Newman Superposition Research Articles

New insight in the nature of surface magnetic anisotropy in iron borate

The theory of surface magnetism of iron borate, FeBO3, has been extended by taking into consideration a crystal field contribution to the surface magnetic anisotropy energy. For this purpose, a model of distortion of the six-fold oxygen environment of iron ions in the near-surface layer of iron borate has been put forward. The spin Hamiltonian parameters for isolated Fe3+ ions in the distorted environment of the near-surface layer have been calculated using the Newman's superposition model. The crystal field contribution to the surface magnetic anisotropy energy has been calculated in the framework of the perturbation theory. The model developed allows concluding that the distortions of the iron environment produce a significant crystal field contribution to the surface magnetic anisotropy constant. The results of experimental studies of the surface magnetic anisotropy in iron borate can be described assuming the existence of relative contractions in the near-surface layer of the order of 1 %.

Spin‐Hamiltonian parameters and local lattice structure for Pd(I) center in γ‐irradiated bis(acetylacetonato)palladium(II)

AbstractThis paper reports on a theoretical analysis of spin‐Hamiltonian parameters and local lattice structure for Pd(I) center in γ‐irradiated Pd(II)(acac)2. Through the crystal‐ and ligand‐field theory, the microscopic spin‐Hamiltonian parameters and local molecular structure for Pd(I) center in the γ‐irradiated Pd(II)(acac)2 system have been studied by using the high‐order perturbation formulas and Newman's superposition model. Based on these calculations, it was found that the distance of the metal‐ligand bonds in the square planar complex for Pd(I) center in the γ‐irradiated Pd(II)(acac)2 system increases by 0.1Å. To understand the detailed physical and chemical properties of the [Pd(I)(acac)2]2– complex, the contributions of the spin‐orbit coupling of ligand to spin‐Hamiltonian parameters for Pd(I) ion are considered. The theoretical results are in reasonable agreement with the experimental values. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A crystal‐field study of erbium oxide and fluoride

AbstractThe energy levels belonging to the three lowest J multiplets of the Er3+ ion in Er2O3 and ErF3 were determined by means of high‐resolution optical absorption spectroscopy. A crystal‐field analysis of these data by means of Newman's Superposition Model was performed, allowing to determine intrinsic parameters which are quite consistent with other oxides and fluorides and to check the validity of the transferability postulate within these compounds. The possibility to use these data to analyze crystalline nanophases in glasses is briefly discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role ofEr3+concentration in high-resolution spectra ofBaY2F8single crystals

Fourier transform absorption spectroscopy with a resolution as fine as $0.02\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ was applied to ${\mathrm{Er}}^{3+}$-doped monoclinic ${\mathrm{BaY}}_{2}{\mathrm{F}}_{8}$ laser crystals in a wide wave number range $(500--24\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ and in the temperature range 9--300 K. The careful analysis of the complex narrow line spectra induced by ${\mathrm{Er}}^{3+}$ allowed us to determine with high accuracy the crystal field splitting of the fundamental $^{4}\mathrm{I}_{15∕2}$ and of the excited $^{4}\mathrm{I}_{13∕2}$, $^{4}\mathrm{I}_{11∕2}$, $^{4}\mathrm{I}_{9∕2}$, $^{4}\mathrm{F}_{9∕2}$, $^{4}\mathrm{S}_{3∕2}$, $^{2}\mathrm{H}_{11∕2}$, $^{4}\mathrm{F}_{7∕2}$, $^{4}\mathrm{F}_{5∕2}$, and $^{4}\mathrm{F}_{3∕2}$ manifolds. On the basis of the experimental data, the crystal-field parameters were determined and Newman's superposition model was applied: in this way a slight displacement of ${\mathrm{Er}}^{3+}$ with respect to the ${\mathrm{Y}}^{3+}$ position was foreseen. The Judd-Ofelt parameters were evaluated: the lifetime values deduced from them were compared to the experimental ones and discussed. The effects caused by increasing ${\mathrm{Er}}^{3+}$ concentrations (0.5%, 2%, 12%, and 20% atomic fraction) were studied in detail. The new lines, the line broadening, and the line-shape changes were explained in terms of ${\mathrm{Er}}^{3+}\text{\ensuremath{-}}{\mathrm{Er}}^{3+}$ interaction.

