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Oxides with the CaFe2O4-type structure have been predicted as being suitable hosts for reactions of intercalation of light cations such as Li and Mg because of their favorable cationic diffusion. A...

Perovskite La_0.3Ca_0.7Fe_0.7Cr_0.3O_(3-delta) (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 degrees C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from delta = 0.272(7) at RT to delta = 0.333(5) at 900 degrees C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 degrees C for 1 h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible.

In a recent theoretical study [Jacobs et al., Adv. Energy Mater., 2018, 8, 1702708], BaFe0.125Co0.125Zr0.75O3−δ was predicted to be a stable phase with outstanding performance as an auspicious cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). It is shown here that the theoretical predictions are valid. The material can be synthesized by the citrate method as a single cubic Pmm phase with a significant amount of oxygen vacancies, randomly distributed in the anionic sublattice facilitating oxygen vacancy conduction. A thermal expansion coefficient of 8.1 × 10−6 K−1 suggests acceptable compatibility with common electrolytes. Electrochemical impedance spectroscopy of symmetrical cells gives an area-specific resistance of 0.33 Ω cm2 at 700 °C and 0.13 Ω cm2 at 800 °C. These values are reduced to 0.13 Ω cm2 at 700 °C and 0.05 Ω cm2 at 800 °C when the material is mixed with 30 wt% Ce0.9Gd0.1O2−δ.

Phase-pure polycrystalline Sn1−δS has been prepared using a one-step microwave-hydrothermal procedure. Its structural characterization by neutron powder diffraction and transmission electron microscopy shows an orthorhombic Pnma phase at room temperature with the presence of a significant amount of Sn vacancies (Sn0.87S), randomly located in the crystal structure. This work proves that this inexpensive 2D material, constituted by high abundance elements, is a promising candidate for thermoelectric applications. This is confirmed by its relatively high carrier density and good mobility (2.5 × 1018 cm−3 and 13 cm2 V−1 s−1 at room temperature, respectively) found in the microwave-hydrothermal-synthesized sample, which favours high thermoelectric performance. In addition, a remarkably high Seebeck coefficient (682 μV K−1) combined with low electrical resistivity (5.6 × 10−4 Ω m) and thermal conductivity (0.57 W m−1 K−1) at 523 K produces a reasonably high figure of merit ZT of 0.76.

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr2YCu2O7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions (magnetic ordering temperature TN = 110 K > superconducting critical temperature Tc = 70 K). The somewhat unexpected A-type AFM structure, with a μ(Fe) ∼ 2 μB magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.

International audience; Metal fluorides with 3D open structures, pyrochlore (pyr) or hexagonal tungsten bronze (HTB), are promising materials as positive electrodes for rechargeable batteries or catalysts. Herein, we have developed a two-step synthesis procedure to obtain new anhydrous mixed-metal-cation fluorides crystallizing in the pyrochlore structure. The first step consists of preparing mixed-metal ammonium fluorides (NH4)M2+Fe3+F6 (M = Mn, Fe, Co Ni) using different synthesis strategies. For M = Mn, three allotropic varieties of (NH4)Mn2+Fe3+F6 are obtained; two phases adopt the expected pyrochlore network with either the cubic Fm3̅m or the orthorhombic Pnma space group, and the third phase exhibits a 3D network with narrow pseudotriangular cavities. 57Fe Mössbauer spectrometry indicates that the crystal structures are governed by the Fe3+/Mn2+ cationic order or disorder. The second step is a topotactic oxidation of pyr-(NH4)M2+Fe3+F6 under a molecular F2 flow. To better understand the reaction mechanism, the topotactic oxidation was followed by thermogravimetry, XRD, FTIR, and Mössbauer spectrometry. The successful synthesis of the first anhydrous pyr-M3+0.5Fe0.5F3 provides a new route to prepare anhydrous mixed-metal fluorides pyrochlore with empty cavities of the open framework.

This communication reports the microwave-assisted hydrothermal synthesis of the x = 0.1 member of the solid solution Ba1–xCaxTiO3 (BCT) materials, its structural and microstructural characterisatio...

