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.     

Potentiometric study of acid–base equilibria in the {KCl + LiCl} eutectic melt at temperatures in the range (873 to 1073) K

Some heterogeneous reactions of oxide ion exchange (carbonate ion dissociation and magnesium oxide dissolution) in the molten {KCl + LiCl} eutectic at temperatures of (873, 973 and 1073) K were studied using an electrochemical cell with an oxygen membrane electrode Pt(O₂)|ZrO₂(Y₂O₃). The dissociation constant of the CO₃²⁻ was found to increase with increasing temperature: pK (873 K)=(2.39 ± 0.05); pK (973 K)=(1.81 ± 0.09); pK (1073 K)=(1.53 ± 0.08). Removal of CO₂ from the gas above the melt allows the complete transformation of CO₃²⁻ to O²⁻. pP_MgO values decrease more from (6.99 ± 0.08) to (5.41 ± 0.04). The oxobasicity indices, pI_(KCl+LiCl), were calculated from the solubility data to be 3.2 at 873 K, 3.4 at 973 K, and 3.6 at 1073 K. This trend suggests an increase in acidity with increasing temperature of {KCl + LiCl}.     

Effect of Fluoride or Chloride Ions on the Morphology of ZrO2 Thin Film Grown in Ethylene Glycol Electrolyte by Anodization

0.3 wt % ammonium fluoride (NH₄F) or ammonium chloride (NH₄Cl) was added to ethylene glycol (EG) as an active ingredient for the formation of anodic oxide comprising of ZrO₂ nanotubes (ZNTs) by anodic oxidation of zirconium (Zr) at 20 V for 10 min. It was observed that nanotubes were successfully grown in EG/NH₄F/H₂O with aspect ratio of 144.3. Shorter tubes were formed in EG/NH₄F/H₂O₂. This could be due to higher excessive chemical etching at the tip of the tubes. When fluoride was replaced by chloride in both electrolytes, multilayered oxide resembling pyramids was observed. The pyramids have width at the bottom of 3-4 μm and the top is 1-2 μm with 10.7 μm height. Oxidation of Zr in EG/NH₄Cl/H₂O₂ was rater rapid. The multilayered structure is thought to have formed due to the re-deposition of ZrO₂ or hydrated ZrO₂ on the foil inside pores formed within the oxide layer. XRD result revealed an amorphous structure for as-anodized samples regardless of the electrolytes used for this work.     

Effective synthesis of novel O-acetylated compounds over ZrO2-Al2O3 solid acid

The solid acids such as ZrO₂, Al₂O₃ and ZrO₂-Al₂O₃ containing different ZrO₂ loadings (10–80 mol%) were prepared by solution combustion method (SCM) and characterized for their total surface acidity by NH₃-TPD/n-butylamine back titration method and crystallinity by powder X-ray diffraction (PXRD) technique. These solid acids were evaluated for their catalytic activity in the synthesis of novel O-acetylated products from substituted phenols, pyridine alcohols and aryl alcohols with acetic anhydride (AA) as an acetylating agent. The reaction conditions were optimized by varying the catalyst, molar ratio of the reactants, reaction temperature and amount of the catalyst. All the solid acids used in this study exhibited good catalytic activity in the reaction. In particular, ZrO₂-Al₂O₃ containing 80 mol% of ZrO₂ was found to be highly active in the acetylation reaction with high yield of acetylated products. Triangular correlation between the surface acidity, crystallinity and catalytic activity of solid acids was observed. These solid acids were found to be reactivable and reusable.     

