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Crystalline films of lead-free all-inorganic Cs3Bi2X9 (X = Br, I) perovskites have been deposited by radio frequency (RF)-magnetron sputtering providing high-quality, single-phase films as confirmed by structural, morphological, and optical property characterization. Progressive tuning of crystal structure characteristics and optical absorbance has been achieved in mixed Br/I phases Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1), highlighting a shift of the band gap from about 2.0 eV for Cs3Bi2I9 to 2.64 eV for Cs3Bi2Br9. X-ray diffraction and Raman scattering allowed defining the range of alloyed compositions where single-phase compositions are found. Finally, preliminary photocatalytic activity tests on the degradation of methylene blue provided solid data indicating the future possible exploitation of Bi-based perovskite derivative materials as active photocatalysts., High-quality films of the Cs3Bi2(I1−xBrx)9 (0 ≤ x ≤ 1) system have been grown by the vapor phase method for the first time. Crystal structure investigation revealed a phase change around x = 0.4, as demonstrated by X-ray diffraction and Raman spectroscopy. Halide alloying provided a continuous modulation of the band gap from 2.0 eV for Cs3Bi2I9 to about 2.64 eV for Cs3Bi2Br9.

The osseointegration of titanium implants is allowed by the TiO2 layer that covers the implants. Titania can exist in amorphous form or in three different crystalline conformations: anatase, rutile and brookite. Few studies have characterized TiO2 covering the surface of dental implants from the crystalline point of view. The aim of the present study was to characterize the evolution of the TiO2 layer following different surface treatments from a crystallographic point of view. Commercially pure titanium and Ti-6Al-4V implants subjected to different surface treatments were analyzed by Raman spectroscopy to evaluate the crystalline conformation of titania. The surface treatments evaluated were: machining, sandblasting, sandblasting and etching and sandblasting, etching and anodization. The anodizing treatment evaluated in this study allowed to obtain anatase on commercially pure titanium implants without altering the morphological characteristics of the surface.

The search for highly performing cathode materials for sodium batteries is a fascinating topic. Unfortunately, Na0.44MnO2 (NMO), the well-known cathode material with good electrochemical performances, suffers from structural degradation due to reduction of Mn4+ to the Jahn–Teller Mn3+ ion, limiting the long-term cyclability. The cation substitution can be a useful way to mitigate the problem, thanks to the possible stabilization of mixtures of different polymorphs. In this paper, NMO was first substituted with Fe ions, obtaining Na0.44Mn0.5Fe0.5O2, with layered structure, then Al, Si and Cu (10% atom) were substituted on both Mn and Fe ions. Mixtures of P3 type phases, in different amount depending on dopant, were obtained and quantified by Rietveld refinements, and relationships between chemical composition, polymorph type and morphology were proposed. Cyclic voltammetry showed broad peaks, due to the complex structural transitions consequent to the intercalation/deintercalation of sodium. Charge discharge cycles disclosed the superior performances of Cu doped sample, which also benefits from improved air stability, a well-known issue of layered compounds. Discharge capacity values of about 63 mAh/g were detected at 1C, and after 50 cycles at C/2, capacities of about 80 mAh/g are obtained, with a capacity retention of 86%.

In the last years of research on new anode materials for advanced Lithium-Ion Batteries (LIBs), niobium-based oxides are raising growing attention due to very high theoretical capacities and high working potential that can prevent the formation of lithium dendrites, ensuring the safety of the batteries. Most of them crystallizes in open Wadsley–Roth shear structures, which show large Li-ion diffusion coefficients and promising applications in energy storage systems. In this paper, the complex structural and electrochemical features of FeNb11O29 are unravelled with in situ Raman Spectroscopy, operando X-Ray Diffraction and electrochemical techniques. The intrinsic pseudocapacitance shown by the iron niobate is correlated to large channels in its structure that cause weak Li+-host interactions and very little charge-transfer resistances. The symmetrisation of the octahedral framework that occurs after the reduction of Nb5+ cations, detected for the first time, seems to be the key of the electrochemistry of FeNb11O29, which shows excellent features for advanced high-power density LIBs.