The M-1212 family of cuprates, with general formula $$\hbox {M}_{{x}}\hbox {Cu}_{1-{{x}}}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{7+{\delta } }$$ (M = transition metal; RE = Rare Earth), can be stabilized at room pressure and continues to be the basis for the study of superconducting cuprates by means of different total or partial substitutions. Here, we explore two interesting possibilities through substitution of the Cu in the chains (forming the charge reservoir layer) by Mo and Fe, leading to the $$\hbox {Mo}_{0.3}\hbox {Cu}_{0.7}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{7+\delta }$$ (RE: Tm and Yb) and $$\hbox {Fe}_{{x}}\hbox {Cu}_{1-{{x}}}\hbox {Sr}_{2}\hbox {YCu}_{2}\hbox {O}_{7+\delta }$$ (x = 0.5) systems respectively—followed by RED/OX processes. Oxidation techniques such as ozonisation and high-pressure-oxygen treatment have been used to increase the critical temperature $$T_\mathrm{c}$$ of the present compounds, looking at the crystal structural evolution of these materials in relation with the $$T_\mathrm{c}$$ enhancement. We have considered, in particular, some of the structural parameters that seem to go in parallel with the $$T_\mathrm{c}$$ increasing: inter and intra-bilayer spacing, apical distance and buckling angle at the superconducting plane. Although the $$\hbox {Mo}_{0.3}\hbox {Cu}_{0.7}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{\mathrm{y}}$$ system does not show superconductivity as synthesized in air, a $${T_\mathrm{c}}\approx 32\,\hbox {K}$$ is achieved after oxygen or ozone flowing at low temperature and it increases by almost three times to a $${T_\mathrm{c}} \approx $$ 83 K under high-pressure-oxygen treatment . On the other hand, the $$T_\mathrm{c}$$ for the $$\hbox {Fe}_{0.5}\hbox {Cu}_{0.5}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{\mathrm{y}}$$ system is increased from $${T_\mathrm{c}} \approx 30\,\hbox {K}$$ up to $${T_\mathrm{c}} \approx 50\,\hbox {K}$$ after oxygenation under ozone annealing.

Solid oxide fuel cells are highly efficient, low-emission, and fuel-flexible energy conversion devices that can also run in reverse as solid oxide electrolysis cells, converting CO2 and/or H2O to useful fuels and pure O2. Our team has recently developed a highly promising mixed conducting oxide catalyst (La0.3Ca0.7Fe0.7Cr0.3O3−δ) that can be used at both the anode and cathode in either the fuel cell or electrolysis mode in a lower-cost symmetrical cell. However, there is still a need to improve material processing and cell manufacturing methods in this field. Here, we report, for the first time, fabrication of a symmetrical solid oxide cell, based on our very promising catalysts, using rapid, low-cost, low-energy, and green microwave (MW) processing techniques. These cells were fabricated with MW-sintered powders and were then MW-sintered without the use of any MW susceptors inside the electrode layers or any additional presintering steps. The catalyst layers show very stable nanostructures and do not delaminate, and the cells exhibit reaction rates that are similar to those obtained using normal ceramic processing methods. Importantly, the powder preparation and cell sintering steps, carried out using MW methods, require only ca. 1/3 and 1/9 of the time/energy, respectively, versus those required in traditional furnace methods, thus translating to significant cost savings.

β-V2O5, obtained by a high-temperature high-pressure method, exhibits a layered structure that favours the insertion of Na+. In this work, we report the electrochemical insertion of sodium in high pressure β-V2O5 and its performance as cathode material for sodium-ion batteries. The material shows a first discharge capacity of 132 mAh g−1 in the 3.6–2.0 V range at a C/20 current density and a maximum capacity of 147 mAh g−1 under equilibrium conditions, corresponding to the insertion of 1 Na+ ion per formula unit. The β-V2O5//Na cell delivers a specific energy as high as 370–410 Wh kg−1. The amount of inserted sodium points to the reduction of 50% of the available V5+ ions. After 20 cycles, the discharge capacity retains 86% of the initial capacity. Concerning the reaction mechanism of high pressure β-V2O5 upon sodium insertion/de-insertion, several phase transitions are inferred from the voltage-composition profile. Ex situ XRD reveals the appearance of several NaxV2O5 phases in the 0 ≤ x ≤ 1 compositional range, which are closely related with the non-sodiated high pressure β-V2O5 structure. An irreversible structural transformation occurs during the very first inserted sodium, at the beginning of the first discharge, and the original high pressure β-V2O5 structure is not recovered upon full Na+ extraction. Electrochemical performances are outstanding though.

New 1212- type Molybdo-Cuprates of with composition Mo0.5Cu0.5Sr2RECu2O7.5 (RE = Rare Earth) have been prepared by High Pressure and High Temperature (HPHT) synthesis. Their crystal structures are characterized by combining the X-Ray/Neutron powder diffraction and electron diffraction techniques. All the materials show tetragonal symmetry, crystallizing in the P4/mmm space group (S.G.). The chain oxygens are randomly distributed in the two different oxygen sites, which are not completely filled and the defect induced by oxygen vacancies in fact makes the chain fragmented and disordered. The microstructure of these compounds is interpreted by a well-known diagonal cell √2ap × √2ap × 3ap, as confirmed by the Electron diffraction (ED) and transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) studies show the predominance of the non magnetic MoVI state over the MoV one. At the same time oxidation state of copper is found to be dominated by CuII.

