Bond covalency in perovskite oxynitrides ATaO2N (A = Ca,Sr, Ba) studied by 14N NMR spectroscopy

Local geometry and bond ionicity around the nitride ions in simple perovskite oxynitrides ATaO₂N (A = Ca, Sr, Ba) have been investigated by solid-state magic-angle spinning (MAS) NMR spectroscopy. From all three compounds, fairly sharp ¹⁴N NMR peaks were observed, suggestive of the symmetric coordination environment of nitride ions. The ¹⁴N chemical shifts of ATaO₂N, δ = 269–272 ppm relative to NH₄Cl (δ = 0 ppm), are correlated to the bond ionicity, based on the N−Ta bond distances and Ta–N–Ta bond angles determined from the Rietveld refinement of neutron diffraction patterns. The ¹H NMR measured for BaTaO₂N presented a peak corresponding to H₂O, implying that the polycrystalline surface of present oxynitride phases is covered by hydroxide terminals.     

Effects of GC temperature and carrier gas flow rate on on‐line oxygen isotope measurement as studied by on‐column CO injection

Although deemed important to δ¹⁸O measurement by on‐line high‐temperature conversion techniques, how the GC conditions affect δ¹⁸O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO–N₂ mix onto the GC column by a six‐port valve and examined the CO yield, CO peak shape, CO–N₂ separation, and δ¹⁸O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N₂ and CO decreases with an increase of GC temperature and carrier gas flow rate. δ¹⁸O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ¹⁸O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ¹⁸O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C¹⁶O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N₂–CO separation, lower He consumption, and insignificant effect on δ¹⁸O value, while a higher‐than‐60 °C GC temperature and a larger‐than‐100 µl CO volume is also recommended. When no N₂ peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrical, electrochemical, and thermomechanical properties of perovskite-type (La1?x Sr x )1?y Mn0.5Ti0.5O3?δ (x?=?0.15–0.75, y?=?0–0.05)

Strontium additions in (La_1?x Sr _x )_1?y Mn_0.5Ti_0.5O_3?δ (x?=?0.15–0.75, y?=?0–0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity and Seebeck coefficient studied at 973–1223?K in the p(O₂) range from 10^?19 to 0.5?atm suggest a dominant role of electron hole hopping and relatively stable Mn³⁺ and Ti⁴⁺ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti⁴⁺ cations and oxygen anions, the tracer diffusion coefficients measured by the ¹⁸O/¹⁶O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum–strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8–15.0?×?10^?6?K^?1 at 300–1370?K and oxygen pressures from 10^?21 to 0.21?atm. The anodic overpotentials of porous La_0.5Sr_0.5Mn_0.5Ti_0.5O_3?δ electrodes with Ce_0.8Gd_0.2O_2-δ interlayers, applied onto LaGaO₃-based solid electrolyte, are lower compared to (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3?δ when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, ~2 Ω?×?cm² in humidified 10?% H₂–90?% N₂ atmosphere at 1073?K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H₂S traces in H₂-containing gas mixtures did not result in detectable decomposition of the perovskite phases.     

A novel mixed-metal borate with large [B12O18(OH)6]6- motif: Synthesis,structure and property

A new mixed-metal polyborate, Na₅Li[B₁₂O₁₈(OH)₆]·2H₂O (1), has been synthesized using solvothermal method and characterized by IR spectroscopy, thermogravimetric analysis, UV–Vis spectroscopy, powder and single-crystal X-ray diffraction, respectively. It crystallizes in the trigonal space group R-3c (No. 167) with unit cell parameters of a = b = 9.6767(6) Å, c = 36.358(5) Å, and Z = 6. Its structure features unprecedented 3D framework constructed from novel honeycomb-shaped inorganic Na-O sheets with unique 12-MR sodium rings and supramolecular polyborate 2D layers of lithium-centered [B₁₂O₁₈(OH)₆]^6-. UV–Vis spectral characterization indicates that compound 1 is a wide-band-gap semiconductor.     

