Preparation,crystalline properties and X-ray absorption spectra of rare earth oxides ROx (R ≡ Ce,PrandTb; 1.5 ⩽ x ⩽ 2)

High purity rare earth (R ≡ Ce, Pr, Tb) sesquioxides, intermediate oxide and dioxide powders and thin flakes were prepared using different methods. Their crystalline properties were controlled by X-ray and electron diffraction. The X-ray absorption spectra (M_IV–V and L_III edges) have been used to determine the rare earth valence in the samples. From R₂O₃ to RO₂ the absorption edges show noticeable changes, indicating the evolution of the valence. The L_III and M_IV–V spectra respectively give information on the sd and 4f symmetry vacant states. The M_IV–V spectra of a given RO_x show that inhomogeneous valence can be exactly deconvoluted into suitably weighted R₂O₃ and RO₂ contributions according to the chemical formula. In the case of L_III edges, remarkable discrepancies in the deconvolution results are observed. This behaviour may be correlated with the change in the sd density of states due to oxygen vacancy ordering in the phase stability region of RO_x.     

A study of conduction band edge states in complex oxides by X-ray absorption spectroscopy

Conduction band edge d-states are compared for complex oxides: (i) mixed tetravalent–trivalent ZrO₂–Y₂O₃ alloys, (ii) tetravalent Zr(Hf)O₂–TiO₂ alloys, and (iii) trivalent La scandate and aluminate. Low Y₂O₃ content cubic ZrO₂–Y₂O₃ alloys display two crystal-field split 4d-features in O K₁ spectra. Alloys with higher Y₂O₃ content, as well as Zr(Hf)O₂–TiO₂ alloys display increased d-state multiplicity. O K₁ spectra of perovskite-structured LaScO₃ and LaAlO₃ indicate Jahn–Teller d-state term-splittings with contributions from both trivalent atomic species.     

Valence and structural transitions in the mixed Ru-Ir perovskites Ba2PrRu1−xIrxO6

The valence state of Pr in the B-site ordered double perovskites Ba₂PrRu_1−xIr_xO₆ is shown to be sensitive to both the precise Ru:Ir content and temperature. Pr L_III XANES measurements show that at room temperature the Pr is trivalent in the Ru-rich compounds with x<0.25. At higher Ir contents the Pr is tetravalent. High-resolution powder synchrotron X-ray and neutron diffraction methods have been used to study the composition and temperature dependence of the crystal structures of these oxides. The Ru and Ir are statistically distributed on one of the two available B-sites. The oxides undergo an apparently first-order monoclinic P2₁/n to tetragonal P4/mnc phase transition in response to the change on the Pr and Ru/Ir valence. High temperatures and Ru contents favor the lower symmetry monoclinic structure, that is arises from the presence of the larger Pr^III cations. The variations in the observed metal-oxygen bond distances are consistent with a simultaneous change in the valence of the Ru/Ir accompanying the Pr^III-Pr^IV valence transition.     

Homogeneity Region Boundaries of Praseodymium Manganite Pr2 − x MnxO3 ± δ in Air

The solubility boundaries of simple praseodymium and manganese oxides and the PrMn₂O₅ double oxide in PrMnO₃ were determined using X-ray powder patterns of homogeneous phases and heterogeneous compositions of the general formula Pr_{2 ? x} Mn_xO_{3 ± δ} (0.90 <- x <- 1.20; Δx = 0.02) obtained by ceramic synthesis from oxides in air over the temperature range 900–1400°C. The results are presented in the form of a fragment of the phase diagram of the Pr-Mn-O system in air. The suggestion was made that the solubility of praseodymium oxide in PrMnO₃ was caused by crystal structure defects, and that of manganese oxides, by structure defects and the partial replacement of praseodymium cations by manganese ions in the cuboctahedral sites of the perovskite-like crystal lattice. The suggestions made can be verified by a systematic study of the oxygen nonstoichiometry of Pr_{2 ? x} Mn_xO_{3 ± δ} manganite depending on x and the temperature of synthesis.     

