Oxidation enthalpies for reduction of ceria surfaces

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow titration and coulometric titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O₂) of 1.6 × 10⁻²⁶ atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from −500 kJ/mol O₂ at low extents of reduction to near the bulk value of −760 kJ/mol O₂ at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm⁺³-doped ceria (SDC) and Y⁺³-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.     

Electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria

The electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria have been determined as a function of temperature (300 < T/°C < 700) and oxygen partial pressure [10^− 24 ≤ p(O₂)/bar ≤ 1, T = 700 °C] by application of impedance spectroscopy. The yttria and ceria contents of Ce_xY_0.2 − xSc_0.6Zr_3.2O_8 − δ (0 ≤ x ≤ 0.2) have been varied systematically. Homogeneous samples have been prepared by means of a sol-gel (glycine-nitrate) combustion process. The ionic conductivity in air is almost independent of composition with typical values around 0.03-0.04 S cm^− 1 for the bulk at 700 °C. A significant decrease of the ionic conductivities of bulk and grain boundaries is found for samples co-doped with ceria at low oxygen partial pressures [p(O₂) < 10^− 15 bar, T = 700 °C]. Activation energies for the ionic transport in oxidizing (air) and reducing (1%-H₂/Ar) atmospheres have been extracted from Arrhenius-plots. The oxygen nonstoichiometry in 1%-H₂/Ar has been investigated by employing thermogravimetry. The decrease of the ionic conductivity under reducing conditions is accompanied by an increase of the corresponding high temperature activation energy of the bulk, which is interpreted in terms of defect association or clustering.     

DC conductivity of silver vanadium tellurite glasses

The mixed electronic-ionic conduction in 0.5[xAg₂O-(1−x)V₂O₅]-0.5TeO₂ glasses with x=0.1-0.8 has been investigated over a wide temperature range (70-425 K). The mechanism of dc conductivity changes from predominantly electronic to ionic within the 30?mol% Ag₂O?40 range; it is correlated with the underlying change in glass structure. The temperature dependence of electronic conductivity has been analyzed quantitatively to determine the applicability of various models of conduction in amorphous semiconducting glasses. At high temperature, T>θ_D/2 (where θ_D is the Debye temperature) the electronic dc conductivity is due to non-adiabatic small polaron hopping of electrons for 0.1?x?0.5. The density of states at Fermi level is estimated to be N(E_F)≈10¹⁹-10²⁰ eV⁻¹ cm⁻³. The carrier density is of the order of 10¹⁹ cm⁻³, with mobility ≈2.3×10⁻⁷-8.6×10⁻⁹ cm² V⁻¹ s⁻¹ at 300 K. The electronic dc conductivity within the whole range of temperature is best described in terms of Triberis-Friedman percolation model. For 0.6?x?0.8, the predominantly ionic dc conductivity is described well by the Anderson-Stuart model.     

Investigations on Fe doped polyvinyl alcohol films with and without gamma (γ)-irradiation

This paper deals with the preparation of pure and ferric chloride (FeCl₃) doped polyvinyl alcohol (PVA) films by solution casting method. Optical and electrical properties were systematically investigated. We have found the decrease in optical band gap energy of PVA films on doping FeCl₃. The optical band gap energy values in the present work are found to be 3.10 eV for pure PVA, 2 eV for PVA:Fe³⁺ (5 mol%), 1.91 eV for PVA:Fe³⁺(15 mol%) and 1.8 eV for PVA:Fe³⁺(25 mol%). Direct current electrical conductivity (σ) of pure, FeCl₃ doped PVA films in the temperature range 70-127 °C has been studied. At 387 K dc electrical conductivity of pure PVA film is 5.5795 μ Ω⁻¹ cm⁻¹, PVA:Fe³⁺ (5 mol%) film is 10.0936 μ Ω⁻¹ cm⁻¹ and γ-Irradiated PVA:Fe³⁺ (5 mol%) film for 900 CGY/min is 22.1950 μ Ω⁻¹ cm⁻¹. The result reveals the enhancement of the electrical conductivity with γ-irradiation. FT-IR study signifies the intermolecular hydrogen bonding between Fe³⁺ ions of FeCl₃ with OH group of PVA.     

Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes

Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La_9.333 + xSi₆O_26 + 1.5x (LSO) were investigated in the oxygen-excess region (x > ca. 0.3). We have found that the oxygen excess-type LSO (OE-LSO), namely La₁₀Si₆O₂₇ on weighted basis, exhibited high conductivity, and substitution of the Si-site of LSO with some dopants (Mⁿ⁺) had a positive effect toward the conducting property. Furthermore, it was also found that addition of a very small amount of iron ions into the M-doped OE-LSO, La₁₀(Si_6-yMⁿ⁺_y)O_27-(2-0.5n)y, improved its conductivity. On the other hand, replacement of the La-site with various ions for La₁₀(Si_6-yMⁿ⁺_y)O_27-(2-0.5n)y did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1-α){La₁₀(Si_5.8Al_0.2)O_26.9}-α(FeO_γ), evaluated with the Hebb-Wagner polarization method were very low: i.e., 1.1 × 10^− 3 and 2.9 × 10^− 3 under P(O₂) = 1.1 × 10⁴ Pa at 1073 K for α = 0.00 and 0.005, respectively. Conductivity for each sample was unchanged under humidified atmosphere at 1073 K sustained for over 50 h, revealing that both compositions were chemically stable. It was concluded that 0.995{La₁₀(Si_5.8Al_0.2)O_26.9}-0.005(FeO_γ) is suitable for the fuel cell electrolytes because of its high and almost pure ionic conductivity, and its good chemical stability under humidified as well as reducing conditions.     

Metal ion-molecule kinetics at combustion temperatures: The reaction of Ca with O2

The high-temperature photochemistry (HTP) technique, previously used for reactions of neutral species, has been adapted to the study of atomic metal ion-molecule reactions. Ca⁺ ions were generated by 193 nm multi-photon photolysis of calcium acetyl acetonate and its pyrolysis fragments. The relative ion concentrations were monitored by laser-induced fluorescence at 393.4 nm. Ar was used as the bath gas. The data for the Ca⁺ + O₂ + M → CaO₂⁺ + M association reaction (1) are fitted by k₁(907-1425 K) = 3.5 × 10⁻³² exp(+3161 K/T) cm⁶ molecule⁻² s⁻¹. Combining with an approximate k₁(296 K) value in the literature leads to k₁(296-1425 K) = 5.8 × 10⁻²² (T/K)^−2.9 exp(−601 K/T) cm⁶ molecule⁻² s⁻¹. Over much of the observed temperature range reaction (1) has much smaller rate coefficients than the corresponding neutral Ca association reaction. Reaction (1) is shown to behave very similarly to the O₂ association reaction with neutral K atoms, with which Ca⁺ is iso-electronic. This suggests that the initial step is ion-pair complex formation of the superoxide Ca²⁺(O₂)⁻, which is also consistent with results from density functional calculations. The k₁ values are rationalized via Troe’s unimolecular formalism, which leads to good accord with the experiments.     

Optical investigation of charge-transfer transitions in spinel Co3O4

Optical transitions in normal-spinel Co₃O₄ have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co₃O₄ by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (Zn_xCo_3−xO₄) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d-d charge-transfer transition between the t_2g states of octahedral Co³⁺ ion and t₂ states of tetrahedral Co²⁺ ion, t_2g(Co³⁺)→t₂(Co²⁺). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O²⁻ ion: p(O²⁻)→t₂(Co²⁺) for the 2.4 eV absorption and p(O²⁻)→e_g(Co³⁺) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for Zn_xCo_3−xO₄ can be explained in terms of the substitution of the tetrahedral Co²⁺ sites by Zn²⁺ ions. The crystal-field splitting Δ_Oh between the e_g and the t_2g states of the octahedral Co³⁺ ion is estimated to be 2 eV.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Oxygen nonstoichiometry, defect structure and electrical properties of LaFe0.7Ni0.3O3 − δ

Oxygen nonstoichiometry (δ), total conductivity (σ) and thermoelectric power (S) of the LaFe_0.7Ni_0.3O_3 − δ sample have been studied as functions of temperature and oxygen partial pressure. Based on the results of the direct reduction of the sample in hydrogen flow at 1100 °C the absolute oxygen content (3 − δ) has been found to vary from 2.999 to 2.974 in the range of 1273-1373 K and 10^− 3-0.21 atm. The point defect equilibrium models have been proposed and fitted to the set of experimental data in the form of log p(O₂) = f(δ)_T dependences. The values of standard thermodynamic quantities of defect formation reactions have been assessed. The joint analysis of oxygen nonstoichiometry, total conductivity and thermoelectric power has been performed using a small-polaron approach. The values of partial conductivity, partial thermopower and mobilities of electronic charge carriers have been calculated. The p-type semiconducting behavior of LaFe_0.7Ni_0.3O_3 − δ has been explained by the higher mobility values of electron holes than those of electrons in the whole range of thermodynamic parameters studied.     

Synthesis and ion conductivity of (Bi2O3)0.75(Dy2O3)0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi₂O₃)_0.75(Dy₂O₃)_0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 followed a parabolic trend, expressed as D² − D₀² = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol^− 1. Dense (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi₂O₃.     

