textured (100) and (111) copper films on MgO and sapphire substrates, respectively. We propose a mechanism to explain the enhanced oxidation of polycrystalline copper originated by oxygen encapsulated by the graphene impermeable layer during graphene growth. The initial oxygen content and the existence of grain boundaries are the main factors that determine the relevance of this process. Graphene is shown to prevent oxidation from the atmosphere for any of the copper substrates but also promotes slow oxidation derived by the release of out-of-equilibrium encapsulated oxygen. The formation of bubbles after several months evidence this slow release. The occluded oxygen in graphene covered copper is demonstrated by comparing the oxygen to copper ratio at different depths using hard X-ray photoelectron spectroscopy for samples with and without graphene., Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and MAT2012-37276-C03-01 and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation facilities at BM25 SpLine The Spanish CRG Beamline at the ESRF. L.A-F. acknowledges a FPI grant (BES-2013-062759) from MINECO.

In addition to the known effect of the substrate on the interfacial properties of perovskite films, here we show that the bulk properties of hybrid lead halide perovskite films depend on the type of substrate used for film growth. Despite the relative large film thickness, ∼600 nm, the roughness and nature of the substrate layer (glass, FTO, TiO and PEDOT:PSS) affect not just the degree of preferential orientation and crystal grain size but also the lattice parameters of CHNHPbI films synthesized from the PbCl precursor. The obtained changes in lattice parameters indicate that the Pb-Pb distance varies by around 0.7%. We suggest that the substrate roughness and chemical nature determine the concentration of defects mainly by varying the chlorine content and probably by the incorporation of oxygen and iodine vacancies during film nucleation and growth. These differences also have consequences in the observed light induced transformations. Upon laser illumination, the formation of additional defects, most probably related to oxygen, is revealed by 110 and 165 cm Raman peaks. With increasing laser power the chemical transformation into PbO is clearly identified by the 140 and 275 cm Raman peaks. The irreversible photoluminescence enhancement observed at low power with illumination time, also dependent on the substrate nature, is proposed to be due to the localization of the electron-hole excitons created in the vicinity of the light generated defects. These results shed light on the performance of the perovskite layer and help to understand how bulk processes, where ion migration is a conspicuous example, are severely affected by interfacial properties such as those imposed by the substrate., Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and 2-R, MAT2014-54852-R and MAT2015-70611-ERC and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 (and Associated Lab LABCADIO belonging to CM net labs ref. 351) is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation facilities at ESRF, and thank María Vila for her assistance in using beamline BM25-SpLine. B. C. H. is grateful to the support of the National Council of Technological and Scientific Development (CNPq), Brazil, through the Science without Borders program.

Stable Graphene/gas/Cu bubbles are formed by the spontaneous oxidation at room conditions of the Cu substrates used as the catalyst agent in chemical vapor deposition growth of graphene. The non homogeneous copper oxidation produces discontinuous CuO films where atmospheric molecules get trapped by the impermeable graphene layer forming bubbles. Raman Interference effects are calculated both for graphene and CuO phonons in graphene/CuO/Cu system using the transfer matrix method finding that CuO thickness is limited to 20 nm and maximum Raman enhancement of graphene is 10. Nevertheless, enhancement values up to 60 are detected for micrometric graphene bubbles on copper which, according to the simulations, are 70 nm height. AFM measurements confirm the bubble dimensions and show that they are mechanically softer than graphene on copper. Consistently, the Raman characteristics of the graphene bubbles coincide with free standing undoped graphene with small strain, in the range 0-0.3%, while graphene on CuO presents hole doping and larger strain fields. The simulations indicate that interference Raman enhancement can reach values >10 in graphene/dielectric/Aluminum trilayers (dielectric = gas, oxide or polymer). These systems are therefore interesting platforms for optical sensing where graphene is the adequate bio-compatible layer for metallic nanoparticles and molecules deposition., A-F. acknowledges a FPI grant from Spanish Ministerio de Economía y Competitividad (MINECO) (BES-2013-062759) and E.C-P from the European Union Seventh Framework Program under grant agreement n° 604391 “Graphene Flagship”. The research leading to these results has received funding from Spanish MINECO (MAT2012-37276-C03-01 and MAT2015-65356-C3-1-R) and Comunidad Autónoma de Madrid (S2013/MIT-2740, PHAMA2.0-CM).