Application of EPR spectroscopy for elucidation of vanadium speciation in VO x /ZrO2 catalysts subject to redox treatment

Application of EPR spectroscopy corroborated by spectra simulation in speciation studies of the tetravalent vanadium in supported VO _x /ZrO₂ catalyst has been discussed. Implementation of genetic algorithms into automated analysis of the EPR spectra has greatly improved the simulation efficiency. The performance of the new procedure has been benchmarked against common simplex method using the multi-component model and real EPR spectra of tetravalent vanadium in VO _x /ZrO₂ catalysts. The analysis has revealed speciation of vanadium into surface isolated and clustered vanadyl entities and isolated bulk V _Zr ^x ions due to formation of Zr_1?x V _x O₂ solid solution in the near to surface region. The structural heterogeneity of vanadium can be controlled by the calcination temperature and the redox treatment.     

Comparative studies in subsolidus areas of ternary oxide systems PbO–V2O5–In2O3 and PbO–V2O5–Fe2O3

Thermal stability of PbO was studied. Reactivity of oxides in the systems PbO–M₂O₃ (M = In, Fe) was investigated up to 650 °C. Using the DTA and XRD methods, parts of investigated ternary oxide systems, labelled by compounds: V₂O₅, Pb₈V₂O₁₃ and M₂O₃ (M = In, Fe), have been divided into partial ternary systems. IR spectra of compounds Pb₂MV₃O₁₁ (M = In, Fe) have been compared.     

New substituted copper titanates with the CaCu3Ti4O12 structure: Li[Cu3−xLix][Ti3−xMV1+x]O12 (MV = Nb: 0.12 ≤ x ≤ 0.33; MV = Ta: x = 0.33)

New substituted copper titanates with the CaCu₃Ti₄O₁₂ perovskite-like structure were synthesized and characterized. Their formula is Li|Cu_3−xLi_x| |Ti_3−xM^V_1+x|O₁₂. There is a homogeneity range for M^V = Nb, 0.12 ≤ x ≤ 0.33, and a unique composition for M^V = Ta, x = 0.33. The unit cell is cubic: a ∼ 2a_p (parameter of the ideal cubic cell) ∼ 7.40 Å. The CaCu₃Ti₄O₁₂-type structure was confirmed from an X-ray powder structural determination. Lithium occupies both square planar sites where it replaces copper and icosahedral sites, with a probable delocalization. Crystal chemistry of the AC₃B₄O₁₂ structure is considered, taking into account the evolution of anion packing and the distortion of polyhedra. A diagrammatic representation is proposed so that precise information on the regularity of the C₃B₄O₁₂ network can be obtained.     

Ba3MIIITiMVO9 (MIII = Fe,Ga, Y,Lu; MV = Nb,Ta, Sb) perovskite oxides: Synthesis,structure and dielectric properties

We describe the synthesis, structures and dielectric properties of new perovskite oxides of the formula, Ba₃M^IIITiM^VO₉, for M^III = Fe, Ga, Y, Lu and M^V = Nb, Ta, Sb. While M^V = Nb and Ta oxides adopt disordered/partially ordered 3C perovskite structures where M^III/Ti/M^V metal-oxygen octahedra are corner-connected, the M^V = Sb oxides show a distinct preference for the 6H structure, where Sb^V/Ti^IV metal-oxygen octahedra share a common face forming (Sb,Ti)O₉ dimers that are corner-connected to the M^IIIO₆ octahedra. The preference of antimony oxides (Sb^V:4d¹⁰) for the 6H structure – which arises from a special Sb^V–O chemical bonding that tends to avoid linear Sb–O–Sb linkages unlike Nb^V/Ta^V:d⁰ atoms which prefer ～180° Nb/Ta–O–Nb/Ta linkages – is consistent with the crystal chemistry of M^V–O oxides in general. The dielectric properties reveal a significant difference among M^III members. All the oxides with the 3C structure excepting those with M^III = Fe show a normal low loss dielectric behaviour with ε = 20–60 in the temperature range 50–400 °C; the M^III = Fe members with this structure (M^V = Nb, Ta) display a relaxor-like ferroelectric behaviour with large ε values at frequencies ≤1 MHz (50–500 °C).     