Adduct formation of [(η7-C7H7)Hf(η5-C5H5)] with isocyanides,phosphines and N-heterocyclic carbenes: An experimental and theoretical study

The reactions of [(η⁷-C₇H₇)Hf(η⁵-C₅H₅)] (1b) with the two-electron donor ligands tert-butyl isocyanide (tBuNC), 2,6-dimethylphenyl isocyanide (XyNC), 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe) and trimethylphosphine (PMe₃) are reported. The 1:1 complexes [(η⁷-C₇H₇)Hf(η⁵-C₅H₅)L] (2b, L = tBuNC; 3b, L = XyNC; 4b, L = IMe, 5b, L = PMe₃) have been isolated in crystalline form, and their molecular structures have been determined by X-ray diffraction analyses. The stabilities of these hafnium complexes were probed via spectroscopic and theoretical methods, and the results were compared to those previously reported for the corresponding zirconium complexes derived from [(η⁷-C₇H₇)Zr(η⁵-C₅H₅)] (1a). The X-ray crystal structure of the PMe₃ adduct [(η⁷-C₇H₇)Zr(η⁵-C₅H₅)(PMe₃)] (5a) was also established.     

PALS study on the defect structure of yttria-stabilized zirconia

Powders of stabilized ZrO₂–8 mol% Y₂O₃ (YSZ) have been obtained by mechanical milling in zirconia vials. The samples were characterized by X-ray diffraction (XRD). Positron annihilation lifetime (PALS) measurements were performed to investigate the lattice defects originated by the incorporation of yttria and those mechanically induced. The XRD results indicate the formation of tetragonal YSZ solid solution. PALS results indicate that positron trapping occurs at different kinds of defects such as vacancy-like defects, grain boundary and associated defects.     

Electrical conductivity and Hf4+ ion substitution range in NaSICON system

In this paper, we present the synthesis and characterizations of NaSICON-type ionic conducting ceramics of the general formula Na_1+xM_1.775Si_x−0.9P_3.9−xO₁₂ with 1.8 ≤ x ≤ 2.2 and M = Zr or Hf. The effect of the total substitution of zirconium by hafnium on electric properties has been studied. The various compositions were prepared by using the sol–gel method and the synthesized precursors were characterized by coupled DTA–TG. The oxides obtained after pyrolysis of the precursors were identified by X-ray diffraction. A sintering study by thermodilatometry permits to select the best thermal cycle adapted to our ceramics. Furthermore, the electric conductivity of the sintered ceramic samples was characterized by complex impedance spectroscopy. These results show that ceramics containing Zr synthesized by soft method, present a higher total conductivity than those obtained in literature (to be around 10⁻⁴ S cm⁻¹). The total substitution of Zr by Hf still improves this conductivity for some compositions.     

Crystal structure of a new small-pore zirconosilicate Na2ZrSi2O7·H2O and its relation to stoichiometrically and topologically similar compounds

The small-pore zirconosilicate Na₂ZrSi₂O₇·H₂O crystallizes in monoclinic system with space group C2/c, a = 5.4715(4); b = 9.4111(6); c = 13.0969(8) Å, β = 92.851(7)°. Its framework consists of layers built of ZrO₆ octahedra and SiO₄ tetrahedra and forming condensed [Si₂O₇] pyrogroups by connection. The sodium ions and water molecules are placed in channels set up between the layers. The stoichiometric and structural similarities of the studied phase with anhydrous compounds having general chemical formula A₂₍₃₎MT₂O₇ (A = Na,K; M = Zr,Lu,Sc; T = Si,Ge) are discussed. The topological relationship of their structures is interpreted in the light of spatial combination of silicon and zirconium polyhedra as basic building units into larger composite building units and their three-dimensional arrangement.     

