Mixed Oxide Systems Research Articles

Impact of Ca2+, Ce3+ codoping on ZnSnO3-SnO2 heterostructure for dielectric, optoelectronic and solar cell applications

We have successfully prepared a new mixed oxide system of Sn-rich ZTO (ZnSn2O5). The impact of coupling Ca2+ and Ce3+ inside the lattice of ZnSn2O5 on the dielectric, optical, and photoelectric conduction properties was investigated. In this regard, the co-precipitation technique was used to prepare three samples of ZTO, Ca-ZTO, and Ca/Ce-ZTO. The samples have been investigated by X-ray diffractometer (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscope (FESEM), X-ray photoelectron spectrophotometer (XPS), and an energy dispersive X-ray spectrometer (EDS). The XRD pattern detected phases that highly interfere as a heterostructured compound. The pristine ZTO particles demonstrate a mixture of two morphologies, hexagonal and quasi-spherical shapes, with particle size distributions of 0.25–1.5 µm, resulting in significant porosity between these particles. Meanwhile, the particles of Ca/Ce-doped ZTO have a homogenous morphology as spherical shape and exhibit larger density, particle size distribution range of 1–2 µm. The dielectric constant as a function of frequency or wavelength is increased with the applied temperature (32 to 140 °C) for both pure and Ca/Ce-ZTO samples. Clearly, Ca/Ce-ZTO displays higher absorption in visible light and the estimated band gaps of ZTO, Ca-ZTO, and Ca/Ce-ZTO are equal to 1.58,1.56 and 1.48 eV, respectively. The pristine ZTO showed a stronger n-type effect than Ca and Ca/Ce-doped ZTO samples. The shift from n-type to being close to N-P junction by multiple doping induces the application of these heterostructured materials as buffering layers in energy conversion applications like thin film solar cells and light emitting diodes.

Synthesis of Magnesium- and Aluminum-Based Mixed Oxide Systems by Low and High Supersaturation Methods

Magnesium–aluminum layered double hydroxides and mixed oxides based on them were obtained by high and low supersaturation methods and analyzed. It was shown that the phase composition and formation of nano-sized particles with a large surface area is significantly affected by the rate of introduction of magnesium–aluminum systems into the medium of the precipitated material. All of the obtained samples were studied by thermogravimetric analysis with mass-spectrometric detection, X-ray diffractometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, and infrared spectroscopy.

Synthesis of Magnesium- and Aluminum-Based Mixed Oxide Systems by Low and High Supersaturation Methods

Synthesis of Magnesium- and Aluminum-Based Mixed Oxide Systems by Low and High Supersaturation Methods

TiO2-CeO2 based composite materials and their application in photocatalysis: A short review

In light of the ever-increasing problem of environmental pollution, photocatalysis represents one of the most promising solutions for the remediation/decomposition of wastewater pollutants. Among the materials used for the photocatalytic degradation of organic pollutants, titanium dioxide has been widely investigated due to its unique and favourable properties. On the other hand, its limitation in absorbing only about 5% of solar light and its relatively wide band gap and rapid recombination of electron-hole pairs restricts its practical application. In order to overcome this drawback and improve photocatalytic ability, various methods have been investigated. Some of the significant methods that have been extensively studied over the past decades involve the preparation of binary oxides, mixed oxide systems, composite materials, etc. This short review provides a comprehensive summary of scientific reports of titania-ceria composite materials and their applications in photocatalytic reactions reported in the literature.