Maghemite-like materials containing Fe(3+) and Cr(3+) in comparable amounts have been prepared by solution-combustion synthesis. The conditions of synthesis and the magnetic properties are described. These materials are ferrimagnetic and are much more stable than pure iron maghemite since their maghemite-hematite transformation takes place at about ∼ 700 °C instead of ∼ 300 °C, as usually reported. These materials were studied by synchrotron radiation X-ray diffraction (XRD) and by X-ray absorption fine structure (XAFS) of the K-absorption edge of two elements. High-resolution XRD patterns were processed by means of the Rietveld method. Thus, maghemites were studied by XAFS in both Fe and Cr K-edges to clarify the short-range structure of the investigated systems. Pre-edge decomposition and theoretical modeling of X-ray absorption near edge structure transitions were performed. The extended X-ray absorption fine structure (EXAFS) spectra were fitted considering the facts that the central atom of Fe is able to occupy octahedral and tetrahedral sites, each with a weight adjustment, while Cr occupies only octahedral sites. Interatomic distances were determined for x = 1, by fitting simultaneously both Fe and Cr K-edges average EXAFS spectra. The results showed that the cation vacancies tend to follow a regular pattern within the structure of the iron-chromium maghemite (FeCrO3).

In this work, we examine the benefits of alternative powder processing methods, with a primary focus on microwave-based synthesis, that could both lower material manufacturing costs and further enhance cathode performance for solid oxide fuel cell applications. La 0.3 Ca 0.7 Fe 0.7 Cr 0.3 O 3− δ (LCFCr), formed using conventional solid-state methods, has been shown in earlier work to be a very promising catalyst for the oxygen reduction reaction. To further increase its performance, microwave methods were used to increase the surface area of LCFCr and to decrease the synthesis time. It was found that the material could be obtained in crystalline form in only 7 h, with the synthesis temperature lowered by roughly 300 °C as compared to conventional methods.

Legislation to replace lead-based ferroelectric ceramics for electromechanical transducers (commonly made of lead titanate-zirconate (PZT)) started in Europe shortly before the year 2000 in the context of elimination of toxic substances from electric and electronic equipment. From that date there has been a large, worldwide research effort to develop new ecological, lead-free compositions that can substitute PZT. Some issues involved in this effort will be reviewed here, from the synthesis procedures to functional properties and trends. New properties on lead-free systems that surpass those of PZT (bismuth sodium titanates for actuators) and enhanced properties of other compositions in which lead-free materials are already at work in devices will be analyzed in this chapter. Present limits of well-known lead-free materials (barium titanate, Aurivillius-type structure compounds, sodium potassium niobate, bismuth sodium titanate, and barium calcium titanate-zirconate) will be summarized, and new and recently studied compositions based on materials with high spontaneous polarization will be considered as future lead-free alternatives.

Hitherto unreported vanadium oxyfluoride VO2F has been synthesized using a solid state reaction at a pressure of 4 GPa and 800 °C. This long awaited vanadium oxyfluoride fills the existing gap of ReO3-type MO2F compounds of Group 5 elements, from which only NbO2F and TaO2F have been known to exist to date. VO2F crystallizes with the VF3-type structure, space group Rc, with a = 5.1226(1) A and c = 13.0686(3) A as determined by powder X-ray diffraction. Highly structured diffuse streaking observed in electron diffraction patterns evidences local O/F ordering. VO2F exhibits two regions upon discharge in a lithium cell, an upper sloped region in the range of 3.9–2.2 V and a lower plateau at 2.15 V. Discharge of VO2F to 1 V provides a gravimetric capacity of 450 mA h g−1. VO2F can reversibly insert up to 1 Li+ per vanadium above 2.15 V without destruction of the host structure, delivering a gravimetric capacity as high as 250 mA h g−1 and pointing to VO2F as a promising intercalation electrode.

Detailed studies of the electronic states for Mo(0.3)Cu(0.7)Sr(2)ErCu(2)Oy samples with different oxygen contents are presented here. The influence of oxygenation on the electronic states for the Mo(0.3)Cu(0.7)Sr(2)ErCu(2)Oy system from the semiconducting to the superconducting state has been investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS studies show that Mo is in a mixed Mo(V) and Mo(VI) oxidation state and Mo(V) is predominant over the Mo(VI) in the as-prepared (AP) sample. Yet annealing under an oxygen atmosphere enhances the Mo(VI) state. At the same time, a reduction in the copper species is observed. In the Cu 2p spectra, a larger energy separation between the satellite and main peaks (E(S)-E(M)) and a lower intensity ratio (I(S)/I(M)) are found to correlate with higher values of the superconducting transition temperature (T(C)). Analysis of these spectra within the Configuration Interaction (CI) model suggests that higher values of TC are related to lower values of the O 2p-Cu 3d charge transfer energy. The change in the Sr 3d and O 1s core level spectra correlates with the oxygen insertion in the (Mo/Cu)O(1+δ) chain site, after oxygenation. The hole concentration (Ph) in the copper plane has been obtained using the room temperature thermoelectric power (TEP) value; this shows an increasing tendency with increasing T(C), after oxygenation. From these experimental results, one observes that T(C)increases with decreasing charge transfer energy. This is, indeed, opposite to the accepted views and occurs in parallel with the shortening of the apical copper-oxygen distance (Cu(2)-O(2)) and the increasing of the CuO(2) plane buckling angle.