The adhesion and proliferation of bacteria on abiotic surfaces pose challenges in both health care and industrial applications. Gold nanostars (GNSs) monolayers grafted on glass have demonstrated to exert antibacterial action due to their photo-thermal features. Here, these GNS layers were further functionalized using thiols monolayers, in order to impart different wettability to the surfaces and thus adding a feature that could help to fight bacterial proliferation. Thiol that has different functional groups was used and the thiol-protected surfaces were characterized by means of UV-vis spectroscopy, contact angles, SEM and surface enhanced Raman spectroscopy (SERS). We verified that (i) coating with the proper thiol allows us to impart high hydrophilicity or hydrophobicity to the surfaces (with contact angle values ranging from 10 to 120&deg, ), (ii) GNS monolayers are strongly stabilized by functionalization with thiols, with shelf stability increasing from a few weeks to more than three months and (iii) photo-thermal features and subsequent antibacterial effects caused by hyperthermia are not changed by thiols layers, allowing us to kill at least 99.99% of representative bacterial strains.

We report the SERS activity of colloidal silver nanoparticles functionalized with a ligand, derived from the siderophore desferrioxamine B (desferal, DFO), an iron chelator widely used in biological and medical applications. The ligand was equipped with a sulfur-containing moiety to ensure optimal binding with silver surfaces. By means of Raman and SERS effects we monitored the route of material preparation from the modified DFO–S molecule to the colloidal aggregates. The results indicate that the functionalization of the chelating agent does not affect its binding ability towards Fe(III). The resulting functionalized silver nanoparticles are a promising SERS tag for operation in biological environments. The Fe–O stretching signature, arising when DFO–S grafted to silver nanoparticles binds Fe(III), could provide a tool for cation sensing in solution.

Carbon films and coatings are industrially very versatile and have a multiplicity of applications, such as conductive coatings or chemical and thermal protection layers. However, in most cases they are fabricated by the expensive vapor process, which has prevented a wider commercialization of these products. Herein, carbon films are fabricated using an inexpensive polymer-solution-based or sol–gel coating process, exemplified by spin coating of thin layers. As carbon precursor, the oligomeric form of acrodam, a high nitrogen-containing and non-volatile compound which is known to have excellent carbon yields, was employed. The oligomer is synthesized from commercially available starting materials (diaminomaleonitrile (DAMN) and acrolein) and dissolves in various organic solvents. The carbonized films were found to be homogeneous, optically flat, void- and crack-free, and were fabricated with up to 800 nm thickness after a carbonization step. High conductivities (up to 334 S cm −1 ) were achieved at the carbonization temperature of 1000 °C.

We report the structural analysis of the highly-doped lithium fulleride Li12C60, performed using low-temperature neutron powder diffraction. Although the main reflections could be initially indexed with a fcc cell, Monte Carlo Simulated Annealing suggests an unusual monoclinic arrangement for the fullerene molecules. In this structure, C60 units with the same orientation are alternatively stacked to form layers, thus maximising their sterical crowding. Rietveld refinement allowed to localise Li ions, which are organised in clusters belonging to the pseudo-tetrahedral voids with a non-complete charge transfer as confirmed by Raman spectroscopy.

The conservation study of the Roman Heritage in Aosta (Italy) is aimed at understanding the causes of the superficial chromatic alteration, which has affected the Roman ruins (I sec a. C.) since the last decades. The Roman monuments are built of pudding stone, a quite irregular and inhomogeneous conglomerate of fluvial origin. The stone in place started showing superficial chromatic alteration as reddish and pinkish areas in the last fifty years only. First results show that the alteration might be due to the oxidation of ferrous ion. The iron oxides spread out in the secondary calcite, thanks to fluid circulation through porous material, and crumble the pudding stone surface.