Ammonia synthesis using Co catalysts supported on MgO–Nd2O3 mixed oxide systems: Effect of support composition

This study presents the use of emerging support materials based on mixed magnesium–neodymium oxides for the dispersion of the Co metal for ammonia synthesis. A series of cobalt catalysts using mixed MgO–Nd2O3 oxide supports with different Mg/Nd molar ratios (2, 4, 7, 10, and 12) was successfully prepared. The N2 physisorption, XRD, HR-TEM, STEM-EDX, TPR/TG-MS, H2-TPD, and CO2-TPD studies of these systems revealed a strong correlation between the composition of the support and the overall catalyst performance. It was found that varying the Mg/Nd molar ratio caused a gradual change in the textural, structural and adsorption properties. The difference in the activity of the Co/MgO–Nd2O3 catalysts towards ammonia synthesis was mainly attributed to the different surface adsorption properties. The highest number of basic sites on the catalyst surface was noted for the support with the Mg/Nd molar ratio of 10. Also, the highest Co dispersion was obtained on this support, leading to the best distribution of Co particles with the smallest Co particle size. As a result, the Co/10MgO–Nd2O3 catalyst demonstrated superior activity for ammonia synthesis compared to other catalysts. The use of a binary composite support consisting of MgO and Nd2O3 with the properly selected ratio of each component is beneficial in terms of catalyst activity, stability and cost compared to single-oxide supports.

Ethoxy Groups on ZrO2, CuO, and CuO/ZrO2 Studied by IR Spectroscopy.

The formation, properties, decomposition and reactions of ethoxy groups on ZrO2, CuO, and CuO/ZrO2 were followed by IR spectroscopy. The reaction of ethanol with terminal Zr-OH groups leads to the formation of monodendate ethoxy groups (type I), whereas the reaction of ethanol with tribridged Zr-OH grups results in the formation of bidendate ethoxyls (type II). In both cases, water is produced. Ethoxy groups of type II were also formed on CuO. The type of the surface species detected after interaction of ethanol with CuO/ZrO2 was the same as detected for both oxides (i.e., ZrO2 and CuO) separately. This suggests that no new phase was formed in the mixed oxide system. At higher temperatures, ethoxy groups were oxidized forming acetate ions. Gaseous ethanol present in the cell was oxidized to acetaldehyde without the intermediacy of ethoxy groups.

On the Sintering Behavior of Nb2O5 and Ta2O5 Mixed Oxide Powders.

A mixed oxide system consisting of Nb2O5 and Ta2O5, was subjected to annealing in air/hydrogen up to 950 °C for 1–4 h to study its sintering behavior. The thermogravimetric–differential scanning calorimetry (TGA–DSC) thermograms indicated the formation of multiple endothermic peaks at temperatures higher than 925 °C. Subsequently, a 30% Ta2O5 and 70% Nb2O5 (mol%) pellet resulted in good sintering behavior at both 900 and 950 °C. The scanning electron microscope (SEM) images corroborated these observations with necking and particle coarsening. The sintered pellets contained a 20.4 and 20.8% mixed oxide (Nb4Ta2O15) phase, along with Ta2O5 and Nb2O5, at both 900 and 950 °C, indicating the possibility of the formation of a solid solution phase. In situ high-temperature X-ray diffraction (XRD) scans also confirmed the formation of the ternary oxide phase at 6 and 19.8% at 890 and 950 °C, respectively. The Hume–Rothery rules could explain the good sintering behavior of the Ta2O5 and Nb2O5 mixed oxides. An oxide composition of 30% Ta2O5 and 70% Nb2O5 (mol%) and a sintering temperature of 950 °C appeared adequate for fabricating well-sintered oxide precursors for subsequent electrochemical polarization studies in fused salts.

Application of Mixed-Metal-Oxides As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity.For example, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic copper electrocatalytic nanoparticles (generated from Cu2O) over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (e.g. CO adsorbates). Mechanistic studies involving chronoamperometric, voltammetric and gas-diffusion electrode measurements are pursued, in addition to spectroscopic and XPS measurements. Results of comparative measurements, involving single components only, will be provided as well. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627.