Simultaneous measurements of oxygen pressure,composition, and electrical conductivity in praseodymium oxides: II. Pr10O18 and PrO2−x phases

Simultaneous measurements of oxygen pressure, composition, and electrical conductivity have been conducted in Pr₁₀O_18±x (epsilon) and PrO_2?x (alpha) phases, between Pr₉O_16±x (zeta) and Pr₁₀O_18±x phases, and between Pr₇O_12±x (iota) and PrO_2?x phases. In Pr₁₀O_18±x phase, the predominant defects are assigned to be neutral oxygen interstitials and neutral or doubly charged oxygen vacancies, and electrical conduction is thought to be governed mainly by the concentration of 7-coordinated praseodymium ions, which are the easiest sites for hopping electrons between Pr³⁺ and Pr⁴⁺ ions. In PrO_2?x phase, the electrical conductivity increases with oxygen pressure and the $\text{O}\text{Pr}$ ratio and the predominant defects are assigned to be neutral oxygen interstitials, indicating that oxygen vacancies are ordered in a short range and this phase is expressed by PrO_1.78+x rather than PrO_2?x in the region measured. The electrical conductivity-composition measurement, as well as the oxygen pressure-composition measurement, shows a reproducible hysteresis loop between Pr₉O_16±x and Pr₁₀O_18±x and it is discussed in terms of a domain model.     

Synthesis and crystal structure of mixed metal(III) tungstenyl(VI) ortho-pyrophosphates

The series of isotypic anhydrous ortho-pyrophosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: Sc, V, Cr, Fe, Mo, Ru, Rh, In, Ir) was obtained via vapor phase moderated solid state reactions in sealed ampoules. The crystal structure of the phosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: V, Ru, Rh) was solved from single crystal X-ray data (C2/c, Z = 16). Fairly regular MO₆ and distorted WO₆ octahedra share vertices with PO₄ and P₂O₇ units to form a 3D network. For the ortho-pyrophosphates with M: V³⁺, Cr³⁺, and Fe³⁺ the oxidation state of M is confirmed by magnetic measurements. ³¹P-MAS-NMR spectra of the diamagnetic phosphates M^III(W^VIO₂)₂(P₂O₇)(PO₄) (M: Sc, In, Ir) show surprisingly different isotropic chemical shifts for the seven phosphorus sites. V^III(W^VIO₂)₂(P₂O₇)(PO₄) occurs as equilibrium phase in the quasi-binary system (V_1–xW_x)OPO₄ at x = 0.67 and exhibits a small homogeneity range 0.60 ≤ x ≤ 0.67. The scandium compound shows a fully inverted occupancy of the M sites according to the formulation W(Sc_1/2W_1/2O₂)₂(P₂O₇)(PO₄).     

The effect of introduction of manganese hydroxide and hydrated aluminum oxide on the pore structure and surface charge of Zr(IV), Ti(IV), and Sn(IV) oxyhydrates

The specific surface area, micropore volume, and pore size distribution for the Zr(IV), Sn(IV), and Ti(IV) oxyhydrates and double hydrates of the composition M _x Mn_1?x O _y ·nH₂O and M _x Al1_?x O _y ·nH₂O [M = Zr(IV), Sn(IV), Ti(IV), x = 0.5–0.9 were calculated. The specific surface charge and zero-charge point in 0.12 M KNO₃ were determined.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Phase formation in the system Zn(VO<Subscript>3</Subscript>)<Subscript>2</Subscript>–HCl–VOCl<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

The phase and chemical compositions of precipitates formed in the system Zn(VO₃)₂–HCl–VOCl₂–H₂O at pH 1?3, molar ratio V⁴⁺: V⁵⁺ = 0.1?9, and 80°C were studied. It was shown that, within the range 0.4 ≤ V⁴⁺: V⁵⁺ ≤ 9, zinc vanadate with vanadium in a mixed oxidation state forms with the general formula Zn_xV⁴⁺ _yV⁵⁺ _2-yO₅ ? nH₂O (0.005 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.3, n = 0.5?1.2). Vanadate Zn_xV₂O₅ ? nH₂O with the maximum tetravalent vanadium content (y = 0.30) was produced within the ratio range V⁴⁺: V⁵⁺ = 1.5?9.0. Investigation of the kinetics of the formation of Zn_xV₂O₅ ? nH₂O at pH 3 determined that tetravalent vanadium ions VO2+ activate the formation of zinc vanadate, and its precipitation is described by a second-order reaction. It was demonstrated that, under hydrothermal conditions at pH 3 and 180°C, zinc decavanadate in the presence of VOCl₂ can be used as a precursor for producing V₃O₇ ? H₂O nanorods 50–100 nm in diameter.     