A novel mixed-conducting perovskite oxide (La0.3Ca0.7Fe0.7Cr0.3O3−δ, LCFCr), prepared using the combustion method, has been developed as a new air electrode material for reversible solid oxide fuel cells (RSOFCs) applications. XRD analysis showed that the LCFCr powder is a pure crystalline phase with an orthorhombic perovskite structure and that its thermal expansion coefficient matches closely with several commonly used solid electrolytes, while TEM analysis confirmed the high purity of the LCFCr powders. The electrochemical activity of LCFCr, screen-printed on a gadolinium doped ceria electrolyte, towards both the oxygen reduction (ORR) and oxygen evolution (OER) reactions, was examined at 600–800 °C in stagnant air using a symmetrical RSOFC configuration. Under open circuit conditions, LCFCr showed a very low polarization resistance (Rp) of only 0.07 Ω cm2 at 800 °C, comparable to some of the best-performing oxide materials reported by others. The LCFCr air electrode was also found to be very stable, with very little loss in performance and no interfacial damage observed, even after 100 h at a 0.4 V (OER) and −0.4 V (ORR) overpotentials.

In this work we report on the microwave assisted synthesis of nano-sized Gd2Ti2O7 (GTO) and Ho2Ti2O7 (HTO) powders from the RE2Ti2O7 pyrochlore family (RE = rare earth). Synchrotron X-ray powder diffraction was used to study RE–Ti cationic anti-site defects with concentrations that decrease in both samples with increasing temperature starting from 1100 °C, and the defects disappear at 1400 °C. SQUID magnetometry measurements revealed that GTO shows a predominantly anti-ferromagnetic structure, whereas HTO exhibits magnetic saturation and a ferromagnetic component at low temperature. Impedance spectroscopy data revealed strongly increased ionic oxygen vacancy conduction in HTO ceramic pellets as compared to GTO, which may be associated with a higher degree of oxygen vacancy disorder. This argument was supported by Raman spectroscopy data.

We describe the synthesis, crystal structure, magnetic and electrochemical characterization of new rock salt-related oxides of formula, Li{sub 3}M{sub 2}RuO{sub 6} (M=Co, Ni). The M=Co oxide adopts the LiCoO{sub 2} (R-3m) structure, where sheets of LiO{sub 6} and (Co{sub 2}/Ru)O{sub 6} octahedra are alternately stacked along the c-direction. The M=Ni oxide also adopts a similar layered structure related to Li{sub 2}TiO{sub 3}, where partial mixing of Li and Ni/Ru atoms lowers the symmetry to monoclinic (C2/c). Magnetic susceptibility measurements reveal that in Li{sub 3}Co{sub 2}RuO{sub 6}, the oxidation states of transition metal ions are Co{sup 3+} (S=0), Co{sup 2+} (S=1/2) and Ru{sup 4+} (S=1), all of them in low-spin configuration and at 10 K, the material orders antiferromagnetically. Analogous Li{sub 3}Ni{sub 2}RuO{sub 6} presents a ferrimagnetic behavior with a Curie temperature of 100 K. The differences in the magnetic behavior have been explained in terms of differences in the crystal structure. Electrochemical studies correlate well with both magnetic properties and crystal structure. Li-transition metal intermixing may be at the origin of the more impeded oxidation of Li{sub 3}Ni{sub 2}RuO{sub 6} when compared to Li{sub 3}Co{sub 2}RuO{sub 6}. Interestingly high first charge capacities (between ca. 160 and 180 mAh g{sup −1})more » corresponding to ca. 2/3 of theoretical capacity are reached albeit, in both cases, capacity retention and cyclability are not satisfactory enough to consider these materials as alternatives to LiCoO{sub 2}. - Graphical abstract: Two new rock salt related oxides of formula, Li{sub 3}M{sub 2}RuO{sub 6}, (M=Co, Ni) have been prepared. The M=Co oxide adopts the LiCoO{sub 2} (R-3m) structure and the M=Ni oxide adopts a similar layered structure related to Li{sub 2}TiO{sub 3,} monoclinic (C2/c), with partial mixing of Li and Ni/Ru atoms. For Li{sub 3}Co{sub 2}RuO{sub 6}, oxidation state for Ru is 4+ and antiferromagnetic (AFM) order is found below 10 K while for the analogous Li{sub 3}Ni{sub 2}RuO{sub 6} , Ru oxidation state is 5+ and a ferrimagnetic (FM) behavior with a Curie temperature of 100 K is found. Electrochemical studies correlate well with both magnetic properties and crystal structure. - Highlights: • New Ruthenium rock salt-related oxides of formula, Li{sub 3}M{sub 2}RuO{sub 6} (M=Co, Ni) were studied. • Structurally different: rhombohedral α-NaFeO{sub 2}-type (Co) and monoclinic Li{sub 2}TiO{sub 3}-type (Ni) • Magnetic behavior is different: the Co sample is AFM while the Ni one is ferrimagnetic. • Ruthenium oxidation states are different: 4+ in the Co sample and 5+ in the Ni one. • Electrochemical studies correlate well with magnetic properties and crystal structures.« less