Perovskite-type oxides for high-temperature oxygen separation membranes

Oxygen permeation through dense ceramic membranes of perovskite-like SrCo_0.9−xFe_0.1Cr_xO_3−δ (x = 0.01–0.05), Sr_1−x−yLn_xCoO_3−δ(Ln = La, Nd, Sm, Gd; x = 0.30–0.35; y = 0–0.10), SrCo_1−xTi_xO_3−δ (x = 0.05–0.20) and LaM_1−xNi_xO_3−δ (M = Ga, Co, Fe; x = 0–0.6) was studied. The SrCoO_3−δ-based solid solutions with cubic perovskite structure were found to exhibit highest permeation fluxes compared to other membranes. However, high thermal expansion coefficients and interaction with gas species such as carbon dioxide may complicate the employment of SrCoO_3−δ membranes for oxygen separation membranes. Alternatively, the LaGa_1−xNi_xO_3−δ (x = 0.2–0.5) perovskites, having significant permeation fluxes as well as thermal expansion coefficients in the range of (10.8–11.6) × 10⁻⁶ K⁻¹, were demonstrated to be suitable as membrane materials at oxygen pressures from 1 × 10⁻² to 2 × 10⁴ Pa. Testing oxygen permeation at oxygen partial pressures of 1–60 atm showed that only oxides with a high oxygen deficiency such as SrCo_0.85Ti_0.15F_3−δ possess sufficient oxygen permeation fluxes. The oxygen permeability of perovskites on the basis of LaGaO₃ and LaCoO_3−δ was found to be negligible at oxygen pressures above 15 atm, caused by low oxygen vacancy concentration and ionic conductivity of such ceramic materials.     

Preparation and characterization of Ce1−xSmxO2−δ (x = 0.1–0.3) as electrolyte material for intermediate temperature SOFC

The effect of Sm doping on CeO₂ for its use as a solid electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been explored here. Ce_1−xSm_xO_2−δ (x = 0.1–0.3) samples are successfully synthesized by carbonate co-precipitation method. TG–DTA, XRD, Raman, UV–Vis, FT-IR, SEM and ac-impedance are used for structural and electrical characterization. From the XRD patterns, well-crystalline cubic fluorite structured solid solution is confirmed. Lattice parameters increased with increase in Sm³⁺ while the crystallite size decreased. The optical absorption spectra exhibits a red shift for Sm³⁺ doped CeO₂. Raman spectra show an intense peak at 463 cm⁻¹, a characteristic peak for doped ceria. SEM shows cluster like particles. Based on ac-impedance data, the total oxygen ionic conductivity is highest for Ce_0.8Sm_0.2O_2−δ in the temperature range of 473–623 K.     

Redox behavior and transport properties of La0.5−2xCexSr0.5+xFeO3−δ and La0.5−2ySr0.5+2yFe1−yNbyO3−δ perovskites

The effects of doping the mixed-conducting (La,Sr)FeO_3−δ system with Ce and Nb have been examined for the solid-solution series, La_0.5−2xCe_xSr_0.5+xFeO_3−δ (x = 0–0.20) and La_0.5−2ySr_0.5+2yFe_1−yNb_yO_3−δ (y = 0.05–0.10). Mössbauer spectroscopy at 4.1 and 297 K showed that Ce⁴⁺ and Nb⁵⁺ incorporation suppresses delocalization of p-type electronic charge carriers, whilst oxygen nonstoichiometry of the Ce-containing materials increases. Similar behavior was observed for La_0.3Sr_0.7Fe_0.90Nb_0.10O_3−δ at 923–1223 K by coulometric titration and thermogravimetry. High-temperature transport properties were studied with Faradaic efficiency (FE), oxygen-permeation, thermopower and total-conductivity measurements in the oxygen partial pressure range 10⁻⁵–0.5 atm. The hole conductivity is lower for the Ce- and Nb-containing perovskites, primarily as a result of the lower Fe⁴⁺ concentration. Both dopants decrease oxide-ion conductivity but the effect of Nb-doping on ionic transport is moderate and ion-transference numbers are higher with respect to the Nb-free parent phase, 2.2 × 10⁻³ for La_0.3Sr_0.7Fe_0.9Nb_0.1O_3−δ cf. 1.3 × 10⁻³ for La_0.5Sr_0.5FeO_3−δ at 1223 K and atmospheric oxygen pressure. The average thermal expansion coefficients calculated from dilatometric data decrease on doping, varying in the range (19.0–21.2) × 10⁻⁶ K⁻¹ at 780–1080 K.     