ZrO2- and Li2ZrO3-stabilized spinel and layered electrodes for lithium batteries

Strategies for countering the solubility of LiMn₂O₄ (spinel) electrodes at 50 °C and for suppressing the reactivity of layered LiMO₂ (M=Co, Ni, Mn, Li) electrodes at high potentials are discussed. Surface treatment of LiMn₂O₄ with colloidal zirconia (ZrO₂) dramatically improves the cycling stability of the spinel electrode at 50 °C in Li/LiMn₂O₄ cells. ZrO₂-coated LiMn_0.5Ni_0.5O₂ electrodes provide a superior capacity and cycling stability to uncoated electrodes when charged to a high potential (4.6 V vs Li⁰). The use of Li₂ZrO₃, which is structurally more compatible with spinel and layered electrodes than ZrO₂ and which can act as a Li⁺-ion conductor, has been evaluated in composite 0.03Li₂ZrO₃ · 0.97LiMn_0.5Ni_0.5O₂ electrodes; glassy Li_xZrO_2 + x/2 (0<x⩽2) products can be produced from colloidal ZrO₂ for surface coatings.     

Solid-state spreading of oxides: Morphology and conductivity of In2O3-based films

Spontaneous solid-state spreading of In₂O₃ over the surface of ceramic and single crystal substrates of Al₂O₃, ZrO₂(0.08Y₂O₃), Y₃Al₅O₁₂ and YAlO₃ at 1380 and 1500 °C has been investigated. The films structure, element and phase composition were studied by means of XRD and SEM/EDS. The phase compositions of all films correspond to that of powder In₂O₃. The most films are dense with monolithic “film/substrate” interface. AC-conductivity in the range 25–250 °C is reported. Depending on the substrate material, the films demonstrate a whole set of conductivity types (semi-conductive, metallic) and wide range of magnitudes (more than 3 orders).Thermodynamic and kinetic aspects of oxide spreading and subsequent crystallization on the surface of primary border film are discussed. It is shown, in particular, that the solid-state spreading is facilitated in systems with high chemical affinity (mutual solubility, formation of intermediate interface compounds). In the case of eutectic-type systems, heteroepitaxy of diffusant at substrate surface results in formation of dense films.     

On oxoacidic properties and solubilities of beryllium and magnesium oxides in molten (CsCs + KCl + NaCl) eutectic at T = 783 K

Reactions of Be²⁺ and Mg²⁺ with O^2– in molten eutectic mixture (CsCs + KCl + NaCl) (0.455:0.245:0.30) at T = 783 K were studied by a potentiometric method using Pt(O₂)|ZrO₂(Y₂O₃) indicator electrode. Addition of O^2– ions to the melt containing Mg²⁺ results in precipitation of MgO (pK_s,MgO = 11.89 ± 0.3, molality) whereas interaction of Be²⁺ with O^2– is accompanied with sequential formation of Be₂O²⁺ (pK = 15.68 ± 0.5, molality) and precipitation of BeO (pK_s,BeO = 9.62 ± 0.3, molality). On the basis of the obtained and known data pK_s,MgO–T⁻¹ dependence in molten (CsCs + KCl + NaCl) eutectic is constructed. The slope of the said dependence in T/K = (from 583 to 1073) range is in good agreement with the value predicted by the Shreder equation, that extends the range of use of the Shreder equation for predictions of metal oxide solubilities in molten halides.     

Synthesis,characterization and properties of microporous lanthanide silicates: K8Ln3Si12O32NO3·H2O (Ln = Eu,Tb, Gd,Sm)

A family of microporous lanthanide silicates, K₈Ln₃Si₁₂O₃₂NO₃·H₂O (denoted LnSiO-CJ3, Ln = Eu, Tb, Gd, Sm), was synthesized under mild hydrothermal conditions at 503 K. The X-ray powder diffraction patterns of these compounds reveal that they are isostructural. The structure of EuSiO-CJ3 was determined by single-crystal X-ray diffraction analysis. It crystallizes in triclinic space group P-1 (No. 2) with a = 11.599(2) Å, b = 12.225(2) Å, c = 13.829(3) Å, α = 112.99(3)°, β = 92.05(3)°, γ = 90.57(3)°. The structure is based on [Si₃O₈]_n^4n? layers with 6-, 8-, 12-rings that are connected by EuO₆ octahedra to form a 3-D framework with 8-ring channels along the [001] direction. Charge neutrality is achieved by the K⁺ and NO₃^? ions located in the channels. The framework of EuSiO-CJ3 shows good thermal stability, which can be stable up to 1273 K. Ion-exchange capacity of EuSiO-CJ3 was investigated by the exchange of NO₃^? ions with halide ions (F^?, Cl^?, Br^?). The peaks in the emission spectra of LnSiO-CJ3 (Ln = Eu, Tb) belong to the characteristic transitions of Ln³⁺ (Ln = Eu, Tb) respectively. The lifetime measurements of LnSiO-CJ3 (Ln = Eu, Tb) suggest the presence of three Ln³⁺ (Ln = Eu, Tb) environments, which are consistent with the crystallographic results.     