H 2 V 3 O 8 nanobelts have been successfully synthesised from commercial V 2 O 5 powder through a fast and environmental friendly microwave-hydrothermal method. X-ray diffraction, field-emission scanning electron microscopy, thermogravimetric analysis, infrared spectroscopy, high-resolution transmission electron microscopy and ICP spectroscopy were used to characterise the morphology and structure–microstructure details. Nanobelts about 100 nm wide and several micrometres long are easily prepared in no more than 2 h. The electrochemical study reveals the reversible insertion of ca. 4 Li per formula unit (400 mAh g −1 ), through several pseudo- plateaus in the 3.75–1.5 V vs Li + /Li voltage range showing the interest of this material produced by a “green” route as an electrode for lithium rechargeable batteries. After the first cycle a significant capacity loss is observed, though a high capacity, ca. 300 mAh g −1 , remains upon cycling. Furthermore, the similarity of discharge and charge curves, pointing to the absence of hydrogen displacement during lithium insertion in H 2 V 3 O 8 , shows that not all protonated systems must be discarded as prospective electrode materials. On the other hand, further reduction down to 1 V is possible to insert up to 5 Li per formula unit (480 mAh g −1 ). Interestingly it corresponds to full reduction of vanadium to V 3+ as it is also confirmed by EELS experiments. However, the full reduction to V 3+ is associated with a fast decay of the extra capacity developed at low voltage with increasing current rate. Then for practical use we may consider only the capacity obtained down to 1.5 V.

Research and development of lead-free piezoelectric materials are still the hottest topics in the field of piezoelectricity. One of the most promising lead-free family of compounds to replace lead zirconate-titanate for actuators is that of BiNaTiO (BNT) based solid solutions. The pseudo-binary (1 - x)BiBiNaTiO-xBaCaTiO system has been proposed for high temperature capacitors and not yet fully explored as piezoelectric material. In this work, the solid solution with x = 0.06 and y = 0.10 was obtained by two different synthesis routes: solid state and Pechini, aiming at using reduced temperatures, both in synthesis (, R. Vivar-Ocampo would like to thank CONACyT-México for providing a PhD scholarship. M.E. Villafuerte-Castrejón also kindly acknowledges the financial support of PAPIIT-UNAM by DGAPA under project IN 102715. E. Morán and D. Ávila thank Ministerio de Economía y Competitividad and Comunidad de Madrid for funding the projects MAT2013-46452-C4-4-R and S2013/MIT-2753. A. M. González thanks to Ministerio de Economia y Competitividad for funding the project MAT2013-40722-R. Villafuerte-Castrejón and the Short Stay of R. Vivar-Ocampo at ICMM-CSIC (Madrid, Spain) with the support of PASPA-UNAM and CONACYT.

International audience; In this work, we describe the use of novel environmentally friendly industrial processing techniques to produce nano-sized crystallite powders and fabricate bulk ceramic pellets of functional materials. We present the application of a microwave (MW)-assisted synthesis process to fabricate the un-doped and doped ceria phases CeO2−δ, Ce0.8Gd0.1Sm0.1O1.9, Ce0.85Gd0.15O1.925, Ce0.85Sm0.15O1.925 and Ce0.8Sm0.18Ca0.02O1.9 for potential application in solid oxide fuel cells. We find that the composition Ce0.8Sm0.18Ca0.02O1.9 may be most beneficial owing to the highest ionic conductivities encountered in the corresponding sintered ceramics. As a next step, we tested a MW-assisted sintering process to fabricate dense Ce0.8Sm0.18Ca0.02O1.9 pellets. We find that both the synthesis and sintering process can be achieved by using MW heat sources and the entire fabrication process may be regarded a “green ceramic processing”.

In this work we investigate the magnetic, dielectric and charge transport properties of LaCoO3 (LCO) synthesized by two different techniques: microwave assisted and conventionally heated ceramic synthesis. The rapid microwave synthesis conditions are far away from thermodynamic equilibrium and are found to lead to modified crystal defect properties as compared to conventional synthesis. The thermally induced magnetic spin state transition at Ts1 ≈ 80 K is exemplified by temperature (T)-dependent dielectric spectroscopy data, which reveal the appearance of an additional dielectric contribution that is correlated to the transition. Magnetisation, M vs T, and electrical resistivity, ρ vs T, curves show that the additional dielectric phase is strongly influenced by magnetic defects and may be associated with higher spin state clusters in a magnetic spin-state coexistence scenario. We suggest that defects such as oxygen vacancies act as magnetic nucleation centres across the spin state transition Ts1 for the formation of higher spin state clusters in LCO perovskites.