Electrical properties of LiBBaTe glass doped with Nd2O3

The dc and ac electrical conductivity of barium tellurite borate glass doped with Nd₂O₃ in the composition 50 B₂O₃- (20-X) BaO- 20TeO₂ 10 LiF or Li₂O where x = 0.5, 1, 1.5 and 2 Nd₂O₃ were measured in the temperature range 303–648 K and in the frequency range 0.1–100 kHz. The dc and ac conductivities values increase, whereas the activation energy of conductivities decreases with increasing Nd₂O₃ content in the glasses containing LiF and by the replacement of LiF by Li₂O the conductivity was found to decrease with addition of Nd₂O₃. The electrical conduction has been observed to be due to small polaron hopping at high temperatures. The frequency dependence of the ac conductivity follows the power law σ_AC (ω) = A ω^s. The frequency exponent (s) values (in the range 0.94 and 0.33) decreases with increasing temperature. The dielectric constant and dielectric loss increased with increasing temperature and decreased with increase in frequency for all glasses studied. In LiF glasses, it is observed that, the values of ?^\ and tan δ are observed to increase with the addition of Nd₂O₃ whereas they decrease in the glasses containing Li₂O. The electrical modulus formalism has been used for studying electrical relaxation behavior in studied glasses. It is for first time that the Nd₂O₃ doped barium tellurite borate glasses have been investigated for dc and ac conductivities and dielectric properties over a wide range of frequency and temperature.     

Dimorphic cerium(III) oxoarsenate(III) Ce[AsO3]

Colourless, water- and air-stable single crystals of cerium(III) oxoarsenate(III) Ce[AsO₃] were prepared by the reaction of cerium metal (Ce) and arsenic sesquioxide (As₂O₃) in the presence of cesium chloride (CsCl) as fluxing agent at 750 °C in an evacuated silica ampoule. Ce[AsO₃] crystallizes monoclinically (a = 902.89(8), b = 782.54(7), c = 829.68(7) pm, β = 103.393(3)°, Z = 8) in the space group P2₁/c and is isotypic with α-Pb[SeO₃]. There are two crystallographically different Ce³⁺ positions. (Ce1)³⁺ is coordinated by nine oxygen atoms (d(Ce–O) = 244–286 pm) and (Ce2)³⁺ by only eight (d(Ce–O) = 239–273 pm). Both crystallographically different As³⁺ cations form discrete ψ¹ tetrahedra [AsO₃]³⁻ (d(As–O) = 174–179 pm), which are attached to the Ce³⁺ cations via edges and corners. The second monoclinic modification of Ce[AsO₃] with the lattice parameters a = 439.32(4), b = 529.21(5), c = 617.34(6) pm and β = 105.369(3)° with Z = 2 was obtained by high-pressure synthesis (11 GPa, 1200 °C) and has both a higher density (6.31 vs. 6.13 g · cm⁻³) and a higher calculated Madelung part of the lattice energy (15,155 vs. 15,132 kJ · mol⁻¹). It adopts the space group P2₁/m, crystallizing isotypically with La[AsO₃], β-Pb[SeO₃], Pb[SO₃] (scotlandite) or K[ClO₃] and exhibits nine-fold coordinated Ce³⁺ cations exclusively (d(Ce–O) = 254–287 pm) along with tripodal [AsO₃]³⁻ anions (d(As–O) = 175–176 pm). Raman spectroscopy on both phases of Ce[AsO₃] shows stretching vibrations between 769 and 731 cm⁻¹ as well as asymmetric vibrations in the range of 659–617 cm⁻¹. The symmetric bending mode vibrations emerge in an interval from 340 to 410 cm⁻¹ and the asymmetric bending modes range between 230 and 290 cm⁻¹.     