Crystal-field and superposition model analysis of :BaY F ( = Er, Dy, Nd)

The experimental values of the energy levels of Er3+, Dy3+, and Nd3+ in BaY2F8 were fitted to a single-ion Hamiltonian containing free-ion and crystal-field interactions. The crystal-field parameters so evaluated were then analyzed by using Newman's Superposition Model. The agreement between the two sets of parameters is good, provided a possible distortion of the F- polyhedron around the rare-earth site is taken into account. The effects of a possible displacement of the rare-earth ion substituting for Y3+ are also evaluated.

Superposition-model analysis of rare-earth doped BaY 2 F 8

The energy level schemes of four rare-earth dopants (Ce 3+ , Nd 3+ , Dy 3+ , and Er 3+ ) in BaY 2 F 8 , as determined by optical absorption spectra, were fitted with a single-ion Hamiltonian and analysed within Newman's Superposition Model for the crystal field. A unified picture for the four dopants was obtained, by assuming a distortion of the F m ligand cage around the RE site; within the framework of the Superposition Model, this distortion is found to have a marked anisotropic behaviour for heavy rare earths, while it turns into an isotropic expansion of the nearest-neighbours polyhedron for light rare earths. It is also inferred that the substituting ion may occupy an off-center position with respect to the original Y 3+ site in the crystal.

Molecular Aspects of Catalytic Reactivity. Application of EPR Spectroscopy to Stuies of the Mechanism of Heterogeneous Catalytic Reactions

Introduction The basic objective of mechanistic studies of real catalytic processes is to dissect the course of the reaction into individual steps; ascertain their sequence; and determine the stoichiometry, structure, and electronic states of active sites and intermediates. The electron paramagnetic resonance (EPR) technique is at present widely used to explore many of these principal aspects of heterogeneous catalysis and surface chemistry. The extreme sensitivity compared to the usual spectroscopic methods is perhaps its most acknowledged advantage and makes EPR best suited to investigate and characterize low-abundance active sites and intermediates appearing during catalytic reaction. Additional information can be drawn from the theoretical analysis of the experimental spin Hamiltonian parameters within the ligand field and from angular overlap or Newman's superposition models as well as by more sophisticated quantum chemical calculations. The purpose of this paper is to show how catalysis benefits fro...

Experimental studies of the EuBa2-xEuxCu3O7- delta compound in the tetragonal phase

EPR and magnetic measurements were carried out on specially prepared samples of the EuBa2-xEuxCu3O7- delta compound in the tetragonal phase, in the temperature range 100-4.2 K. The EPR spectra of Eu2+, Cu2+, coupled pairs of divalent copper ions and the superexchange interaction between Cu ions over oxygen bridges have been observed. The EPR spectrum of europium ions indicates the existence of a crystal-field splitting of the 8S7/2 ground state of Eu2+ ions with a value of the spin-Hamiltonian parameter b20 of about 14*10-2 cm-1. This parameter is analysed within the framework of Newman's superposition model. Anomalies in the temperature variation of the magnetic susceptibility are attributed to the presence of divalent and trivalent europium ions. The susceptibility of Eu3+ ions is non-zero because the excited states are thermally populated and they contribute to the result of the ground state with J=0.