A Kinetic Study of Photocatalytic Degradation of Phenol over Titania–Silica Mixed Oxide Materials under UV Illumination

A set of titania–silica mixed oxide materials were prepared by a cosolvent-induced gelation method using ethanol and toluene as solvent and cosolvent, respectively. These materials were extensively characterized by utilizing several characterization techniques and assessed for phenol degradation under UV illumination. The degradation of phenol follows first-order kinetics, and fragmented products formed during the phenol degradation were qualitatively identified by using high performance liquid Chromatographic (HPLC) and atomic pressure chemical ionization mass spectroscopic (APCI-MS) techniques. The complete mineralization of phenol was further evidenced by the measurement of the total organic contents that remained in the solution after irradiation. The pore diameter of the materials was found to be the key factor for phenol degradation, whereas surface area and pore volume play a role among the mixed oxide materials. In addition, in the mixed oxide system there was an inverse correlation obtained with the particle size of the materials and the degradation efficiency. The smaller particle size of titania in the mixed oxide material was found to be a requirement for an effective degradation of phenol.

Electrical Properties of Compositional Al<sub>2</sub>O<sub>3</sub> Supplemented HfO<sub>2</sub> Thin Films by Atomic Layer Deposition

With advanced research for dielectrics including capacitors in DRAMs, decoupling filters in microcircuits and insulating gates in transistors, a lot of demand for the new challenging of high-k materials in semiconductor industries has been emerged. This study explores and addresses the experimental approach for composite materials with one of the major concerns of high capacitance, and low leakage, as well as ease of integration technology. The characteristics of Al2O3 supported HfO2 (AHO) thin films for a series of different Hf ratios with Al2O3 dielectrics by atomic layer deposition demonstrated as a candidate material. A composite AHO films with the homogeneous compositions of Al and Hf atoms into the Al-Hf-O mixed oxide system could stabilize the polycrystalline structure with increasing of dielectric constant (k) and decreasing of leakage current density, as well as a higher breakdown voltage than HfO2 film on its own. 70 nm thick AHO thin films with different composition of Al and Hf contents were prepared by atomic layer deposition technique on titanium nitride (TiN) and silicon dioxide (SiO2) coated Si substrates. Photolithography and metal lift-off technique were used for the device fabrication of the metal-insulator-metal (MIM) capacitor structures. AHO films on TiN/SiO2/Si were measured by semiconductor analyzer and source/ measure system with probe station in the voltage range from -5 to 5 V with a frequency range from 10 kHz to 1 MHz were used to conduct capacitance-voltage (C-V) measurements with low/medium frequency range and current-voltage (I-V) measurements. It was found that Au/AHO/TiN/SiO2/Si MIM capacitors demonstrate a capacitance density of 1.5 - 4.5 fF/μm2 at 10 kHz, a loss tangent of 0.02 - 0.04 at 10 kHz, dielectric constant of 11.7 - 35.5 depending on the composition and a low leakage current of 1.7 × 10-9 A/cm2 at 0.5 MV/cm at room temperature. The acquired experimental results could show the possibility of compositional alloy thin films that could potentially replace or open new market for high-k challenges in semiconductor technology.

Development of cobalt catalyst supported on MgO–Ln2O3 (Ln = La, Nd, Eu) mixed oxide systems for ammonia synthesis

The binding of hydrogen with nitrogen in the ammonia molecule has enormous potential as a hydrogen storage method. The development of efficient ammonia synthesis catalysts with high activity and stability is essential for this reaction. In this study, a series of MgO–Ln2O3 (Ln = La, Nd, Eu) mixed oxides was designed and applied as the support of cobalt catalysts. The tuning of the support composition by selecting lanthanide elements differing in electronegativity led to changes in the physicochemical properties of the catalysts, such as the active phase dispersion, accessibility of active sites for hydrogen, surface density of basic sites. The Co/MgO–Nd2O3 catalyst showed better activity than other studied catalysts in NH3 synthesis reaction at 470 °C, 6.3 MPa. Its excellent performance is ascribed to the increased density of medium basic sites and the enhancement of electron-donating ability.