Monthly sea surface temperature records reconstructed by δ<Superscript>18</Superscript>O of reef-building coral in the east of Hainan Island,South China Sea

Stable oxygen isotopic compositions of a coral colony ofPorites lutea obtained on a core allowed the reconstruction of a 56-a (1943–1998) proxy record of the sea surface temperatures. This coral δ¹⁸O data are from the east of Hainan Island water (22°20’N, 110°39’E), South China Sea. The relationship between δ¹⁸O in the skeletal aragonite carbonate and the sea surface temperature (SST) is SST = -5.36 δ¹⁸O_PDB-3.51 (r = 0.73,n = 470), dδ¹⁸O/d(SST) = -0.187?/ °C; and the thermometer was set at monthly resolution. The 56-a (1943–1998) proxy record of the sea surface temperatures reflected the same change trend in the northern part of South China Sea as the air temperature change trend in China.     

Lanthanide complexes derived from hexadentate macrocyclic ligand: Synthesis,spectroscopic and thermal investigation

The lanthanide complexes derived from (3,5,13,15-tetramethyl 2,6,12,16,21-22-hexaazatricyclo[15.3.I^1-17I^7-11]cosa-1(21),2,5,7,9,11(22),12,15,17,19-decane) were synthesized. The complexes were found to have general composition [Ln(L)X₂·H₂O]X, where Ln = La³⁺, Ce³⁺, Nd³⁺, Sm³⁺ and Eu³⁺ and X = NO₃^? and Cl^?. The ligand was characterized by elemental analyses, IR, Mass, and ¹H NMR spectral studies. All the complexes were characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, IR, Mass, electronic spectral techniques and thermal studies. The ligand acts as a hexadentate and coordinates through four nitrogen atoms of azomethine groups and two nitrogen of pyridine ring. The lanthanum complexes are diamagnetic while the other Ln(III) complexes are paramagnetic. The spectral parameters i.e. nephelauxetic ratio (β), covalency factor (b^1/2), Sinha parameter (δ%) and covalency angular overlap parameter (η) have been calculated from absorption spectra of Nd(III) and Sm(III) complexes. These parameters suggest the metal–ligand covalent bonding. In the present study, the complexes were found to have coordination number nine.     

Offline oxygen isotope analysis of organic compounds with high N:O

Although the advantages of online δ¹⁸O analysis of organic compounds make its broad application desirable, researchers have encountered NO⁺ isobaric interference with CO⁺ at m/z 30 (e.g. ¹⁴N¹⁶O⁺, ¹²C¹⁸O⁺) when analyzing nitrogenous substrates. If the δ¹⁸O value of inter‐laboratory standards for substrates with high N:O value could be confirmed offline, these materials could be analyzed periodically and used to evaluate δ¹⁸O data produced online for nitrogenous unknowns. To this end, we present an offline method based on modifications of the methods of Schimmelmann and Deniro (Anal. Chem. 1985; 57: 2644) and Sauer and Sternberg (Anal. Chem. 1994; 66: 2409), whereby all the N₂ from the gas products of a chlorinated pyrolysis was eliminated, resulting in purified CO₂ for analysis via a dual‐inlet isotope ratio mass spectrometry system. We evaluated our method by comparing observed δ¹⁸O values with previously published or inter‐laboratory calibrated δ¹⁸O values for five nitrogen‐free working reference materials; finding isotopic agreement to within ±0.2‰ for SIGMA® cellulose, IAEA‐CH3 cellulose (C₆H₁₀O₅) and IAEA‐CH6 sucrose (C₁₂H₂₂O₁₁), and within ±1.8‰ for IAEA‐601 and IAEA‐602 benzoic acids (C₇H₆O₂). We also compared the δ¹⁸O values of IAEA‐CH3 cellulose and IAEA‐CH6 sucrose that was nitrogen‐'doped' with adenine (C₅H₅N₅), imidazole (C₃H₄N₂) and 2‐aminopyrimidine (C₄H₅N₃) with the undoped δ¹⁸O values for the same substrates; yielding isotopic agreement to within ±0.7‰. Finally, we provide an independent analysis of the δ¹⁸O value of IAEA‐600 caffeine (C₈H₁₀N₄O₂), previously characterized using online systems exclusively, and discuss the reasons for an average 1.4‰ enrichment in δ¹⁸O observed offline relative to the consensus online δ¹⁸O value. Copyright © 2010 John Wiley & Sons, Ltd.     