A straightforward PEGylation of SWCNTs has been performed by photomediated addition of various PEG derivatives onto the nanotube surfaces. The reaction was promoted both by UV and by solar light and took place under very mild conditions using commercial PEGs as PEGylating agents. The process relies on the formation of a-oxy radicals directly from PEGs by a photochemical hydrogen abstraction reaction mediated by a decatungstate salt. Actually, low-molecular-weight PEGs (e.g. PEG 200) added more efficiently than those having long polyethylene glycol chains (e.g. PEG 600). Interestingly, the functionalization caused a partial dissolution of the resulting CNTs in the reaction medium. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The paper describes the preparation of micropatterned zinc oxide films through a synthetic route based on the thermal degradation of metal-loaded poly(ethylene glycol)-based hydrogels. This approach requires very simple apparatuses and involves only simple, inexpensive and environmentally friendly chemicals. Highly transparent and crack free nanocrystalline films were obtained through this route. The evolution of the films during the thermal degradation and the influence of synthesis parameters on the film characteristics were investigated by thermal analysis, X-ray grazing incidence diffraction, scanning electron microscopy, atomic force microscopy and micro-Raman. The response toward reducing gasses was tested at different temperatures. Finally, the micropatterning of ZnO films through soft lithography has been characterized.

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Ionizing radiations from a 60-Co source have been employed for inducing radical reactions finally leading to the grafting of unsaturated olygomers and vinyl monomers onto the surface and micropores of precipitated silica Zeosil 116 5 (150- 170 m 2 /g). The target was to obtain modified silica with enhanced polar compatibility with respect to polybutadiene and styrene-butadiene copolymers rubber matrices and suited to favour the formation of chemically bound rubber in a vulcanization process. The latter property was expected to follow from the characteristics of free radical reactivity arising from the unsaturations present in the organic coating. Three classes of modified silica have been prepared: A)silica coated with polybutadiene olygomers with M n in the range 1000-5000 and having a different relative content of 1,4-cis, 1 , 4 -trans and vinyl double bonds [1,2] ; B)Silica coated with polybutadiene olygomers further modified by subsequent grafting of styrene monomer. The type A samples were obtained by γ irradiation under vacuum of silica samples preimpregnated with the oligomers in the dose range up to 30-200 kGy. Type B samples were obtained via three different steps as outlined in the scheme below: SiO 2 (PB) γ vacuum→/298K SiO 2 -PB γ air→/298K SiO 2 -PB-OOH Fe 2+ →/Sty SiO 2 -PB-O-PS The characterization of the modified silica was carried on by elemental and thermogravimetric analysis, FTIR (diffuse reflectance), Raman spectroscopy, inverse gas chromatography (IGC), CP/MAS 29-Si NMR, granulometry and TEM microscopy.

The last decade has come across an increasing demand for theranostic biocompatible nanodevices possessing the double ability of diagnosis and therapy. In this work, we report the design, synthesis and step-by-step characterization of rationally coated gold nanostars (GNSs) for the SERS imaging and photothermal therapy of HeLa cancer cells. The nanodevices were realized by synthesizing GNSs with a seed growth approach, coating them with a controlled mixture of thiols composed of a Raman reporter and a polyethylene glycol with a terminal amino group, and then reacting these amino groups with folic acid (FA), in order to impart selectivity towards cancer cells which overexpress folate receptors on their membranes. After a complete characterization, we demonstrate that these FA-functionalized GNSs (FA-GNSs) are able to bind selectively to the membranes of HeLa cells, acting as SERS tags and allowing SERS imaging. Moreover, we demonstrate that once bound to HeLa cell membranes, FA-GNSs exhibit photothermal effect which can be exploited to kill the same cells in vitro using laser irradiation in the NIR at a very low and safe irradiance. We thus demonstrate that the FA-GNSs designed following the described approach are an efficient prototype of theranostic nanodevices.