CoFe2O4 particles of 16 nm and 17 nm embedded in a silica matrix have been prepared through the hydrothermal method and the sol-gel method, respectively. From neutron powder diffraction a cation distribution of (Fe(0.72)Co(0.28))[Fe(1.28)Co(0.72)]O4 has been determined for Co-ferrite particles of 17 nm, which is in agreement with its particle size taking into account the reported x values for other nanometric Co-ferrite particles. Magnetic measurements were performed up to 700 K as the prepared ferrite samples present blocking temperatures above room temperature. The temperature dependence of the superparamagnetic moment has been analyzed and presents for both samples an abrupt drop in the magnitude once the blocking temperature is overcome. The temperature dependence of the calculated magnetic field needed to reach the magnetic saturation of the samples allows us to determine the temperature range for which the nanoparticles show superparamagnetic behaviour. The ordering temperature is in both cases lower than the tabulated one for bulk Co-ferrite (793 K) which has been ascribed mainly to two factors: a different cation distribution and the nanometric particle size, both contributing to lowering of the strength of the superexchange interactions.

The scheelite-type polymorph of GdCrO 4 has been obtained from the corresponding zircon-type compound under high pressure and temperature conditions, namely 4 GPa and 803 K. The crystal structure has been determined by X-ray powder diffraction. This GdCrO 4 scheelite crystallizes in a tetragonal symmetry with space group I 4 1 / a (No. 88, Z =4), a =5.0501(1) A, c =11.4533(2) A and V =292.099(7) A 3 . The thermal decomposition leads to the formation of the zircon–polymorph as intermediate phase at 773 K to end in the corresponding GdCrO 3 distorted perovskite-structure at higher temperatures. Magnetic susceptibility and magnetization measurements suggest the existence of long-range antiferromagnetic interactions which have been also confirmed from specific heat measurements. Neutron powder diffraction data reveal the simultaneous antiferromagnetic Gd 3+ and Cr 5+ ordering in the scheelite-type GdCrO 4 with a T N ∼20 K. The magnetic propagation vector was found to be k =(0 0 0). Combined with group theory analysis, the best neutron powder diffraction fit was obtained with a collinear antiferromagnetic coupling in which the m Cr 5 + and m Gd 3 + magnetic moments are confined in the tetragonal basal plane according to the mixed representation Γ 6 ⊕ Γ 8 .

Cu5SbO6 is found to have a monoclinic, Delafossite-derived structure consisting of alternating layers of O–Cu(I)–O sticks and magnetic layers of Jahn–Teller distorted Cu(II)O6 octahedra in an edge sharing honeycomb arrangement with Sb(V)O6 octahedra. This yields the structural formula Cu(I)3Cu(II)2Sb(V)O6. Variants with ordered and disordered layer stacking are observed, depending on the synthesis conditions. The spin 1/2 Cu2+ ions form dimers in the honeycomb layer. The magnetic susceptibility measured between 5 and 300 K is characteristic of the presence of a singlet–triplet spin gap of 189 K. High resolution synchrotron X-ray diffraction studies indicate that changes in the intra- or interdimer distances between 300 and 20 K, such as might indicate an increase in strength of the Peierls-like distortion through the spin gap temperature, if present, are very small. A comparison to the NaFeO2-type Cu2+ honeycomb compounds Na3Cu2SbO6 and Na2Cu2TeO6 is presented.