Study of CuZnMOx oxides (M = Al,Zr, Ce,CeZr) for the catalytic hydrogenation of CO2 into methanol

CuO–ZnO–Al₂O₃ catalysts were synthesized by two methods, sol–gel and co-precipitation syntheses. Al₂O₃ was then substituted with other supports, such as ZrO₂, CeO₂ and CeO₂–ZrO₂ in order to have a better understanding of the support's effect. These catalysts containing 30 wt% of Cu were then tested for CO₂ hydrogenation into methanol. The effect of reaction temperature and GHSV on the catalytic behaviour was also investigated. The best results were obtained with a 30 CuO–ZnO–ZrO₂ catalyst synthesized by co-precipitation and calcined at 400 °C. This catalyst presents a good CO₂ conversion rate (23%) with 33% of methanol selectivity, leading to a methanol productivity of 331 g_MeOH.kg_cata⁻¹·h⁻¹ at 280 °C under 50 bar and a GHSV of 10,000 h⁻¹.     

Structural and Mössbauer characterization of the ball milled Fex(Cr2O3)1−x system

The Fe_x(Cr₂O₃)_1?x system, with 0.10 ≤ X ≤ 0.80, was mechanically processed for 24 h in a high-energy ball-mill. In order to examine the possible formation of iron–chromium oxides and alloys, the milled samples were, later, thermally annealed in inert (argon) and reducing (hydrogen) atmospheres. The as-milled and annealed products were characterized by X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy and magnetization. The as-milled samples showed the formation of an Fe_1+YCr_2?YO_4?δ nanostructured and disordered spinel phase, the α₁-Fe(Cr) and α₂-Cr(Fe) solid solutions and the presence of non-exhausted precursors. For the samples annealed in inert atmosphere, the chromite (FeCr₂O₄) formation and the recrystallization of the precursors were verified. The hydrogen treated samples revealed the reduction of the spinel phase, with the phase separation of the chromia phase and retention of the Fe–Cr solid solutions. All the samples, either as-milled or annealed, presented the magnetization versus applied field curves typical for superparamagnetic systems.     

Standard enthalpies of formation of selected XYZ half-Heusler compounds

The standard enthalpies of formation of selected ternary half-Heusler type compositions XYZ (X = Au, Co, Fe, Ir, Ni, Pd, Pt, Rh, Ru; Y = Hf, Mn, Ti, Zr; Z = Ga, Sn) were measured using high temperature direct reaction calorimetry. The measured standard enthalpies of formation (in kJ/mole of atoms) of the half-Heusler compounds (prototype MgAgAs, Pearson symbol cF12, space group F-43m) are, IrMnSn (−29.4 ± 1.8); NiTiSn (−52.6 ± 2.4); PtHfSn (−98.8 ± 3.4); PtMnSn (−55.8 ± 2.6); PtTiSn (−93.6 ± 3.3); PtZrSn (−104.9 ± 3.8); for the B2 compound (prototype CsCl, Pearson symbol cP2, space group Pm-3m), RuMnGa (−26.9 ± 1.7); for the C1 structured (prototype CaF₂, Pearson symbol cF12, space group Pm-3m) or the C1_b structured compound IrMnGa (−40.9 ± 1.7). Indicative standard enthalpies of formation of the following compounds were obtained, half-Heusler compounds AuMnSn, CoTiSn, IrZrSn, NiHfSn, NiZrSn, PdHfSn, PdZrSn, RhTiSn; Heusler compound (prototype Cu₂MnAl, Pearson symbol cF16, space group Fm-3m) RhMnSn; hexagonal compound (prototype BeZrSi, Pearson symbol hP6, space group P6₃/mmc) PtMnGa and another type of hexagonal compound (prototype RhHfSn, Pearson symbol hP18, space group P-62c) RhHfSn, IrZrsn, RhZrSn. Values were compared with ab initio calculations from AFLOW and OQMD. Lattice parameters of these compounds were determined using X-ray diffraction (XRD) analysis. Microstructures were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Selected alloys were further annealed to investigate phase transformations and phase relationships.     