Thermal stability and phase homogeneity of the triphylite LiFePO4 compound were investigated by means of Raman spectroscopy. A detailed check of phase homogeneity of undoped samples obtained from different preparation routes—hydrothermal, sol–gel, and solid state synthesis—and Mn-doped compounds from solid-state synthesis was performed by means of a mapping of the Raman spectra. The triphylite compositional and structural properties were carefully investigated also with the help of structural refinements and magnetic techniques, which also allowed us to reveal and identify the impurity phases formed together with the olivine LiFePO4. The effect of laser irradiation on the triphylite thermal stability was thoroughly investigated and related to the synthesis route, to the doping, and to the sample homogeneity. The thermal stability of iron oxides, present both as synthesis products and as consequence of the irradiation itself, was also analyzed following the magnetitemaghemitehematite phase transformation. All the experimental observations concur in indicating that the effectiveness of the laser heating on these compounds mainly depends on grain size and the degree of order of the olivine structure, the highest thermal stability being displayed in the case of the nonhomogenous undoped samples obtained from solid-state preparation, which show a highly ordered triphylite phase. Copyright © 2010 John Wiley & Sons, Ltd.

We present the results of a systematic study performed by micro-Raman spectroscopy on pure anatase, pure rutile and mixed anatase–rutile TiO2 thin films, deposited by radio frequency magnetron sputtering on quartz substrates, with different thicknesses. The crystal structures of the as-deposited films were unambiguously determined and a good crystalline homogeneity was revealed by a systematic mapping of the samples. In the mixed-phase films, the relative amount of the two phases was monitored by a simple analysis of the components of the multi-Lorentzian fitting curves. For the single-phase films, the influence of the thickness and the effect of different thermal treatments, carried out to obtain series of thin films differing only for oxygen content, are discussed. The analysis of the scattered light has provided indication about the presence of an interface layer between the substrate and the film, which can play a role in driving the interesting magnetic properties exhibited by our samples, which are of potential usefulness for spintronics application. The results obtained from other techniques are briefly reported and discussed in relation to our systematic Raman characterization. This study points out how Raman investigation can provide suggestions toward the understanding of the complex physical phenomena leading to room-temperature ferromagnetism in TiO2 thin films. Copyright © 2009 John Wiley & Sons, Ltd.

Micro Raman and static magnetization data on manganese or cobalt-implanted (at 3 or 5 at.%) strontium titanate (Mn,Co):SrTiO3 single crystals are presented. Micro Raman results reveal a cubic perovskite structure at 300 K with no evidence of any secondary phase. All samples show ferromagnetism both at 300 K and at 10 K. Moreover, micro Raman spectra at low temperature reveal the presence of polar ordering, which allow to consider the investigated materials as possible host matrices for multiferroic phenomena. Obtained results are discussed in connection to supplementary outcomes derived by content, optical absorption and electron paramagnetic resonance (EPR) measurements.

From the analysis of the temperature dependence of the Electron Paramagnetic Resonance spectra for “as grown” and reduced SrTiO 3 :Mn crystals and ceramics it was proposed that the manganese incorporates into the strontium titanate mainly as small particles of MnO 2 or MnO.

Ferromagnetic Co:TiO 2 thin films have been grown by Co evaporation on a sputtered TiO 2 (100) surface. Annealing in ultra-high vacuum induced a recovery of the oxygen stoichiometry and consequently the oxidation of titanium to Ti 4+ at the surface, while oxidation of the Co layer has not been observed. Room temperature ferromagnetism is observed, along with a paramagnetic contribution. Differences of the hysteresis cycle with respect to that usually observed for bulk cobalt are ascribed both to the reduced film thickness and to the disorder induced by the sputtering treatment.

Structural, microstructural, and spectroscopic properties of Ba6Zr2Ta8O30 (BZT) and Ba6Zr1.94M0.06Ta8O30 (M = Ni, Co, Fe, Mn, Cr) related doped compounds are reported. The distribution of Zr, Ta, and M dopant ions on the cationic sites and the M valence states are discussed on the basis of the results obtained by the application of the Rietveld method on X-ray powder diffraction (XRPD) patterns and by electron paramagnetic resonance. The micro-Raman spectra of pure and doped BZT, reported and discussed for the first time, confirm the capability of the lattice to host transition cations in a glasslike matrix. The particle size effects of the samples are investigated by analyzing XRPD line broadening and scanning electron microscopy observations. Comparisons with the isostructural pure and doped Ba6Ti2Nb8O30 compounds are also reported.