Hybrid functional/embedded cluster study of uranium and actinide (actinide = Np, Pu, Am or Cm) mixed dioxides bulk and {110} surfaces

Understanding actinide (An) mixed oxides (MOX) is important for the development of minor An bearing U-Pu MOX nuclear fuel, and the long-term storage of spent nuclear fuel. In this work, we systematically simulate AnO2 (An = U, Np, Pu, Am, and Cm) and U-An MOX (An = Np, Pu, Am, and Cm) bulk and {110} surfaces using hybrid density functional theory within an embedded cluster model, focusing on the spin density of An and the density of states of AnO2 bulk and surface for the An series from U to Cm; spin density transfer is found between O and An and is discussed in detail. For U-An MOX bulk and surface, geometric structure, U and An spin densities and the densities of states are presented. Clear conclusions can be drawn regarding the oxidation state of An in the U-Np and U-Cm MOX systems, while U-Pu MOX and U-Am MOX are less clear but and sensitive to the simulation method, suggesting they are in a transition region between the U-Np and U-Cm systems.

Kinetic analysis of the reduction of a ternary system of Bi, Sb and Te oxides by hydrogen for BiSbTe3 synthesis

Reduction in a hydrogen atmosphere of Bi2O3, Sb2O3 and TeO2 mixes oxides for the synthesis of BiSbTe3 was analysed. The reduction reactions of Sb2O3 and Sb2O4 oxides, as well as Bi2O3+Sb2O3 and Sb2O3+3TeO2 mixtures, were also evaluated. The reduction of Sb2O4 is investigated for the first time. The reactions of the mixed oxides systems also were not the subject of research so far despite being used for synthesis of the (Bi,Sb)2Te3 material. The study comprised of kinetic analysis of TGA curves performed with non-parametric and isoconversional methods. Insight into powders’ structure during the process was taken to identify processes taking place. The results show, that Sb2O3 present in the reduced oxidizes to Sb2O4. This redox process between Sb oxide and another oxide takes place directly. The self-heating effect caused by elemental tellurium is diminished in the presence of Bi, because Bi-Te phases decrease the content of the elemental Te.

Exceptional Catalytic Activity of Cu−Zn/ZrO2 Mixed Metal Oxide towards the Oxidation Reaction

AbstractHerein, we report the catalytic activity of the copper and zinc doped ZrO2‐based mixed metal oxide system towards the oxidation reaction. To study this attribute, different weight % loadings of copper and zinc supported on zirconia catalysts were synthesized by co‐precipitation followed by the hydrothermal method. The synthesized catalysts were characterized by various physicochemical methods. X‐ray analysis revealed that mainly cubic zirconia (111) planes were formed, and up to 0 to 25 wt.% Cu and Zn were doped in the zirconia lattice. Also, elemental mapping revealed uniform dispersion of Cu, Zn, Zr, and O elements. The catalytic activity of the synthesized catalyst was studied for olefin epoxidation and alcohol oxidation reactions using aqueous tert‐butyl hydrogen peroxide as an oxidant. Among all the synthesized catalysts, 2 wt.% doping of 1 : 1 Cu: Zn on ZrO2 gave almost complete olefin conversion with the best selectivity towards epoxides under optimized reaction conditions. Furthermore, the selected catalyst showed excellent substrate scope for different olefins and alcohol oxidation. Also, the catalyst was successfully recycled up to five cycles without affecting the conversion and selectivity.

Theoretical aspects on doped-zirconia for solid oxide fuel cells: From structure to conductivity

Solid oxide fuel cells (SOFCs) are regarded to be a key clean energy system to convert chemical energy (e.g. H2 and O2) into electrical energy with high efficiency, low carbon footprint, and fuel flexibility. The electrolyte, typically doped zirconia, is the “state of the heart” of the fuel cell technologies, determining the performance and the operating temperature of the overall cells. Yttria stabilized zirconia (YSZ) have been widely used in SOFC due to its excellent oxide ion conductivity at high temperature. The composition and temperature dependence of the conductivity has been hotly studied in experiment and, more recently, by theoretical simulations. The characterization of the atomic structure for the mixed oxide system with different compositions is the key for elucidating the conductivity behavior, which, however, is of great challenge to both experiment and theory. This review presents recent theoretical progress on the structure and conductivity of YSZ electrolyte. We compare different theoretical methods and their results, outlining the merits and deficiencies of the methods. We highlight the recent results achieved by using stochastic surface walking global optimization with global neural network potential (SSW-NN) method, which appear to agree with available experimental data. The advent of machine-learning atomic simulation provides an affordable, efficient and accurate way to understand the complex material phenomena as encountered in solid electrolyte. The future research directions for design better electrolytes are also discussed.