Simulation of 59Co NMR Chemical Shifts in Aqueous Solution

⁵⁹Co chemical shifts were computed at the GIAO‐B3LYP level for [Co(CN)₆]^3?, [Co(H₂O)₆]³⁺, [Co(NH₃)₆]³⁺, and [Co(CO)₄]^? in water. The aqueous solutions were modeled by Car–Parrinello molecular dynamics (CPMD) simulations, or by propagation on a hybrid quantum‐mechanical/molecular‐mechanical Born–Oppenheimer surface (QM/MM‐BOMD). Mean absolute deviations from experiment obtained with these methods are on the order of 400 and 600 ppm, respectively, over a total δ(⁵⁹Co) range of about 18 000 ppm. The effect of the solvent on δ(⁵⁹Co) is mostly indirect, resulting primarily from substantial metal–ligand bond contractions on going from the gas phase to the bulk. The simulated solvent effects on geometries and δ(⁵⁹Co) values are well reproduced by using a polarizable continuum model (PCM), based on optimization and perturbational evaluation of quantum‐mechanical zero‐point corrections.     

Minimization of sample requirement for δ18O in benzoic acid

The measurement of the oxygen stable isotope content in organic compounds has applications in many fields, ranging from paleoclimate reconstruction to forensics. Conventional High‐Temperature Conversion (HTC) techniques require >20 µg of O for a single δ¹⁸O measurement. Here we describe a system that converts the CO produced by HTC into CO₂ via reduction within a Ni‐furnace. This CO₂ is then concentrated cryogenically, and 'focused' into the isotope ratio mass spectrometry (IRMS) source using a low‐flow He carrier gas (6–8 mL/min). We report analyses of benzoic acid (C₇H₆O₂) reference materials that yielded precise δ¹⁸O measurement down to 1.3 µg of O, suggesting that our system could be used to decrease sample requirement for δ¹⁸O by more than an order of magnitude. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of Nitrogen and Oxygen Isotopes in Nitrates: A Minireview

Nitrogen and oxygen isotope ratios (δ¹⁵N and δ¹⁸O) in nitrates are important in hydrology, oceanography, atmospheric chemistry, and agriculture. This paper reviews current isotope ratio mass spectrometry methods for the determination of nitrogen and oxygen isotopes in nitrates that include graphite combustion, AgNO₃-ion exchange, Ba(NO₃)₂-acetone, two-step chemical conversion, bacterial approaches, and off-axis integral cavity output laser spectroscopy. This paper introduces the principles, processes, advantages, and disadvantages of these procedures. Future studies should focus on the determination of oxygen isotopes. The development of novel spectroscopic or other isotopic methodology may also be new research directions. In addition, in some nitrates, δ¹⁷O is more sensitive than traditional δ¹⁸O and further development for this isotope is of interest. In addition, the determination of δ¹⁷O may be used to more accurately evaluate δ¹⁸O.     

Synchronous ATR infrared and NIR-spectroscopy investigation of sepiolite upon drying

A new environmental cell allowing for the independent synchronous collection of the near- and mid-infrared spectra (12,000–600 cm⁻¹) in the diffuse reflection and attenuated total reflection (ATR) modes, respectively, is reported. The cell is employed to study in real time the dehydration of the phyllosilicate mineral sepiolite, Mg₈Si₁₂O₃₀(OH)₄(OH₂)₄·wH₂O, in both its natural form and after in situ deuteration at ambient. The spectra are obtained under dynamic purging with dry N₂ and compared to those of the same material conditioned over saturated salt solutions. Sepiolite is an important industrial mineral with a modulated structure of alternating tunnels and ribbons. Its mild drying is associated with pronounced vibrational spectral changes due to the removal of surface and zeolitic H₂O and the concomitant structural relaxation of the ribbons. Detailed assignments are provided for the fundamental, combination and overtone spectrum of H₂O confined in the tunnels of sepiolite, SiOH groups on the external surface of the particles, and Mg₃OH groups in the 2:1 ribbons. The spectra are discussed in comparison to those of palygorskite (modulated phyllosilicate with narrower ribbons and tunnels), talc (trioctahedral magnesian phyllosilicate without modulation) and high-surface area silica. It is demonstrated that sepiolite exhibits three discrete states of zeolitic hydration at ambient temperature: Besides the previously known hydrated (w = 7–8) and dry (w = 0–1) states which dominate the spectra above 30% and below 3% relative humidity, respectively, a hitherto unknown intermediate (w = 4–5) is found in the 3–10% range. The new state is most conveniently identified in the near-infrared by a ν₀₂ Mg₃O-H stretching mode at 7205 cm⁻¹ (ν₀₁ = 3686 cm⁻¹, X = 83.5 cm⁻¹) and a characteristic H₂O combination band at 5271 cm⁻¹ (D₂O: 3908 cm⁻¹).     