Microstructural modifications, induced by femtosecond laser irradiation on Lithium Niobate (LN) crystals were investigated by means of optical microscopy and micro-Raman spectroscopy. Commercial z-cut congruent LN substrates were irradiated with focused ultra-short laser pulses at 810 nm. At the focus region either structural changes or material removal were observed by varying the irradiation conditions. The mapping of Raman spectrum revealed the insurgence of Raman modes, which are forbidden in the used scattering geometry. This phenomenon increases gradually by approaching the ablation region. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The light-induced charge transport process in as grown and thermally treated in oxygen atmosphere nominally pure KTaO3 (KTO) single crystals and KTO doped by Cu (KTO:Cu) and Fe (KTO:Fe) were studied by electron paramagnetic resonance (EPR). A sizeable increasing of the EPR signal of Cu2+ centers and reducing for Fe3+ centers was observed under UV illumination (365nm) in the oxidized KTO:Cu and KTO:Fe crystals respectively. This effect is considered as a sequence of the light induced charge exchange between Cu+ and Cu2+, and between Fe3+ and Fe2+ centers. Correlation between temperature dependencies of the EPR signals and photoconductivity was found and discussed too. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we report on micro-Raman analysis on lithium niobate (LN) substrates in order to study the compositional homogeneity of the crystals and to clear up the effects of etching and polishing processes on the surface of wafers and crystals. The fact that the linewidth of some Raman modes scale with the composition of LN crystals, together with the use of a confocal microscope, allowed a three-dimensional determination of the sample stoichiometry and of the crystalline quality. This local tool can supply additional information, which can be complementary to the electro-optic coefficients, carefully measured as well in order to check functional parameters. Raman spectra from buried regions were obtained on as-grown, etched and polished crystals and wafers. The depth profile of the peak energy and the linewidth of the Raman mode at 872 cm −1 indicate that mechanical processing of surfaces causes, in some cases, structural modifications till a depth of 15 μm.

We report a broadband H-1 NMR study of the spin dynamics of coated maghemite and goldmaghemite hybrid nanostructures with two different geometries, namely dimers and coreshells. All the samples have a superparamagnetic behavior, displaying a blocking temperature (T-B similar to 80 K (maghemite), similar to 105 K (dimer), similar to 150 K (coreshell)), and the magnetization reversal time follows the VogelFulcher law. We observed three different anomalies in H-1 NMR T(1)1 versus T that decrease in amplitude when increasing the applied magnetic field. We suggest that the anomalies are related to three distinct system dynamics: molecular rotations of the organic groups (240 < T < 270 K), superparamagnetic spin blockage (100 < T < 150 K), and surfacecore spin dynamics (T < 25 K). By fitting the T(1)1 data with a heuristic model, we achieved a good agreement with magnetic relaxation data and literature values for methyl group rotation frequencies.

In this work, we showed the results of the characterization of ether based electrolytes for Li-O2 batteries, prepared by mixing TEGDME and N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid (IL). The mixtures having different ratio TEGDME/IL and containing LiCF3SO3 salt, were thoroughly characterized by thermal analysis, conductivity and electrochemical stability measurements. Their potential use as electrolytes in Li-O2 batterieswas checked investigating Li/TEGDMEIL/ O2 cells by means of cycling voltammetry (CV) and galvanostatic charge-discharge tests. In this work, we showed the results of the characterization of ether based electrolytes for Li-O2 batteries, prepared by mixing TEGDME and N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid (IL). The mixtures having different ratio TEGDME/IL and containing LiCF3SO3 salt, were thoroughly characterized by thermal analysis, conductivity and electrochemical stability measurements. Their potential use as electrolytes in Li-O2 batterieswas checked investigating Li/TEGDMEIL/O2 cells by means of cycling voltammetry (CV) and galvanostatic charge-discharge tests.