Minerals from Macedonia: XVII. Vibrational spectra of some common appearing amphiboles

The vibrational (infrared and Raman) spectroscopy is used in order to identify and characterize the following amphibole minerals with general formula W_0–1X₂Y₅Z₈O₂₂(OH)₂ (W = Na, K; X = Na, Ca; Y = Mg, Fe²⁺, Fe³⁺, Al; Z = Si, Al) originating from the localities in the Republic of Macedonia: glaucophane, Na₂(Mg,Fe²⁺)₃(Fe³⁺,Al)₂Si₈O₂₂(OH)₂; tremolite–actinolite, Ca₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂; hornblende (Na,K)_0–1Ca₂(Mg,Fe²⁺,Fe³⁺,Al)₅(Si,Al)₈ O₂₂(OH)₂ and arfvedsonite, NaNa₂(Mg,Fe²⁺)₄(Fe³⁺,Al)Si₈O₂₂(OH)₂. The chemical composition of these minerals is not necessarily fixed. It is due to the possibility to form solid solution series with other minerals being their end-members (for example, tremolite–ferro-actinolite series, Ca₂Mg₅Si₈O₂₂(OH)₂–Ca₂Fe²⁺₅Si₈O₂₂(OH)₂). In this context, it is shown that the intensity and especially the number of the IR bands in the ν(OH) region could serve as a tool for exact mineral identification. Namely, it is based on the presence of different Y cations in various octahedral sites (M1 and M3), which is manifested by different spectral view. On the other hand, the expressed similarities in the 1300–370 cm⁻¹ (IR) and 1200–100 cm⁻¹ regions (Raman) of the spectra are observed due to their common structural characteristics (double chains of SiO₄ tetrahedra). Thus, the bands in this region are tentatively prescribed mostly to the vibrations of the SiO₄ tetrahedra. The results of our study are compared with the corresponding literature data for the analogous mineral species originating all over the world.     

Vacuum ultraviolet spectroscopic properties of rare earth (RE) (RE = Eu,Tb, Dy,Sm, Tm)-doped K2GdZr(PO4)3 phosphate

The luminescent characteristics of RE (RE³⁺ = Eu, Tb, Dy, Sm and Tm)-doped K₂GdZr(PO₄)₃ have been investigated. The band in the range of 130–157 nm in the VUV excitation spectra of these compounds is attributed to the host lattice or PO₄^3? group absorption and the band from 157 nm to 215 nm with the maximum at 188 nm is due to the O–Zr charge transfer transition. For Eu³⁺-doped sample, the relatively weak band of O^2?–Eu³⁺ charge transfer (CTB) at 222 nm is observed and for Tb³⁺-doped sample, the band at 223 nm is related to the 4f–5d spin-allowed transition of Tb³⁺. For Dy³⁺- and Sm³⁺-doped samples, the O^2?–Dy³⁺ and O^2?–Sm³⁺ CTBs have not been observed, probably due to the 2p electrons of oxygen tightly bound to the zirconium ion in the host lattice. In Tm³⁺-doped sample, the weak O^2?–Tm³⁺ CTB is located at 170 nm. It is observed that there is energy transfer between the host and the luminescent activators (e.g. Eu³⁺, Tb³⁺ and Sm³⁺) except for Tm³⁺.     