Ce 0.85 RE 0.15 O 1.925 (RE = Gd, Sm), Ce 0.8 (Gd 0.1 Sm 0.1 )O 1.9 and CeO 2− δ nano-powders were synthesized by microwave-assisted hydrothermal synthesis in a time and energy efficient way. The fluorite-type crystal structure of the synthesized nano-powders was confirmed by X-ray diffraction (XRD) and Rietveld refinement, and the nano-metric particle sizes calculated from the XRD line width broadening were consistent with transmission electron microscopy observations. The Brunauer–Emmett–Teller method was used to confirm large powder surface area. Scanning electron microscopy confirmed high density and low surface porosity of the sintered ceramics as a result of the high sintering activity of the large surface area nano-powders. Impedance spectroscopy was carried out to separately analyze grain boundary (GB) and bulk dielectric relaxations where GB areas were found to constitute ionic charge transport barriers, because their resistance was larger than that of the bulk. Such barriers were found to be comparatively low due to high GB and bulk ceramic ionic conductivities, with the highest values encountered in the composition Ce 0.85 Sm 0.15 O 1.925 . The optimal ionic conductivities encountered were associated with the effect of high sintering activity of the nano-powders.

The title compound has been synthesized by the hydroxide route. The crystal structure has been investigated at room temperature from high-resolution neutron powder diffraction (NPD) data. It crystallizes in a cubic spinel structure, space group F d 3 ¯ m , Z = 8, with a = 8.3940(2) A at 295 K. The crystallographic formula is (Ni 0.202(1) Mn 0.798(1) ) 8a (Ni 0.790(1) Mn 1.210(1) ) 16d O 4 where 8 a and 16 d stand for the tetrahedral and octahedral sites of the spinel structure, respectively. There is a significant inversion degree of the spinel structure, λ = 0.80. In fact, the variable parameter for the oxygen position, u = 0.2636(4), is far from that expected ( u = 0.25) for normal spinels. From a bond-valence study, it seems that the valence distribution in NiMn 2 O 4 spinel is not as trivial as expected (Ni 2+ and Mn 3+ ), but clearly the tetrahedral Mn ions are divalent whereas the octahedral Mn and Ni are slightly oxidized from the expected +3 and +2 values, respectively. The mixed valence observed at the octahedral sites provides the charge carriers that, by a hopping mechanism between Mn 3+ /Mn 4+ adjacent sites, leads to a significant conductivity, up to 0.85 S cm −1 at 800 °C in air.

Rare-earth chromite ErCrO 3 powder was synthesized from metal nitrate precursors using microwave synthesis. (Micro-) structural characterizations were performed using X-ray diffraction, Rietveld refinement, High Resolution Transmission Electron Microscopy and Electron Micro-Diffraction. Magnetization vs. temperature measurements revealed anti-ferromagnetism with T Neel ≈ 135 K. An anti-ferrimagnetic moment of ≈0.4 μ B was determined from magnetization vs. applied field measurements. Temperature dependent impedance spectroscopy (IS) indicated 3 dielectric relaxation processes: electrode interface, grain boundary and bulk. The intrinsic bulk activation energy was found to be 0.27 eV and the dielectric permittivity ɛ r was ≈23 in excellent agreement with Clausius–Mossotti predictions and showed no perceptible temperature dependence. This and the low ɛ r value suggested that ErCrO 3 is a dielectric rather than ferroelectric compound. IS measurements with applied dc bias revealed the signs of an unconventional type of Schottky barrier at the metallic Au electrode/ceramic ErCrO 3 interface.

The half-metallic ferromagnet K(2)Cr(8)O(16) with the hollandite structure has been chemically modified using soft chemistry methods to increase the average oxidation state of chromium. The synthesis of the parent material has been performed under high pressure/high temperature conditions. Following this, different redox reactions have been carried out on K(2)Cr(8)O(16). Oxidation to obtain potassium-de-inserted derivatives, K(2-x)Cr(8)O(16) (0 ≤x≤ 1), has been investigated with electrochemical methods, while the synthesis of sizeable amounts was achieved chemically by using nitrosonium tetrafluoroborate as a highly oxidizing agent. The maximum amount of extracted K ions corresponds to x = 0.8. Upon oxidation the hollandite structure is maintained and the products keep high crystallinity. The de-insertion of potassium changes the Cr(3+)/Cr(4+) ratio, and therefore the magnetic properties. Interestingly, the Curie temperature increases from ca. 175 K to 250 K, getting therefore closer to room temperature.

Multiferroic BiFeO3 has been rapidly synthesized by a microwave – hydrothermal method using nitrates as the metallic source. Structural characterization was performed by thermal analysis, X-ray diffraction and transmission electron microscopy. Generally accepted trigonal space group R3c, as well as recently suggested monoclinic symmetries, were assayed in the search for the best fit. Due to the ambiguity of the Rietveld refinement to distinguish between crystal systems, a micro-diffraction and HRTEM study has been performed. The best solution was obtained with the trigonal model. The room-temperature Mossbauer spectra reveal the presence of a small fraction (2%) of iron in low spin configuration.