Pure and doped Ba 6 Ti 2 Nb 8 O 30 (BTN), obtained by substituting M = Cr, Mn, or Fe on the Ti site (Ba 6 Ti 2-x M x Nb 8 O 30 , x = 0.06 and 0.18) and Y and Fe on the Ba and Ti sites, respectively (Ba 6-x Y x Ti 2-x Fe x Nb 8 O 30 , x = 0.18), are synthesized. The influence of cation doping on the local structure, the cation oxidation state, and the possible defect formation able to maintain the charge neutrality are investigated by spectroscopic (electron paramagnetic resonance (EPR) and micro-Raman), structural (X-ray powder diffraction) and transport (impedance spectroscopy, thermoelectric power) measurements, in the temperature range of 300-1200 K in air and N 2 flow. Starting from the valence state of the doping ions (Fe 3+ , Cr 3+ , and Mn 2+ ), determined by EPR, and from thermoelectric power measurements, evidencing a negative charge transport, different charge-compensating defect equilibria, based on the creation of positive electron holes or oxygen vacancies and electrons, are discussed to interpret the conductivity results.

EPR and optical absorption measurements have been performed on nearly stoichiometric lithium niobate single crystals with iron in traces and in 0.1% molar nominal amount. Thermal treatments in reducing atmosphere allowed to obtain samples with different Fe2+/Fe3+ ratios. The Fe2+/Fe3+ dynamic has been observed by irradiating the samples with UV and visible light in the temperature range 4-300 K: the photochromic effect resulted to be more evident in the strongly reduced crystals. A simple model to explain the photochromic behavior in nearly stoichiometric LiNbO3:Fe is proposed.

The UV illumination effects in KTaO3 single crystals with Cu2+ impurity ions have been studied by electron paramagnetic resonance method. Single crystals both “as-grown” and thermally treated in oxygen atmosphere were investigated. It was found that the EPR signal of the Cu2+ centers decreases in the “as-grown” crystals during UV-irradiation at 365 nm, in the wholeinvestigated temperature range (3.7- 300 K). On the contrary, for the oxidized specimens a sizeable increase of the EPR signal of Cu2+ centers (except the signal from exchange coupled pair centers) is observed under UV illumination in some temperature regions.

In order to understand the manner of incorporation of oxygen vacancies into perovskite structure oxides, KTaO3 crystals containing Cu2+ impurity ions were investigated following thermal treatments in atmosphere of H2, air, and vacuum. The results of thermal treatments were controlled by EPR. The influence of light illumination on copper centres in these crystals was studied also. The thermal treatment and light illumination effects in EPR spectra of Cu impurity centres are studied for the incipient perovskite-like ferroelectric KTaO3 single crystals. It was found that thermal treatment at ∼950°C in hydrogen atmosphere and in vacuum strongly depresses EPR signal of both types of Cu2+(1) and Cu2+(2) axial centres substituting Ta5+ in the B-sublattice positions. Subsequent high-temperature treatment in air restores EPR signals of Cu2+ centres, but changes the ratio between Cu2+(1) and Cu2+(2) centres concentration. These effects, together with UV light illumination effects observed for T < 80 K, are...