Hydrothermal syntheses of materials constructed from vanadium oxide and secondary metal-tetrapyridylpyrazine subunits. Structural consequences of the identity of the secondary metal

Hydrothermal reactions of V₂O₅, tetra-2-pyridylpyrazine (tpyprz) and an appropriate M(II) starting material yield a series of oxides of general composition [{M_x(tpyprz)}_yV₄O₁₂] [x=1, y=2, M=Co(II), Ni(II); x=2, y=2, M=Cu(I); x=2, y=1, M=Zn(II)]. The Co(II) and Ni(II) analogues (1 and 2) are isostructural and consist of one-dimensional ribbons constructed from {V₄O₁₂}⁴⁻ clusters linked through {M(tpyprz)}₂⁴⁺ binuclear units of edge sharing {MO₃N₃} octahedra. In contrast, the structure of [{Cu₂(tpyprz)}₂V₄O₁₂] (3) is two-dimensional and constructed of {Cu₂(tpyprz)}_n²ⁿ⁺ chains linked in the second dimension through the {V₄O₁₂}⁴⁻ clusters. The structure of [{Zn₂(tpyprz)V₄O₁₂] (4) is also two-dimensional but may be described as {Zn₂V₄O₁₂} chains interconnected through the binucleating tpyprz ligands. The roles of the coordination preferences of the secondary metal cations as well as the nature of the organic components are discussed.     

Iron(III)-titanium(IV)-oxide electrodes: Their structural,electrochemical and photoelectrochemical properties

Iron(III)-titanium(IV)-oxides of the general composition Fe_xTi_1-xO_y were prepared in the composition range x=0–0.9 by thermal decomposition of the corresponding metal salt solutions. For a medium range of composition, 0.1≤x≤0.7, amorphous oxides were formed under the given conditions of preparation. Electrochemical properties such as reduction and re-oxidation of the oxide, electrode capacity behaviour, oxygen evolution and reduction and the redox reaction, Fe²⁺/Fe³⁺, were investigated. Photoelectrochemical properties were obtained from photocurrent spectra and the dependence on potential of the photocurrent. Annealing experiments showed that crystallization yields lower photocurrents and a shift of the photocurrents spectra to shorter wavelengths. Thus, amorphous semiconductors seem worth being investigated for a possible application in electrochemical solar cells. An attempt is made to describe the impact of non-crystallinity on the photoelectrochemical behaviour of semiconductors. A model of the oxide is proposed to explain the electrochemical and photoelectrochemical properties of the oxides Fe_xTi_1-xO_y.     

Magnetic and dielectric properties of YbFe2−xMnxO4 (0?x?1)

We have investigated the magnetic and dielectric properties of YbFe_2−xMn_xO₄ (0?x?1), which is an Fe-site-substituted system of new multiferroic oxides RFe₂O₄ (R=Y, Ho-Lu). X-ray diffraction measurements show that a solid solution is formed between YbFe₂O₄ (x=0) and YbFeMnO₄ (x=1) for 0?x?1, whereas only compounds with x=1 (i.e., RM₁M₂O₄; M₁ and M₂=trivalent and divalent cations, respectively) have been known for the Fe-site substitution in RFe₂O₄. The valence of the Mn ion is determined to be ∼2+, consistently with the suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature (T_N) and the dielectric constant (ε′) decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe₂O₄ are governed by the electron transfer between Fe-site ions, unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed as well.     

New vanadium(IV) and titanium(IV) oxyfluorotellurates(IV): V2Te2O7F2 and TiTeO3F2