Stability, oxygen permeability and chemical expansion of Sr(Fe,Al)O3 − δ- and Sr(Co,Fe)O3 − δ-based membranes

Perovskite-type SrFe_0.7Al_0.3O_3 − δ and SrCo_0.8Fe_0.2O_3 − δ, and two related dual-phase composites with nominal compositions (SrFeO_3 − δ)_0.7(SrAl₂O₄)_0.3 and (SrCo_0.8Fe_0.2O_3 − δ)_0.7(SrAl₂O₄)_0.3, were comparatively studied employing controlled-atmosphere dilatometry, thermogravimetry, Mössbauer spectroscopy, and measurements of steady-state oxygen permeation fluxes through dense ceramic membranes. The composite materials display lower thermal and chemical expansion compared to the parent single-phase perovskites. The thermal expansion coefficients at 1023-1223 K are however still high, (20-23) × 10^− 6 K^− 1 at atmospheric oxygen pressure and (17-18) × 10^− 6 K^− 1 at p(O₂) = 10 Pa, thus limiting the range of possible membrane reactor configurations. Sr(Co,Fe)O_3 − δ-based materials exhibit extensive vacancy-ordering processes in inert atmospheres, resulting in a slow relaxation of the oxygen nonstoichiometry, chemical expansion and oxygen permeation fluxes. In comparison to Sr(Fe,Al)O_3 − δ, the stability of cobalt-containing ceramics in CO₂ is also poor, which leads to a partial blocking of the membrane surface by decomposition products and degradation of the oxygen transport. Thermogravimetric analysis showed that the interaction with carbon dioxide occurs even at elevated temperatures, up to 1223 K. Under high oxygen chemical potential gradients such as air/(H₂-H₂O), the composite membranes showed kinetically stable operation without bulk decomposition at 1073 K. The kinetic stabilization associated with surface-limited oxygen permeation was confirmed by the conversion-electron Mössbauer spectroscopy analysis of one (SrFeO_3 − δ)_0.7(SrAl₂O₄)_0.3 membrane exposed to dry CH₄ at 1173 K, where no traces of Fe²⁺ and metallic iron were detected in the reduced surface layer.     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Influence of Al2O3 additions on crystallization mechanism and conductivity of Li2O-Ge2O-P2O5 glass-ceramics

The crystallization mechanism and conductivity of lithium aluminum germanium phosphate [LAGP] glass-ceramics fabricated from Li_1+xAl_xGe_2−x(PO₄)₃ (x=0.0-0.7) glass system were investigated as a function of Al₂O₃ additions. A non-isothermal analysis was performed to study the crystallization behavior of LAGP glass-ceramics at various heating rates (5-25K min⁻¹) by the Kissinger equation and the Augis-Bennett equation, illustrating volume crystallization for the glass-ceramics. The crystal identification and microstructure in glass-ceramics containing various Al₂O₃ contents were analyzed by means of XRD and FESEM. The main phase of the glass-ceramics was found to be LiGe₂(PO₄)₃, with AlPO₄ as the impurity phase. Additionally the highest total ionic conductivity (5.8×10⁻⁴ S/cm) at room temperature was obtained when x=0.5 for Li_1+xAl_xGe_2−x(PO₄)₃ (x=0.0-0.7) glass-ceramics, suggesting that it was a promising electrolyte for practical application in all-solid-state lithium batteries.     

Electrode reaction at platinum / ceria surface modified YSZ interface

The electrode reaction was examined on ceria coated YSZ by a platinum point electrode in H₂-H₂O atmosphere at 973 K- 1173 K. The thickness of the ceria coating layer was altered from 0 to 2.5 μm, fabricated by a laser ablation and by a vacuum vapor deposition method on YSZ single crystals. The electrode / electrolyte interface conductivity increased with 1/4 powers ofp(H₂) andp(H₂O) on both ceria coated and non-coated YSZ. The interface conductivity was significantly improved on a thicker ceria coating surface than 1 μm. The effective electrode reaction radius also increased in a thick ceria coating. The¹⁸O/¹⁶O exchange experiment at low oxygen partial pressure revealed that the oxygen surface exchange rate of ceria is not high compared with that of YSZ. It can be concluded that the bulk ionic conduction of ceria makes a more effective contribution to the electrode reaction than the surface catalytic activity in H₂-H₂O atmosphere. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Structural, optical and electronic properties of P doped p-type ZnO thin film