We present the first growth of KTaO3:Be (6% at. in the batch) single crystals, together with photoconductivity and thermally stimulated currents studies of this material. It is probably that very small Be2+ ions (ionic radius ∼0.35 A ) mostly substitute K+(1.64 A) and bounded and/or free electronic states formation including polaronic states. As a results Be-doping effect can strongly influence phase instability and electronic properties of KTaO3. It was found that Be2+ doping leads to appreciable photocurrents already at room temperature under irradiation of the samples with photons even at energies just below the fundamental optical gap. Photoconductivity increases strongly at low temperature. Intense thermally stimulated current peaks between 15 and 100 K are probably due to the presence of shallow hole traps and polaronic centres. The obtained results are compared to past observations in pure KTaO3 and KTaO3:Li (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron paramagnetic resonance experimental results of potassium tantalate single crystals doped by Co2+ ions as well as new results of our continuous studies of KTaO3:Cu single crystals are presented. Divalent impurity ions substitute for Ta5+ in the octahedron position in the KTO3 crystal and oxygen vacancies are required to compensate the extra charge of Ta5+. The Cu2+ ions associated with oxygen vacancies form only axial centres (included exchange coupled pair centre), the symmetry axes being along cubic directions. A new axial centre of Co2+ in the crystal of KTaO3:Co has been studied. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An unusual sequence of phase transitions (PT) and reentrant dipole glass-like phase formation at low temperatures was found recently in KTaO{sub 3} weakly doped with Li and Nb (K{sub 0.9986}Li{sub 0.0014}Ta{sub 0.976}Nb{sub 0.024}O{sub 3}) [Phys. Rev. B 63 (2001) 172]. We report on detailed low frequency (100 Hz-1 MHz) permittivity and Raman light scattering studies of similar composition, but without Li admixture, KTa{sub 1-x}Nb{sub x}O{sub 3} with x = 0.018 (KTN1.8). The aim of the study is to answer the question if the reentrant dipole glass-like phase exists in KTN1.8 and what is the microscopic origin of this phase. A detailed study of the sharp low-temperature PT observed at T{sub C} {approx} 27 K revealed properties inherent to the reentrant glass-type state at lower temperatures. The substitution of Nb for Ta influences the TO{sub 1} soft lattice mode and leads to PT with the long-range ferroelectric ordering. A crossover to an order-disorder polar microregion dynamics with a non-standard {var_epsilon}(T) behaviour and dipole glass-like formation were found below T{sub C} (at {approx}15 K), which is attributed to the randomness of the Nb distribution. A crossover to the long-range order was found under a dc bias field.

EPR results of potassium tantalate single crystals doped by Co2+ ions as well as new results of our continuous studies of KTaO3: Cu single crystal have been submitted. Divalent impurity ions substitute for Ta5+ in the octahedron position in the KTO3 crystal and oxygen vacancies are required to compensate the extra charge of Ta5+. The Cu2+ ions associated with oxygen vacancies form only axial centres (included exchange coupled pair centre), the symmetry axes being along cubic directions. New axial centre of Co2+ in the crystal of KTaO3: Co have been studied.

The high temperature behavior of Electron Paramagnetic Resonance (EPR) signals of Cu2+ centers and Cu2+-Cu2+ exchanged coupled pair centers in potassium tantalate (KTaO3, and related K1 − xLixTa1 − yNbyO3) single crystals has been studied. EPR signals were observed in the range of 300–1000 K. The dynamic behavior of oxygen vacancies and the influence of its on Cu2+ ions EPR spectra were found. A model for divalent impurity ions in the potassium tantalate crystal is proposed.

Electron paramagnetic resonance (EPR) of exchange-coupled pairs of C2+ ions was observed in KTaO3:Cu, K1−xLixTaO3:Cu and K1−xLixTa1−yNbyO3:Cu single crystals. The spectra are typical of centres with S = 1 and tetragonal symmetry with the pair axis parallel to the [100], [010] or [001] directions. The angular and temperature dependences of the signals have been studied. The experimental results are explained in terms of a pair centre model, which consists of a chain of two equivalent copper ions and three oxygen vacancies. The two copper ions occupy neighbouring tantalum sites and one of the oxygen vacancies lies between them, the chain running in the 100 direction. Ferromagnetic exchange coupling takes place between the copper ions, as confirmed by the thermal behaviour of the corresponding EPR spectra.

The Y2BaNiO5 compound, prototypical Haldane system characterized by anti-ferromagnetic behaviour, and its analogs Y2BaNi12xMxO5 (M ¼ Mg, Zn; 0:05 # x # 0:65) are investigated by X-ray diffraction and Raman spectroscopy. Structural features and phase stability ranges of the doped samples are first determined. The first order Raman scattering for the different solid solutions is measured at room temperature in order to gain new insight on lattice dynamics of the Y2BaNiO5 compound when Zn 2þ or Mg 2þ ions progressively substitute into Ni site.