Reactivity of intramolecularly coordinated aluminum compounds to R3EOH (E = Sn,Si). Remarkable migration of N,C,N and O,C,O pincer ligands

The reaction of organoaluminum compounds containing O,C,O or N,C,N chelating (so called pincer) ligands [2,6-(YCH₂)₂C₆H₃]AlⁱBu₂ (Y = MeO 1, ^tBuO 2, Me₂N 3) with R₃SnOH (R = Ph or Me) gives tetraorganotin complexes [2,6-(YCH₂)₂C₆H₃]SnR₃ (Y = MeO, R = Ph 4, Y = MeO, R = Me 5; Y = ^tBuO, R = Ph 6, Y = ^tBuO, R = Me 7; Y = Me₂N, R = Ph 8, Y = Me₂N, R = Me 9) as the result of migration of O,C,O or N,C,N pincer ligands from aluminum to tin atom. Reaction of 1 and 2 with (ⁿBu₃Sn)₂O proceeded in similar fashion resulting in 10 and 11 ([2,6-(YCH₂)₂C₆H₃]SnⁿBu₃, Y = MeO 10; Y = ^tBuO 11) in mixture with ⁿBu₃SnⁱBu. The reaction 1 and 3 with 2 equiv. of Ph₃SiOH followed another reaction path and ([2,6-(YCH₂)₂C₆H₃]Al(OSiPh₃)₂, Y = MeO 12, Me₂N 13) were observed as the products of alkane elimination. The organotin derivatives 4–11 were characterized by the help of elemental analysis, ESI-MS technique, ¹H, ¹³C, ¹¹⁹Sn NMR spectroscopy and in the case 6 and 8 by single crystal X-ray diffraction (XRD). Compounds 12 and 13 were identified using elemental analysis,¹H, ¹³C, ²⁹Si NMR and IR spectroscopy.     

The crystal structure of potassium ammonium hexamolybdotellurate with telluric acid K5NH4[TeMo6O24].Te(OH)6.6H2O

An inorganic compound formulated as K₅NH₄[TeMo₆O₂₄].Te(OH)₆.6H₂O (1) has been isolated by conventional solution method and structurally characterized by single-crystal X-ray diffraction methods, scanning electron microscopy (SEM), IR, UV–vis spectra, and cyclic voltammetry measurements. This compound crystallizes in the monoclinic system, space group C2/c with unit a = 18.6841(1) Å, b = 10.0513(1) Å, c = 21.1065(1) Å, β = 116.495(1)°, V = 3547.49(4) ų, Z = 4, R = 0.033 and wR (F²) = 0.087 for 3432 unique observed reflexions [I > 2σ(I)]. The crystal structure of (1) is built up from an Anderson clusters connected through hydrogen-bonding interactions into a three-dimensional supramolecular network.     

Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation

Micro-tubular solid-oxide fuel cell consisting of a 10-μm thick (ZrO₂)_0.89(Sc₂O₃)_0.1(CeO₂)_0.01 (ScSZ) electrolyte on a support NiO/(ScSZ) anode (1.8 mm diameter, 200 μm wall thickness) with a Ce_0.8Gd_0.2O_1.9 (GDC) buffer-layer and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)/GDC functional cathode has been developed for intermediate temperature operation. The functional cathode was in situ formed by impregnating the well-dispersed nano-Ag particles into the porous LSCF/GDC layer using a citrate method. The cells yielded maximum power densities of 1.06 W cm⁻² (1.43 A cm⁻², 0.74 V), 0.98 W cm⁻² (1.78 A cm⁻², 0.55 V) and 0.49 W cm⁻² (1.44 A cm⁻², 0.34 V), at 650, 600 and 550 °C, respectively.