Optical absorption and near infrared emission properties of Nd3+ ions in alkali lead tellurofluoroborate glasses

Nd³⁺ doped H₃BO₃–PbO–TeO₂–RF (R = Li, Na and K) glasses were prepared through melt quenching technique. Optical absorption and near infrared (NIR) fluorescence spectra were recorded at room temperature. The spectral intensities were analyzed in terms of the Judd–Ofelt (J–O) parameters (Ω_λ = 2, 4, 6). The covalency effect of Nd–O bond on the J–O parameters was estimated from the relative absorbance ratio (R) between ⁴I_9/2 → ⁴F_7/2 and ⁴I_9/2 → ⁴S_3/2 transitions. The effect of Nd–O covalency on the Ω₄ and Ω₆ intensity parameters as well as on the spontaneous emission probabilities (A_R) was discussed. Lomheim and Shazer hybrid method was applied to determine the fluorescence branching ratios (β_R) of each emission transition from the ⁴F_3/2 metastable level to its lower lying levels. The evaluated total radiative transition probabilities (A_T), stimulated emission cross-sections (σ_e) and gain bandwidth parameters (σ_e × Δλ_P) were compared with the earlier reports.     

Interplay between Cu and Fe Valences in BaR(Cu0.5Fe0.5)2O5+δ Double Perovskites with R=Lu, Yb, Y, Eu, Sm, Nd, and Pr

We have conducted a systematic ⁵⁷Fe Mössbauer study on BaR(Cu_0.5Fe_0.5)₂O_5+δ double perovskites with various oxygen contents and rare-earth elements (R=Lu, Yb, Y, Eu, Sm, Nd, and Pr). In samples based on R=Lu, Yb, Y, Eu, Sm the oxygen content remained at δ≈0, upon reductive or oxidative heat treatments under normal pressure. The larger rare-earth elements, i.e. Nd or Pr, readily allowed for continuous oxygen content tuning up to δ≈0.3. By employing high-pressure heat treatments higher oxygen contents were achieved for all samples. The Néel temperature of the samples was found to decrease with increasing amounts of oxygen entering the lattice. In high-pressure oxygenated samples the decrease was less severe indicating that despite the incorporation of oxygen a large amount of Fe still remains in the high-spin trivalent state. By using charge-neutrality arguments together with the relative intensities of the Mössbauer spectral components the average valences of Fe and Cu were obtained. Oxygenation under normal pressure led to a corresponding increase of the valence of Fe, while Cu remained divalent. Upon high-pressure heat treatment equal amounts of Fe³⁺ and Cu²⁺ were found to be oxidized to Fe⁵⁺ and Cu³⁺, respectively.     

Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

A comparative study on the oxidation and charge compensation in the A_xCoO_2−δ systems, A=Na (x=0.75, 0.47, 0.36, 0.12) and Li (x=1, 0.49, 0.05), using X-ray absorption spectroscopy at O 1s and Co 2p edges is reported. Both the O 1s and Co 2p XANES results show that upon removal of alkali metal from A_xCoO_2−δ the valence of cobalt increases more in Li_xCoO_2−δ than in Na_xCoO_2−δ. In addition, the data of O 1s XANES indicate that charge compensation by oxygen is more pronounced in Na_xCoO_2−δ than in Li_xCoO_2−δ.