We made Thermally Stimulated Conductivity (TSC), Thermoluminescence (TL) and Electron Spin Resonance (ESR) measurements on single crystals of potassium tantalate in the temperature range 4.2-290 v K. We revealed two sorts of O m shallow hole centers which are responsible for Photoconductivity (PC) and Photoluminescence (PL) enhancement. Both O m centers were identified by their ESR spectra. We show that these centers serve as radiative electron-hole recombination centers. The measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18-30 v K and 65-70 v K. Both TSC and TL are attributed to the thermal ionization of the same shallow donor centers related with oxygen vacancies. Experimental data were treated in a simple one-trap/one-recombination center model, which takes into account the presence of "thermally disconnected" deep electron traps.

We present a detailed investigation of structural properties of manganese doped strontium titanate ceramics and Verneuil grown single crystals, the last prepared with Sr deficiency compensated by proper Mn amount. EXAFS, TEM, ESR and Raman techniques, combining local and lattice probes, allows to inspect the local chemical environment and electronic structure of manganese taking into account possible chemical non-homogeneities and/or phase segregation. On the basis of experimental evidences, structural models for Mn insertion in SrTiO3 are discussed considering possibility for Mn ions to occupy both Ti4+ and Sr2+ sites as well as manganese segregation and Mn incorporation related non-homogeneities.

Raman investigations on single crystals of K 1-x Li x Ta 1-y Nb y O 3 were performed to identify structural effects arising from the simultaneous presence of off-centre Li and Nb ions in the incipient ferroelectric KTaO 3 . Two soft-mode transitions were evidenced at 45 K and 30 K in a sample with x = 0.0014 and y = 0.024 after both zero-field and field cooling. These temperatures are close to the maximum and to the extrapolated Curie temperature for non-relaxing permittivity reported in the same system. A sample with x = 0.0028 and y = 0.013 displayed a ferroelectric transition just below 30 K only after field cooling, also being consistent with permittivity data.

Polycrystalline EuBa 2 Cu 3 O y (EuBCO) samples with various oxygen contents ( y =6.25, 6.38 and 6.8) have been analysed by means of microRaman spectroscopy. The usual Raman features of REBa 2 Cu 3 O y high temperature superconductors (RE=Y, Eu, Sm, Er…) have been confirmed also for EuBCO. A correlation between the crystal structure and the conductivity properties as a function of the oxygen content has been established by studying in detail the features of the different Raman spectra. In particular, the analysis of the peak profiles of lattice vibrations involving oxygen atoms provides important information to understand the anomalous superconducting properties found in the sample with y =6.38. In addition, the appearance in the superconducting samples of a new mode centred at 223 cm −1 may be correlated to ordering phenomena which have been called to justify the conductivity results.

Raman scattering observations were performed on samples of K1-xLixTa1-yNbyO3, with x in the range 0.6-1.0 mol% and y in the range 0.2-0.3 mol%. Concentrations of Li and Nb, determined by analytical methods, were below the known thresholds at which these substituents can separately induce a ferroelectric transition in the quantum paraelectric KTaO3. The temperature evolution of TO1 and TO4 spectra was monitored. The results show a critical behaviour below 50 K, characterized by an abrupt increase of first-order scattering strengths and by spontaneous splitting of the non-softening TO1 mode. From our data, we infer the occurrence of a ferroelectric phase transition of order-disorder character. This proves that small additions of Nb enhance the tendency of the Li subsystem towards cooperative ordering in the KTaO3 matrix.

Electron paramagnetic resonance (EPR) of copper centers in KTaO3:Li:Cu single crystals have been studied. Superhyperfine (SHF) structure of an exchange pair Cu2+-Cu2+ center, associated with the eight surrounding nearest-neighbor potassium ions was found. The EPR signal behaviour with the temperature may characterize the dielectric properties of crystals at the microwave frequency.