Chemical Imaging of Mixed Metal Oxide Catalysts for Propylene Oxidation: From Model Binary Systems to Complex Multicomponent Systems

AbstractIndustrially‐applied mixed metal oxide catalysts often possess an ensemble of structural components with complementary functions. Characterisation of these hierarchical systems is challenging, particularly moving from binary to quaternary systems. Here a quaternary Bi−Mo−Co−Fe oxide catalyst showing significantly greater activity than binary Bi−Mo oxides for selective propylene oxidation to acrolein was studied with chemical imaging techniques from the microscale to nanoscale. Conventional techniques like XRD and Raman spectroscopy could only distinguish a small number of components. Spatially‐resolved characterisation provided a clearer picture of metal oxide phase composition, starting from elemental distribution by SEM‐EDX and spatially‐resolved mapping of metal oxide components by 2D Raman spectroscopy. This was extended to 3D using multiscale hard X‐ray tomography with fluorescence, phase, and diffraction contrast. The identification and co‐localisation of phases in 2D and 3D can assist in rationalising catalytic performance during propylene oxidation, based on studies of model, binary, or ternary catalyst systems in literature. This approach is generally applicable and attractive for characterisation of complex mixed metal oxide systems.

Prediction of crystallized phases of amorphous Ta2O5-based mixed oxide thin films using a density functional theory database

The genomics approach to materials, heralded by increasingly accurate density functional theory (DFT) calculations conducted on thousands of crystalline compounds, has led to accelerated material discovery and property predictions. However, so far, amorphous materials have been largely excluded from this as these systems are notoriously difficult to simulate. Here, we study amorphous Ta2O5 thin films mixed with Al2O3, SiO2, Sc2O3, TiO2, ZnO, ZrO2, Nb2O5, and HfO2 to identify their crystalline structure upon post-deposition annealing in air both experimentally and with simulations. Using the Materials Project open database, phase diagrams based on DFT calculations are constructed for the mixed oxide systems and the annealing process is evaluated via grand potential diagrams with varying oxygen chemical potential. Despite employing calculations based on crystalline bulk materials, the predictions agree well with the experimentally observed crystallized phases of the amorphous thin films. In the absence of ternary phases, the dopant acts as an amorphizer agent increasing the thermal stability of Ta2O5. The least efficient amorphizer agent is found to be Nb2O5, for which the cation has similar chemical properties to those of Ta in Ta2O5. These results show that DFT calculations can be applied for the prediction of crystallized structures of annealed amorphous materials. This could pave the way for accelerated in silico material discovery and property predictions using the powerful genomic approach for amorphous oxide coatings employed in a wide range of applications such as optical coatings, energy storage, and electronic devices.

Role of Mixed Oxides in Hydrogen Production through the Dry Reforming of Methane over Nickel Catalysts Supported on Modified γ-Al2O3

H2 production through dry reforming of methane (DRM) is a hot topic amidst growing environmental and atom-economy concerns. Loading Ni-based reducible mixed oxide systems onto a thermally stable support is a reliable approach for obtaining catalysts of good dispersion and high stability. Herein, NiO was dispersed over MOx-modified-γ-Al2O3 (M = Ti, Mo, Si, or W; x = 2 or 3) through incipient wetness impregnation followed by calcination. The obtained catalyst systems were characterized by infrared, ultraviolet–visible, and X-ray photoelectron spectroscopies, and H2 temperature-programmed reduction. The mentioned synthetic procedure afforded the proper nucleation of different NiO-containing mixed oxides and/or interacting-NiO species. With different modifiers, the interaction of NiO with the γ-Al2O3 support was found to change, the Ni2+ environment was reformed exclusively, and the tendency of NiO species to undergo reduction was modified greatly. Catalyst systems 5Ni3MAl (M = Si, W) comprised a variety of species, whereby NiO interacted with the modifier and the support (e.g., NiSiO3, NiAl2O4, and NiWO3). These two catalyst systems displayed equal efficiency, >70% H2 yield at 800 °C, and were thermally stable for up to 420 min on stream. 5Ni3SiAl catalyst regained nearly all its activity during regeneration for up to two cycles.