Spinel-type (S.G.= Fd 3 m ) ZnFe 2 O 4 fine particles with sizes from 4 to 19 nm prepared by solvothermal and microwave-assisted solvothermal methods have been studied by neutron powder diffraction at room temperature. The cation distribution corresponding to mixed spinel structure (Zn 2+ 1− x Fe 3+ x )[Fe 3+ 2− x Zn 2+ x ]O 4 along with the unit cell parameter has been estimated after Rietveld refinement of the obtained neutron diffraction data for all the samples. It has been found that the inversion degree parameter ( x ) takes values between 0.11 and 0.20 depending not only on the particle size but also on the synthesis conditions as well. All the samples behave as superparamagnetic with an effective magnetic moment per particle ( μ SP ) from 7.0×10 2 to 7.7×10 3 μ B . The sample obtained by microwave assistance displays a different magnetic behavior as the ZFC and FC magnetic susceptibility and the magnetization versus applied field hysteresis loop measured at 5 K suggest. This is related with the dipole interactions that are a consequence of the higher inversion degree and μ SP .

The use of nitrite sodalite as a precursor to prepare new blue ultramarine analogs has been investigated. The encapsulation of the chromophores inside of beta cages was achieved by heating the nitrite sodalite precursor with a mixture of sodium carbonate, sulfur, and a reducing agent at about 1000 K under airtight conditions. The obtained new blue ultramarine-type material was characterized by means of X-ray and neutron powder diffraction, transmission electron microscopy, 29Si magic-angle-spinning NMR, IR, Raman, and diffuse reflectance spectroscopies. Thermogravimetric analysis and magnetic measurements were performed in order to determine water and chromophore contents, respectively. On the basis of the spectroscopic results and Rietveld analysis of neutron powder data, the beta cage filling was found to be yellow S2(-) and blue S3(-) chromophores along with carbonate anions. The determined chromophore concentration, about 0.31 mol per formula unit, is enough to have greenish-blue ultramarines, which proves that ultramarine analogs can be obtained from nitrite sodalite.

Blue ultramarine pigments with composition Na 7.7 [(Al,Si) 12 O 24 ] · (SO 4 ) 0.9 (S 3 ) 0.8 and Na 6.5 K 0.5 [(Al,Si) 12 O 24 ] · (SO 4 ) 0.9 (S 3 ) 0.7 were synthesized in order to get a deep knowledge of its structure and microstructure. These pigments were prepared following the current industrial method synthesis, which consists of the calcination of the metakaolin, sulphur, sodium carbonate and charcoal under controlled atmosphere. The structural characterization of the obtained pigments was performed in detail by the complementary use of different techniques such as X-ray powder diffraction and transmission electron microscopy (SAED and HRTEM) together with 29 Si MAS-NMR and IR spectroscopies. The obtained results have allowed to propose a structural model with the lowest symmetry for ultramarine pigments. The suitable space group to describe this structure is P 23 (No.195), which takes account the observed β-cage cluster ordering at the unit cell level and the framework adaptation to the different size of the β-cage clusters.

We report a new method to produce high-quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/AlO. Graphene on sapphire is obtained in situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and optical transmission have been used to assess the quality of graphene in a metal free area. We used helium atom scattering as a sensitive probe of the crystallinity of the graphene on sapphire. The observation of high reflectivity and clear diffraction peaks demonstrates the presence of flat and homogeneous graphene domains over lateral scales of microns, consistent with the AFM results. Surprisingly, putting graphene on sapphire improves the quality of the He-diffraction spectra. Graphene forms a moiré pattern with a (11 × 11) periodicity, aligned with the (1 × 1) sapphire unit cell. The lattice constant of graphene on sapphire is a = (2.44 ± 0.02) Å. The phonon dispersion of the graphene flexural mode has been measured. This allowed the determination of the bending rigidity k = 0.61 ± 0.15 eV, and the graphene-sapphire coupling strength g = (5.8 ± 0.4) × 10 N m. The uniformity of the graphene has also been investigated by Raman mapping. Judging by the ratio of the 2D to G peaks, the quality of the graphene is not degraded by Cu removal. The high transparency (80%) measured in the visible range makes this system suitable for many applications that require hybrid properties commonly associated with metals (conductivity) and insulators (transparency). Our study shows that He-diffraction and Raman provide crucial information on quite different, complementary aspects of the same samples., This project was sponsored by the European Union, Seventh Framework Programme: Theme NMP.2012.1.4-3 Grant no. 309672. AMW and HKY acknowledge support from the Alexander von Humboldt Foundation, the Max Planck EPFL Center for Molecular Nanoscience and Technology as well as the Deutsche Forschungsgemeinschaft under CRC 1073. Additionally, a part of the research has been supported by the Spanish MINECO, under project MAT2012-37276-C03-01, and by Comunidad de Madrid, under project S2013/MIT-2740 (PHAMA_2.0-CM). S2013/MIT-2740/PHAMA_2.0-CM