As part of a continuing study of oxyfluorotellurates(IV), materials likely to present interesting nonlinear optical properties, two new phases, titanium(IV) tellurium(IV) trioxide difluoride, TiTeO₃F₂, and divanadium(IV) ditellurium(IV) heptaoxide difluoride, V₂Te₂O₇F₂, have been characterized and present, respectively, titanium and vanadium in the tetravalent state. The TiTeO₃F₂ structure is based on linear double rows of TiO₃F₃ polyhedra sharing vertices. These rows are connected to adjacent rows via two vertices of Te₂O₅ bipolyhedra. The Te, Ti, one F and two O atoms are on general positions, with one O and F statistically occupying the same site with half‐occupancy for each anion. One O and one F occupy sites with .m. symmetry. The V₂Te₂O₇F₂ structure consists of zigzag chains of VO₄F₂ octahedra alternately sharing O—O and F—F edges. These chains are connected via Te₂O₅ bipolyhedra, forming independent mixed layers. The Te, V, one F and three O atoms are on general positions while one O atom occupies a site of symmetry. In both phases, the electronic lone pair E of the Te^IV atom is stereochemically active. A full O/F anionic ordering is observed in V₂Te₂O₇F₂, but in TiTeO₃F₂ one of the six anionic sites is occupied by half oxygen and half fluorine, all the others being strictly ordered. These compounds represent new members of a growing family of oxyfluorotellurates(IV), including the recently characterized members of formula MTeO₃F, M being a trivalent cation. As was true for the previous members, they are characterized by an unusually high thermal and chemical stability in relation to the absence of direct Te—F bonds.     

Crystal structure, optical properties and colouring performance of karrooite MgTi2O5 ceramic pigments

Karrooite, MgTi₂O₅, is a promising ceramic pigment due to its high refractoriness and refractive indices, as well as its ability to host transition metal ions in two crystallographically distinct octahedral sites. The colouring performance was investigated combining X-ray powder diffraction with UV-vis-NIR spectroscopy on karrooite doped with V, Cr, Mn, Fe, Co or Ni (M) according to the formula Mg_1−xTi_2−xM_2xO₅, with x=0.02 and 0.05. Transition metals solubility in the karrooite lattice is not complete and a second phase is always present (geikielite or rutile). Structural data proved that incorporation of different chromophore ions into the karrooite structure affects unit cell parameters, bond length distances and angles, site occupancies and therefore cation order-disorder. Optical spectra exhibit broad absorbance bands of Co(II), Cr(IV), Fe(III), Mn(II), Mn(III), Ni(II), V(IV) with distinct contributions by cations in the M₁ and M₂ sites. Karrooite pigments have colours ranging from orange to brown-tan (Cr, Fe, Mn, V) to green (Co) and yellow (Ni) that are stable in low-temperature (<1050 °C) ceramic glazes and glassy coatings.     

Oxygen Nonstoichiometry and Transport Properties of Mixed-Conducting Ce<Subscript>0.6–<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>0.4</Subscript>Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2–δ</Subscript>

The oxygen nonstoichiometry and electrical conductivity of fluorite-type solid solutions Ce_0.6?xLa_0.4Pr _x O_2–δ (x = 0.1–0.2) were studied in the oxygen partial pressure range 10^–19–0.35 atm at 1023–1223 K. It was confirmed that the Pr^4+/3+ and Ce^4+/3+ redox pairs, which determine the concentration of p- and n-type electron charge carriers, play the dominant roles under oxidizing and reducing conditions, respectively. The conductivity vs. charge carrier concentration dependencies in these conditions are almost linear. Increasing praseodymium content leads to a substantially higher hole conductivity and an expanded range of the oxygen nonstoichiometry variations at high oxygen partial pressures. Under reducing conditions when praseodymium cations become trivalent opposite trends are observed on doping.     

Vanadium-oxide nanotubes: Synthesis and template-related electrochemical properties

The electrochemistry of mixed-valent-containing vanadium-oxide nanotubes (VO_x-NTs) is first reported. Using dodecylamine and hexadecylamine as templates, two kinds of VO_x-NTs were synthesized and characterized. Both VO_x-NTs contain V₂O₃, VO₂ and V₂O₅ species. Dodecylamine-templated VO_x-NTs (C₁₂-VO_x-NTs) are much more stable than hexadecylamine-templated VO_x-NTs (C₁₆-VO_x-NTs). C₁₂-VO_x-NTs demonstrate stable redox behavior at ∼−0.2 V, which is arising from the electrochemical transfer between V(III) and V(IV), whereas C₁₆-VO_x-NTs exhibit unstable and weak redox peaks. In both cases, the reduction reaction of V(V)–V(IV) is never observed. The oxidation peak for V(IV) appeared only after a relatively long time immersion, suggesting that some small molecules such as water intercalate into the layered nanostructures before the electrochemical reaction. The difference may be an origin of the different template-induced the electroactivity of vanadium-oxide species in the nanotube-microenvironment.