СИНТЕЗ ТА ФОТОКАТАЛІТИЧНА АКТИВНІСТЬ ПОКРИТТІВ TI/TInOm∙ZRO2 ДЛЯ ОЧИЩЕННЯ ПРОМИСЛОВИХ СТІЧНИХ ВОД ВІД ОРГАНІЧНИХ АРОМАТИЧНИХ ЗАБРУДНИКІВ

It is shown that photocatalytic processes on semiconductor materials are promising for use in technologies for purifying industrial waste water and air from toxic organic impurities for solving important environmental problems. Studies on the formation of coatings with titanium(IV) oxide supplemented with zirconium(IV) oxide have been carried out. The Ti/TiO2 coverings were formed by anodic oxidation of technical alloys of VT1-0 grade titanium and E-125 zirconium from aqueous electrolyte solutions based on 0.5 M sulfuric acid and 1 M potassium pyrophosphate. To obtain mixed oxide coatings of the Ti/TinOm·ZrO2 composition, zirconium(IV) oxide of a given concentration was additionally introduced into the electrolyte solutions. The photocatalytic activity of the obtained systems was assessed by the phenol oxidation reaction. It is shown that, as a result of anodic oxidation of the VT1-0 alloy in sulfuric and pyrophosphate electrolytes, it is possible to obtain mixed oxide systems of the Ti/TinOm·ZrO2 composition with a porous and microcrystalline surface structure and a zirconium content of up to 2 % by weight. It was found that an increase in the pH of the electrolyte leads to a significant decrease in the content of zirconium in the coatings. It is shown that the contact masses Ti/TiO2, Zr/ZrO2, Ti/TinOm·ZrO2 are photocatalytically active in the oxidation of phenol under the action of UV radiation, and the mixed Ti/TinOm·ZrO2 coatings formed from a sulfuric acid electrolyte exhibit a higher catalytic activity with respect to compared with both individual oxides and Ti/TinOm·ZrO2 deposited from pyrophosphate electrolytes. The results obtained indicate the possibility of creating photocatalytic converters using mixed oxide systems formed on metal supports for purifying wastewater from organic aromatic compounds. Keywords: coatings, titanium(IV) oxide, electrochemical anodizing, photocatalytic activity, zirconium(IV) oxide, organic aromatic pollutants, phenol, waste water, purification. Corresponding author Panasenrj V.A. е-mail: office@niochim.kharkov.ua

Modelling of plutonium diffusion in (U,Pu)O2±x mixed oxide

The modelling of the thermo-kinetic properties of uranium-plutonium mixed oxide (MOX) is of utmost importance for optimizing its synthesis and for predicting its behaviour in Fast Breeder Reactors. Despite the stakes and likely because of experimental issues, little or no experimental data are available for the entire MOX system. We circumvent here the difficulties by developing a mobility database for plutonium using the DICTRA code. A well-established model of MOX formalized within the Compound Energy Formalism ensures the thermodynamic description. Rationalisation of the mobility parameters combined with the use of both cBΩ model and the few experimental data lead to a full and comprehensive description of plutonium self-diffusion in MOX for any plutonium content, O/M ratio and temperature. Additionally, we show that the observed plateau of the self-diffusion as a function of the oxygen to metal ratio (O/M) is related to the constant Pu3+ fraction for very low O/M ratio. Moreover, the observed minimum close to O/M = 2 is found for the lowest mobility of Pu4+.