The Pr2BaCuO5 oxide has been prepared by using a precursor method. X-ray and neutron diffraction data reveal that this mixed oxide crystallizes with the so-called Nd2BaPtO5-type structure, showing tetragonal symmetry and space group P4/mbm. Magnetic susceptibility, magnetization and specific heat measurements reveal that Pr2BaCuO5 oxide behaves as antiferromagnet, where the Pr3+ and Cu2+ sublattices become simultaneously ordered at 15 K. Preliminary neutron diffraction studies confirm this proposed antiferromagnetic ordering and it can be described by the wave vector k = [0,0,0].

This paper reports the specific conditions used in the preparation of the dimorphic phases of HoCrO4 oxide. The scheelite form has been obtained from the room pressure stable HoCrO4–zircon heated at 823 K at 40 kbar. The structures of both the polytypes have been refined from X-ray powder diffraction data using the Rietveld method. The zircon type of HoCrO4 oxide crystallizes with tetragonal symmetry, S.G. I41/amd and lattice parameters a = 7.119(10) and c = 6.2557(5) A; while the scheelite–HoCrO4 derivative shows tetragonal symmetry, S.G. I41/a and lattice parameters a = 5.0017(1) and c = 11.2664(2) A. Magnetic susceptibility measurements show that both zircon and scheelite forms of HoCrO4 oxide present a very different magnetic behaviour at low temperatures. The zircon form behaves as a ferromagnet with a Curie temperature of 17.6 K, while the scheelite form is antiferromagnetic with a Neel temperature of 7.6 K. A metamagnetic transition for the scheelite form with a critical field of 1 T at 2 K has been also observed. The changes in the sign of the interactions have been also analyzed by considering the possible super-exchange mechanisms and the differences found in the Ho–O–Cr distances and bond angles in going from zircon to scheelite structural type.

The crystal and magnetic structure of Sr 2 ErRuO 6 has been studied by means of neutron powder diffraction as well as magnetization and susceptibility measurements. Neutron diffraction profile measured at 50 K shows that the Ru 5+ and Er 3+ are ordered in the B-sites of the perovskite-type structure, while the Sr atoms occupy the A-site. This compound crystallizes with a monoclinic unit cell, space group P 2 1 / n and lattice parameters are approximately √2 a p × √2 a p × 2 a p . Magnetic susceptibility measurements reveal the existence of antiferromagnetic interactions in which Ru 5+ and Er 3+ sublattices are involved. The field dependence of the magnetization indicates the presence of a weak ferromagnetic component at the transition temperature, arising from the spin canting of the antiferromagnetically ordered Ru 5+ and Er 3+ moments. Thermal evolution of the neutron diffraction patterns indicate that the Neel temperature is 36 K and the magnetic reflections can be indexed on the basis of a propagation vector k = [0, 0, 0]. The spin arrangement is described by the A x A z magnetic modes where the Ru 5+ and Er 3+ moments are mainly aligned along the c -axis of the structure, forming an angle of ∼6° with the c -axis in the case of the Er 3+ sublattice and ∼15° for the Ru 5+ moment.