P doped ZnO films were grown on quartz by radio frequency-magnetron sputtering method using a ZnO target mixed with 1.5 at% P₂O₅ in the atmosphere of Ar and O₂ mixing gas. The as-grown P doped ZnO film showed n-type conductivity, which was converted to p-type after 800 °C annealing in Ar gas. The P doped ZnO has a resistivity of 20.5 Ω cm (p∼2.0×10¹⁷ cm⁻³) and a Hall mobility of 2.1 cm² V⁻¹ s⁻¹. XRD measurement indicated that both the as-grown and the annealed P doped ZnO films had a preferred (0 0 2) orientation. XPS study agreed with the model that the P_Zn-2V_Zn acceptor complex was responsible for the p-type conductivity as found in the annealed P-doped ZnO. Temperature-dependent photoluminescence (PL) spectrum showed that the dominant band is located at 3.312 eV, which was attributed to the free electronic radiative transition to neutral acceptor level (FA) in ZnO. The P_Zn-2V_Zn acceptor complex level was estimated to be at E_V=122 meV.     

Electrical and optical properties of thermally-evaporated thin films from A2[TiO(C2O4)2] (A = K, PPh4) and 1,8-dihydroxyanthraquinone

In this work, the synthesis of molecular materials formed from A₂[TiO(C₂O₄)₂] (A = K, PPh4) and 1,8 dihydroxyanthraquinone is reported. The synthesized materials were characterized by atomic force microscopy (AFM), infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy. IR spectroscopy showed that the molecular-material thin-films, deposited by vacuum thermal evaporation, exhibit the same intra-molecular vibration modes as the starting powders, which suggests that the thermal evaporation process does not alter the initial chemical structures. Electrical transport properties were studied by dc conductivity measurements. The electrical activation energies of the complexes, which were in the range of 0.003-1.16 eV, were calculated from Arrhenius plots. Optical absorption studies in the wavelength range of 190-1090 nm at room temperature showed that the optical band gaps of the thin films were around 1.9-2.3 eV for direct transitions Eg_d. The cubic NLO effects were substantially enhanced for materials synthesized from K₂[TiO(C₂O₄)₂], where χ⁽³⁾ (−3ω; ω, ω, ω) values in the promising range of 10⁻¹² esu have been evaluated.     

Interface properties of an O2 annealed Au/Ni/n-Al0.18Ga0.82N Schottky contact

We oxidized a Ni/Au metal bi-layer contact fabricated on HVPE Al_0.18Ga_0.82N from 373 K to 573 K in 100 K steps. In the range 1 kHz to 2 MHz, the Capacitance–Voltage–Frequency (C–V–f) measurements reveal a frequency dispersion of the capacitance and the presence of an anomalous peak at 0.4 V owing to the presence of interface states in the as deposited contact system. The dispersion was progressively removed by O₂ anneals from temperatures as low as 373 K. These changes are accompanied by an improvement in the overall quality of the Schottky system: the ideality factor, n, improves from 2.09 to 1.26; the Schottky barrier height (SBH), determined by the Norde [1] method, increases from 0.72 eV to 1.54 eV. From the Nicollian and Goetzberger model [2], we calculated the energy distribution of the density of interface states, N_SS. Around 1 eV above the Al_0.18Ga_0.82N valence band, N_SS, decreases from 2.3×10¹² eV⁻¹ cm⁻² for the un-annealed diodes to 1.3×10¹² eV⁻¹ cm⁻² after the 573 K anneal. Our results suggest the formation of an insulating NiO leading to a MIS structure for the oxidized Au/Ni/Al_0.18Ga_0.82N contact.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Superconducting molybdenum nitride epitaxial thin films deposited on MgO and α-Al2O3 substrates by molecular beam epitaxy

Molybdenum nitride Mo₂N_x films were grown on MgO(0 0 1) and on α-Al₂O₃(0 0 1) substrates by molecular beam epitaxy under nitrogen radical irradiation. X-ray photoelectron spectroscopy revealed that the composition of the film varied in the range of Mo₂N_1.4-Mo₂N_2.8 depending on the growth temperature. The deposition at 973 K gave well-crystallized films on both substrates. The high-resolution reciprocal space mapping by X-ray diffraction showed that the nitrogen-rich γ-Mo₂N crystalline phase (the composition: Mo₂N_1.4) was epitaxially grown on MgO at 923 K with a slight tetragonal distortion (a = 0.421 and c = 0.418 nm) to fit the MgO lattice (a = 0.421 nm). On α-Al₂O₃(0 0 1), nitrogen-rich γ-Mo₂N (Mo₂N_1.8) was grown at 973 K with (1 1 1) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the γ-Mo₂N on α-Al₂O₃(0 0 1) had a slight rhombohedral distortion (a = 0.4173(2) and α = 90.46(3)°). Superconductivity was observed below 2.8-3 K for the films grown at 973 K on MgO and on α-Al₂O₃(0 0 1).