In this work, the sol-gel synthesis, structural characterization, and transport properties of a new solid solution of the general formula La(x)Bi(1-x)Fe(0.5)Cr(0.5)O(3) (0.4 ≤ x ≤ 1) are presented. The solubility limit x has been determined and variation of the lattice parameters measured through profile fitting. The cell parameters, space group, and atomic positions, as obtained by the Rietveld refinement of X-ray diffraction data, are reported. This analysis and electron diffraction studies as well do not reveal any evidence of Fe/Cr ordering. Regarding the transport properties, magnetic and electric characterizations are described. The electrical response with the temperature and frequency has been studied, and a "positive temperature coefficient" for the resistivity has been found for temperatures between 270 and 400 °C. The magnetic behavior is striking because, for all materials studied, zero-field-cooling curves appear above field-cooling ones, an anomalous feature that is interpreted as being due to complex ferromagnetic/antiferromagnetic interactions in the B perovskite sublattice.

A new procedure to prepare oxides materials from oxalates complexes precursors is presented. Detailed description of the method is reported, illustrated with examples of the preparation of oxides. A solid-solid reaction between the oxalic acid and nitrate salt of metal leads to the formation of the corresponding oxalate. These oxalates are used as precursors and are heated to form the corresponding oxides by thermal-decomposition. This article relates the first examples of oxides prepared by this new method: the synthesis of the copper oxide CuO, the hexagonal phase of lanthanum oxide La2O3, and the lanthanum cuprate La2CuO4. All the samples are characterized by SEM and XRD. Thermal analysis TGA and DTA show the expected thermo-decomposition behavior for both copper and lanthanum oxalates. This study reveals also the formation mechanism of the cuprate La2CuO4 from the used precursor.

Electrodes consisting of composites of nanosized-LaPO4/Carbon-Super P yield irreversible capacities as large as 872 mAh/g carbon (down to 0.2 V), which would be equivalent to the reaction of 0.9 lithium ions with LaPO4. Increasing the crystallite size of LaPO4 drastically decreases the specific capacity delivered by the cell, reaching a minimum value of 330 mAh/g carbon for micrometer-LaPO4. A significant capacity is also detected in nanosized LaPO4 /Carbon-KS6 and LaPO4/Ni electrodes. An in situ XRD investigation proves that the bulk of LaPO4 does not react with lithium, evidencing that LaPO4 is useless as electrode material for lithium cells. The reactivity of the electrode is likely due to the electrolyte decomposition on the surface of both the electronic conductor additives (C or Ni) and the LaPO4 nanoparticles.

Double perovskites which exhibit 1:2 B-site ordering are under focus because of their interesting structure−property relationships and high chemical versatility. Perovskites with a big cation, such as Sr2+ or Ca2+, occupying the octahedral B site are very scarce because the calculated Goldschmidt tolerance factor becomes quite small as these types of cations usually occupy the A-site. This work describes the synthesis and structural and microstructural characterization of a new calcium and strontium antimoniate which shows face-centered cubic symmetry and a rock-salt ordered distribution of Sb5+ and Sr2+. The ordered-cation distribution occurs due to the differences in both the ionic sizes and the bonding character of the two B-site cations. As a consequence of the cation stoichiometry of the title phase and assuming a pentavalent state for Sb, a large concentration of oxygen vacancies are created (y ∼ 0.25). Diffraction studies reveal that these vacancies tend to arrange in a short-ordered way and are re...

A series of lanthanum perovskites, LaMO 3 (M = Al, Cr, Mn, Fe, Co), having important technological applications, have been successfully prepared by a very fast, inexpensive, reproducible, environment-friendly method: the microwave irradiation of the corresponding mixtures of nitrates. Worth to note, the microwave source is a domestic microwave oven. In some cases the reaction takes place in a single step, while sometimes further annealings are necessary. For doped materials the method has to be combined with others such as sol–gel. Usually, nanopowders are produced which yield high density pellets after sintering. Rietveld analysis, oxygen stoichiometry, microstructure and magnetic measurements are presented.

Microstructural, magnetothermal and dielectric properties of YCrO3 powders prepared by combustion and solid state methods have been studied by a combination of XRD, specific heat, magnetization and permittivity measurements. The TEM and XRD characterization confirm that the combustion powders are amorphous plate-like agglomerates of nano-sized crystalline particles. A more uniform grain size along with an increase of the relative density is observed by SEM in the sintered samples prepared by combustion route with respect to those produced by solid state reaction. Similar to the material obtained through solid state synthesis, the material prepared by the combustion method also shows spin canted antiferromagnetic ordering of Cr+3 (S=3/2) at ∼140 K, which is shown by magnetization as well as λ-type anomaly in the total specific heat. Furthermore, the magnetic contribution to the total specific heat reveals spin fluctuations above TN and a spin reorientation transition at about 60 K. Both YCrO3 compounds show a diffuse phase transition at about 450 K, typical of a relaxor ferroelectric, which is characterized by a broad peak in the real part of the dielectric permittivity as a function of temperature, with the peak decreasing in magnitude and shifting to higher temperature as the frequency increases. The relaxor dipoles are due to the local non-centrosymmetric structure. Furthermore, the high loss tangent in a broad range of temperature as well as conductivity analysis indicates a hopping mechanism for the electronic conductivity as we believe it is a consequence of the outer d3-shell, which have detrimental effects on the polarization and the pooling process in the YCrO3 bulk material. The more uniform particle size and higher density material synthesized through the combustion process leads to an improvement in the dielectric Properties.

