Fine structures in the X-ray photoemission spectra of MnO,FeO, CoO,and NiO single crystals

The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced.     

Quantitative XPS of NiO, CoO and MnO. The effects of elastic and inelastic electron scattering

In order to check whether the method for background correction developed by Tougaard can be applied to oxides we measured XPS spectra of single crystalline NiO, MnO and CoO samples. The materials for which background treatment was developed and tested originally were metals and alloys. In contrast to that, the oxides we study in the present work are insulators. We have chosen these compounds because their structure is simple and by cleaving them in vacuo it is possible to obtain surfaces with well-defined stoichiometry.

The inelastic background for these oxides is likewise described well by the above mentioned model. The ratio of metal to oxygen concentration calculated from the XPS intensities so obtained oscillates around 1.0 with a maximum deviation of about ±20% in the worst case. These deviations originate from elastic electron scattering, namely forward focusing (X-ray photoelectron diffraction). Taking this into account, the perfect stoichiometry is obtained within a few percent.     

Application of three body force shell model to the lattice dynamics of transition metal oxides MnO,CoO and NiO

Lattice dynamics of some transition metal oxides has been studied using the three body force shell model. Reasonably good fits with the experimental phonon dispersion curves have been obtained by choosing values of molecular electronic polarisability substantially lower than those derived from the dielectric constant. This result is similar to those obtained in many other crystals which are not ideally ionic.     

Matrix-isolated FeO,NiO, and CoO: Ground-state vibrational frequencies

The infrared vibrational spectra of the FeO, NiO, and CoO molecules have been observed in Ar matrices at 14 K. Frequencies for each of the isotopomers ⁵⁴Fe¹⁶O, ⁵⁴Fe¹⁸O, ⁵⁶Fe¹⁶O, ⁵⁶Fe¹⁸O, ⁵⁸Ni¹⁶O, ⁵⁸Ni¹⁸O, ⁶⁰Ni¹⁶O, ⁶⁰Ni¹⁸O, ⁵⁹Co¹⁶O, and ⁵⁹Co¹⁸O were measured. The derived vibrational constants are:

$\text{w}{}_{\mathrm{e}}\text{= 880.02 ± 1.2 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 3.47 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{56}\text{Fe}{}^{16}\text{O; w}{}_{\mathrm{e}}\text{= 837.61 ± 1.1 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 5.92 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{58}\text{Ni}{}^{16}\text{O; and w}{}_{\mathrm{e}}\text{= 853.75 ± 1.2 and w}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{= 3.65 ± 0.6 cm}{}^{\mathrm{?1}}\text{for}{}^{59}\text{Co}{}^{16}\text{O.}$

These results support the assignment of ⁵Σ⁺ as the ground state of FeO, ³Σ^? as the ground state of NiO, and ⁴Σ^? as the ground state of CoO.     

Electrical properties of CoO,NiO, CuO and ZnO doped beta″-alumina

The measurements of ionic conductivity of sintered beta″-alumina samples doped with CoO, NiO, CuO and ZnO were caried out. It was found that conductivities of these samples are lower than conductivity of Li₂O stabilized beta″-alumina. For CoO, NiO, CuO as well as Li₂O stabilized beta″-alumina the bending of Arrhenius plots was observed. For samples doped with ZnO the plots were linear in whole 20°C–450°C temperature range. The doping effect on bulk conductivity was stronger than on grain boundary conductivity.     

Low temperature μSR studies in NiO and CoO

Recent measurements on NiO and CoO show a complex temperature dependence. In this work, the local μ⁺ fields are examined over an extended temperature range. Only one precession frequency of 61.3 MHz was observed in the μ⁺ spin precession in zero field in NiO, similar to the case in MnO. The signal broadens at 200 K and is difficult to observe above 250 K. In contrast, in CoO, at least three lines are observed: two sharp lines at 54.2 MHz, and at 78 MHz, with an additional small peak at 162 MHz. Above 40 K, the 54.2 MHz signal (CoO) vanishes, but the 78 MHz signal survives to 110 K. However, at 270–280 K, a new signal at 13 MHz is also observed. Dipole field calculations of these 3d‐oxides (MnO, NiO and CoO) for various lattice sites (symmetric sites and O‐bonded positions) are examined and compared with the experimental results, with considerations toward the dynamics of the muons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Splitting of the zone-center phonon in MnO and NiO

We develop a model based on the superexchange interaction and the Heisenberg Hamiltonian to understand the zone-center transverse optical (TO) phonon splittings in the late transition-metal oxides with type-II antiferromagnetic ordering. Even for a cubic rocksalt lattice, the anisotropic magnetic coupling due to the non-cubic distribution of the spin density can produce a splitting of the TO phonon energies. To calculate these splittings for MnO and NiO, we apply a recently implemented LSDA+U method based on a pseudopotential plane-wave formalism and obtain results for both MnO and NiO which are consistent with our model predictions. The calculated splitting for MnO is in excellent agreement with a recent experiment, while the result for NiO appears to be inconsistent with current experimental results and entails further investigation.     

Laser modulated reflectance of gallium telluride

Photoreflectance spectrum of Gallium Telluride in the exciton absorption region is presented and interpreted invoking a light-induced modulation of the exciton Rydberg. An alternative explanation assuming electroreflectance- like effect due to surface field modulation seems to be not adequate to justify the measured line-shape.     

Photoacoustic and reflectance spectra of zirconium-silicate pigments

Photoacoustic spectra and diffusion reflectance spectra of powdered zirconium-silicate ceramic pigments (Zr_0·97V_0.03Si₄, Zr_0·98Pr_0·02SiO₄ and ZrSiO₄ 0·05 -Fe₂O₃) have been measured and found to be closely correlated. The signal saturation problem connected with both spectral techniques is extensively discussed with respect to a particle size of powdered samples. The photoacoustic technique enables one to establish the bulk optical absorption coefficients of the powdered samples and thermal parameters, knowing the particle size diameters.     

Magnetostructural phase transitions in NiO and MnO: Neutron diffraction data

Structural and magnetic phase transitions in NiO and MnO antiferromagnets have been studied by high-precision neutron diffraction. The experiments have been performed on a high-resolution Fourier diffractometer (pulsed reactor IBR-2), which has the record resolution for the interplanar distance and a high intensity in the region of large interplanar distances; as a result, the characteristics of both transitions have been determined simultaneously. It has been shown that the structural and magnetic transitions in MnO occur synchronously and their temperatures coincide within the experimental errors: T_str ≈ T_mag ≈ (119 ± 1) K. The measurements for NiO have been performed with powders with different average sizes of crystallites (~1500 nm and ~138 nm). It has been found that the transition temperatures differ by ~50 K: T_str = (471 ± 3) K, T_mag = (523 ± 2) K. It has been argued that a unified mechanism of the “unsplit” magnetic and structural phase transition at a temperature of T_mag is implemented in MnO and NiO. Deviation from this scenario in the behavior of NiO is explained by the quantitative difference—a weak coupling between the magnetic and secondary structural order parameters.     

Revisiting the valence-band and core-level photoemission spectra of NiO

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.