This work was supported by the Spanish MINECO (FIS2014-54498-R, MAT2011-27470-C02-02, and CSD-2009-00013), by the European Union (Graphene Flagship Contract No. CNECT-ICT-604391 and COST MP1201 action), and by the Comunidad de Madrid through programs Nanofrontmag-CM (S2013/MIT-2850) and MAD2D-CM (S2013/MIT-3007).We acknowledge MINECO and CSIC for financial support and for provision of synchrotron radiation facilities and would like to thank the SpLine BM25 staff for assistance in using the beamline

We propose a strategy to study the elastic properties of extremely thin graphene oxide (GO) films using Brillouin spectroscopy. The dependence of the surface acoustic wave of a gold capping layer on the structural, chemical and morphological changes occurring to the underneath GO film with temperature is reported and analyzed. At room temperature the shear constant c44 is ∼17 GPa and hardens up to 28 GPa at 100 °C due to the partial elimination of embedded water layers and to interlayer distance shrinking. At 200 °C the almost complete elimination of water induces layer stacking disorder, further GO–GO distance reduction and a significant increase of all elastic constants. The in-plane constants harden due to the partial restoration of the sp2 C network (c11: from 268 to 620 GPa) and the out of plane constants harden due to the H bonds that now directly connect the neighbouring GO layers (c44 ≈ 80 GPa). The obtained Young's moduli are significantly higher than those reported for GO paper because the ultra-thin GO films are highly ordered and there is no macroscopic applied strain during the measurement. The results obtained here are associated with the intrinsic properties of GO as in-plane and inter-layer bonding., X.D. acknowledges a FPI scholarship from Spanish MINECO. The research leading to these results has received funding from Spanish MINECO under project MAT2012-37276-C03-01, from Comunidad de Madrid, project S2013/MIT-2740 (PHAMA_2.0-CM) and from the European Union Seventh Framework Program under grant agreement no. 604391 “Graphene Flagship.” info:eu-repo/grantAgreement/EC/FP7/604391

The title compound is obtained from zircon-type GdCrO4 (Pt crucible, 4 GPa, 803 K, 30 min).

The title compound is characterized by powder XRD, EPR spectroscopy, and magnetic susceptibility measurements.

Cu(5)SbO(6) is found to have a monoclinic, Delafossite-derived structure consisting of alternating layers of O-Cu(I)-O sticks and magnetic layers of Jahn-Teller distorted Cu(II)O(6) octahedra in an edge sharing honeycomb arrangement with Sb(V)O(6) octahedra. This yields the structural formula Cu(I)(3)Cu(II)(2)Sb(V)O(6). Variants with ordered and disordered layer stacking are observed, depending on the synthesis conditions. The spin ½ Cu(2+) ions form dimers in the honeycomb layer. The magnetic susceptibility measured between 5 and 300 K is characteristic of the presence of a singlet-triplet spin gap of 189 K. High resolution synchrotron X-ray diffraction studies indicate that changes in the intra- or interdimer distances between 300 and 20 K, such as might indicate an increase in strength of the Peierls-like distortion through the spin gap temperature, if present, are very small. A comparison to the NaFeO(2)-type Cu(2+) honeycomb compounds Na(3)Cu(2)SbO(6) and Na(2)Cu(2)TeO(6) is presented.

We report the characterization of Bi${}_{2}$Te${}_{2}$Se crystals obtained by the modified Bridgman and Bridgman-Stockbarger crystal-growth techniques. X-ray-diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($g1\phantom{\rule{0.28em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) and low carrier concentration ($\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}5\ifmmode\times\else\texttimes\fi{}{10}^{16}$/cm${}^{3}$) at 4 K were found in the central region of the long Bridgman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature-dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase.