Nanocrystalline (between 30 and 40 nm) LiCoO2 and LiCo1−xGaxO2 (0 ≤ x ≤ 0.15) powders have been prepared by using a standard microwave furnace. Depending on the power and irradiation time, nanosized LT-LiCo1−xGaxO2 (cubic) or HT-LiCo1−xGaxO2 (hexagonal) (0 ≤ x ≤ 0.15) materials have been obtained and studied by means of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electrochemical characterization and magnetic behaviour of the electrochemical active polymorphs (HT-phases) have also been investigated.

A novel Pr2Cu2O5 phase has been prepared under high-pressure and high-temperature conditions (P ∼6 GPa and T ∼1673 K) in a Belt-type apparatus and characterized by X-ray diffraction and electron microscopy. The crystal structure appears to be an orthorhombic “oxygen-deficient perovskite” (M.T. Anderson, J.T. Vaughey, and K.R. Poeppelmeier, Structural similarities among oxygen-deficient perovskites, Chem. Mater. 5 (1993), pp. 151–165) isostructural with La2Cu2O5 (J.F. Bringley, B.A. Scott, S.J. La Placa, R.F. Boheme, T.M. Shaw, M.W. McElfresh, S.S. Trail, and D.E. Cox, Synthesis of the defect perovskite series LaCuO 3−δ with copper valence varying from 2+to 3+, Nature 347 (1990), pp. 263–265) and Nd2Cu2O5 (B.-H. Chen, D. Walker, E. Suard, B.A. Scott, B. Mercey, M. Hervieu, and B. Raveau, High pressure synthesis of NdCuO3−δ perovskites (0≤δ≤0.5). Inorg. Chem. 34 (1995), pp. 2077–2083).

The energetic stability of FePO4 polymorphs (berlinite, heterosite, monoclinic, and high-pressure forms) is investigated combining first-principles and experimental methods. Calculations at the den...

In this work, three different methods for preparing BiFeO3 polycrystals are compared: hydrothermal synthesis, microwave heating in the solid state and the combination of both, that is a hydrothermal method using microwave heating. The best materials, without high purity reactants, are obtained in few minutes by the last procedure, a new, very fast, reproducible and environment-friendly method of synthesis, which is described and discussed here.

The behavior of Li 3 PO 4 and Li 2 MSiO 4 ( M = Mn, Co) compounds under high pressure/high temperature is investigated. Pmn2 1 -Li 3 PO 4 remains stable up to the higher experimental limit of 80 kbar (900 °C). A sample of Li 2 MnSiO 4 consisting of a mixture of Pn2 1 and Pmnb polymorphs converts upon high pressure/high temperature treatment into the Pmn2 1 polymorph; the latter being stable at 80 kbar and 900 °C. A sample of Li 2 CoSiO 4 - P2 1 /n transforms to the denser Pmn2 1 polymorph at 40 kbar/900 °C, but decomposes at higher pressure (60 kbar/900 °C). No evidence on any novel Li 2 MSiO 4 polymorph is detected in any of the high-pressure products.

Olivine-LiCoXO4 (X = P, As) compounds might transform to the denser spinel-type and Na2CrO4-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO4 is predicted to transform to the Na2CrO4-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO4 treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO4 transforms to the Na2CrO4-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO4 when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na2CrO4-type polymorph. Crystallographic data of the new compound LiCoPO4 within the Na2CrO4 structural type are provided.

Silicates LiMSiO 4 are potential positive electrode materials for lithium ion batteries. In this work we analyse from first principles calculations the relative stability of possible LiFeSiO 4 -polymorphs within four structural types. Olivine-LiFeSiO 4 is predicted to be more stable than the LiFeSiO 4 prepared by delithiation of Li 2 FeSiO 4 ; the latter being the only LiFeSiO 4 compound reported so far. Attempts to prepare olivine-LiFeSiO 4 from a mixture of reactants at ambient pressure (600–1100 °C) resulted in a mixture of quartz-SiO 2 , Li 2 SiO 3 , LiFe 5 O 8 and LiFeSi 2 O 6 phases. Conducting the reaction under HP conditions (40 kbar) leads to the formation of LiFeSi 2 O 6 as a majority phase, regardless the nature of the reactants/precursors. First principles calculations indicate that the preparation of the olivine-LiFeSiO 4 is thermodynamically hindered due to the competition with the more stable LiFeSi 2 O 6 pyroxene, in the range of pressure/temperature investigated.