WO${}_{3\ensuremath{-}x}$F${}_{x}$ ($x$ $\ensuremath{\le}$ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (teflon). The symmetry of the crystal structures of WO${}_{3\ensuremath{-}x}$F${}_{x}$ changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO${}_{3}$ to a metallic conductor, and superconductivity (${T}_{c}$ $=$ 0.4 K) was discovered in the samples with fluorine contents of 0.41 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.

We report the structural, transport, and thermodynamic properties of the semiconducting, layered rare-earth iron oxyselenides $R$${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$ ($R=\text{Ce, Pr, Nd, and Sm}$), which have crystal structures related to the arsenide superconductors except with in-plane oxygens within the Fe square planes. The activation energies are estimated to be between 0.15 and 0.26 eV for these four compounds. The antiferromagnetic Fe sublattice ordering, which appears as an anomaly in the magnetic susceptibility and a sharp peak in the heat capacity, occurs at 87.2, 88.6, 87.7, and 85.3 K, for the Ce to Sm variants, respectively, and the structural analysis suggests a possible correlation between ${T}_{N\ensuremath{-}\mathrm{Fe}}$ and the Fe-Se distance, rather than the in-plane Fe-O distance. The $R$ sublattice orders antiferromagnetically at 21.8 K for the Pr variant and 5.8 K for the Sm variant; no $R$ sublattice ordering is observed above 2 K for $R=\text{Ce}$ and Nd. The existence of short-range spin correlations above the Fe sublattice ordering temperature is revealed by magnetic entropy analysis, which also suggests that the ordered magnetic states of the $R$ sublattice arise from triply degenerate crystal-electric-field (CEF) states for Pr and doubly degenerate CEF states for Sm. The neutron-scattering measurements performed on Pr${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$ reveals that the ordered Fe sublattice has the same magnetic structure as the one in La${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$, with a propagation vector of $\mathbf{k}=(0.5,0,0.5)$ and an ordered magnetic moment of 2.23(3)${\ensuremath{\mu}}_{B}$/Fe at 5 K.

Using a combination of high-resolution and high-sensitivity neutron and x-ray diffraction measurements, it is shown that BaPb${}_{1\ensuremath{-}x}$Bi${}_{x}$O${}_{3}$ is dimorphic for the critical superconducting composition regime of $x$ between 0.2 and 0.3. The system is not chemically phase separated; the simultaneous existence of tetragonal and orthorhombic polymorphs of the same composition is demonstrated. The polymorphs differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a second-order phase transition. It is established that the tetragonal symmetry polymorph is the superconducting phase.

We report the characterization of the layered rare-earth manganese oxyselenide ${\text{La}}_{2}{\text{O}}_{3}{\text{Mn}}_{2}{\text{Se}}_{2}$. The susceptibility data show a broad maximum near 350 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. A sharp feature associated with a short-range to long-range antiferromagnetic phase transition is seen at 163 K. A very small heat-capacity anomaly is detected around 163 K, indicating that most of the magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 200, 150, 100, and 6 K. The structure was refined in space group $I4/mmm$ with $a=4.13\text{ }939(3)\text{ }\text{\AA{}}$ and $c=18.8511(2)\text{ }\text{\AA{}}$ at ambient temperature. No structural distortion was detected. The resulted magnetic structure is G-type with a propagation vector of $\mathbf{k}=(0,0,0)$ and an ordered magnetic moment of $4.147(28)\text{ }{\ensuremath{\mu}}_{B}/\text{Mn}$ along $c$ is found at 6 K. Warren peak shape analysis of the neutron-diffraction data near $22\ifmmode^\circ\else\textdegree\fi{}$ is employed to characterize the increase in correlation length in the 2D magnetic state on approaching the three-dimensional ordering transition.

7 páginas, 6 figuras, 3 tablas.-- PACS number(s): 75.25.-j, 62.50.-p, 61.66.Fn, 75.30.Cr.-- et al., The scheelite form of the TbCrO4 oxide has been obtained by treating TbCrO4-zircon at 4 GPa and 823 K. X-ray and neutron diffraction data reveal that the high-pressure polymorph of TbCrO4 crystallizes with tetragonal symmetry space group I41/a and lattice parameters a=5.036 74(10) Å and c=11.3734(4) Å. Although bisdisphenoids [TbO8] and tetrahedra [CrO4] are present in both scheelite and zircon polymorphs, the remarkable changes observed in both polyhedra appear to support the reconstructive model for the zircon-scheelite first-order phase transition. Specific heat and neutron diffraction measurements confirm the antiferromagnetic ordering previously proposed from magnetic susceptibility measurements, in which both Tb3+ and Cr5+ sublattices are involved. The magnetic structure has been determined and can be described on the basis of the coincidence between the chemical and magnetic cells, k=(0,0,0), where the magnetic moments of the Tb3+ and Cr5+ cations are antiferromagnetically aligned along the c axis of the structure, with TN=29 K. Their values at 2 K are 8.97(3)μB and 1.089(9)μB, respectively., The authors are grateful to Spanish MICINN and FEDER for financial support under Research Projects No. MAT2007- 63497 and No. MAT2007-61621, CONSOLIDER-Ingenio 2010 (Grant No. CSD 2007-00010).

Neutron diffraction data of DyCrO4 oxide, prepared at 4 GPa and 833 K from the ambient pressure zircon-type, reveal that crystallize with the scheelite-type structure, space group I41/a. Accompanying this structural phase transition induced by pressure the magnetic properties change dramatically from ferromagnetism in the case of zircon to antiferromagnetism for the scheelite polymorph with a T N= 19 K. The analysis of the neutron diffraction data obtained at 1.2 K has been used to determine the magnetic structure of this DyCrO4-scheelite oxide which can be described with a k = [0, 0, 0] as propagation vector, where the Dy and Cr moments are lying in the ab-plane of the scheelite structure. The ordered magnetic moments are 10 µB and 1 µB for Dy+3 and Cr+5 respectively

We report on the destabilization of the 6H-SrIrO3 polymorph through partial substitutions of zinc and lithium for iridium to form perovskites. The perovskites crystallize in the orthorhombic space group Pbnm: SrIr1−xZnxO3 is found for 0.25 ≤ x ≤ 0.33, while SrIr1−xLixO3 is found only for x = 0.25. The Zn and Li ions are randomly distributed in the B-site lattice. Analysis shows that the perovskite stabilization is not driven by changes in average ionic size but rather is due to destabilization of the face-sharing octahedra that are present in 6H-type SrIrO3. Magnetic susceptibility measurements show Curie–Weiss behavior, with relatively large temperature independent contributions, and that the iridium atoms have low effective moments, 0.52 to 1.08 μB per Ir. The resistivity of SrIr0.67Zn0.33O3, characterized by Mott variable range hopping type semiconducting behavior, indicates that substituted Zn ions introduce significant disorder into the system. SrIr0.75Li0.25O3 has a significant linear contribution to the specific heat at low temperatures.

The magnetic behaviour of Pr2BaCuO5 oxide has been studied by means of magnetic susceptibility, magnetization, specific heat and neutron powder diffraction at low temperatures. This oxide behaves as an antiferromagnetic with a Neel temperature of 13.7 K and the magnetic structure can be described on the basis of a wavevector k =( 0, 0, 0) .T he Cu 2+ magnetic moments are aligned along the c-axis of the crystal structure, while the Pr 3+ magnetic moments lie in the ab-plane with a small component along the c-axis. The experimental values of the ordered moments at 2 K are 0.9(3) and 0.8(3 )μ B for copper and praseodymium ions, respectively. The obtained set of crystal field parameters reproduces the paramagnetic susceptibility measurements and justifies the low experimental ordered moment of the Pr 3+ ion. (Some figures in this article are in colour only in the electronic version)