Crystal field potential of NdCu 2Si 2: A comparison with CeCu 2Si 2

The results of an inelastic neutron scattering investigation of the crystal field potential of NdCu 2Si 2 are reported. We have observed three ground state and three excited state transitions which establish unambiguously the crystal field level scheme with an overall splitting of 11.45 meV. By performing a profile refinement of the measured spectra we have determined the Hamiltonian parameters for the tetragonal crystal field: B 0 2 = (-3.1 ± 0.1) × 10 -2 meV, B 0 4 = (1.1 ± 0.1) × 10 -3 meV, B 4 4 = (1.33 ± 0.04) × 10 -3 meV, B 0 6 = (-3.17 ± 0.04) × 10 -5 meV and B 4 6 = (6.62 ± 0.02 × 10 -4 meV. The parameters are analysed in terms of Newman's superposition model and compared to recent results on the isostructural heavy fermion compound CeCu 2Si 2.

EPR study of tetragonal and monoclinic Fe3+ centres in Rb2ZnF4 crystals

EPR measurements have been made on tetragonal and monoclinic Fe3+ centres in Rb2ZnF4 at room temperatures. The fine-structure terms for the monoclinic centre are separated into uniaxial and cubic terms for comparison with the Fe3+-VK centre in KZnF3 and for the uncompensated tetragonal centre. The monoclinic centre is ascribed to a Fe3+ ion associated with the nearest Rb+ vacancy and the principal z-axis is found to be 7 degrees from the (001)-axis in the opposite direction to that of the Rb+ vacancy. Rudowicz's NCC parameters for the monoclinic Fe3+ and Cr3+ centres are examined in relation to the separation of the spin-Hamiltonians. The calculated metal-ligand distances and deviation angles for the front and back fluorines by Newman's superposition model are in good agreement with, respectively, the distances for the uncompensated centre and angles for the Fe3+-VK centre in KZnF3 determined from 19F-ENDOR measurements. The relationships between the bnm and the axial and cubic parameters obtained by the superposition model are similar to those obtained from the spin-Hamiltonian separation.

EPR Study of Tetragonal and Orthorhombic Fe3+Centres in Rb2CdF4Crystals

EPR measurements for the substitutional Fe 3+ ions in layered perovskite-type Rb 2 CdF 4 crystals have been made at room temperatures using a X-band spectrometer. A tetragonal spectrum is ascribed to an uncompensated Fe 3+ centre. An orthorhombic spectrum is analysed by separating the fine structure terms into uniaxial terms and a cubic one. Obtained uniaxial parameters are well compared to those for the other tetragonal Fe 3+ centres. By the analysis, the orthorhombic centre is ascribed to a Fe 3+ ion associated with a nearest-Cd 2+ vacancy. The fine structure parameters are analysed further by the Newman's superposition model to obtain metal-ligand distances. Results are discussed together with those for the Cr 3+ and Gd 3+ centres in A 2 MF 4 crystals.

EPR of Gd3+-doped single crystals of LiY1-xYbx

Extensive X-band EPR measurements have been carried out on ${\mathrm{Gd}}^{3+}$-doped single crystals of ${\mathrm{LiY}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{F}}_{4}$ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0), covering the temperature range 4.2--290 K. The spin-Hamiltonian parameters (SHP) are evaluated by the use of a rigorous least-squares fitting procedure. The systematics of the SHP are studied both as functions of x and of temperature. Newman's superposition model has been applied to the calculation of SHP; it is found that the calculated values agree quite well with the experimental values, provided that one considers minor distortions of the positions of the eight fluorine ions surrounding the ${\mathrm{Gd}}^{3+}$ ion. The temperature variation of SHP has been explained to be predominantly due to spin-phonon interaction and partially due to thermal expansion of the crystal lattice. Finally, the average ${\mathrm{Gd}}^{3+}$-${\mathrm{Yb}}^{3+}$ exchange interaction, averaged over the nearest and next-nearest neighbors, has been estimated, in ${\mathrm{LiY}}_{0.9}$${\mathrm{Yb}}_{0.1}$${\mathrm{F}}_{4}$, to be 3.8\ifmmode\pm\else\textpm\fi{}2.5 GHz, as found by the application of the molecular-field model using the g shift from the isostructural diamagnetic host, ${\mathrm{LiYF}}_{4}$, at liquid-helium temperature.

EPR studies of Gd 3+ centres in the cubic phase of CsCaCl 3 and CsPbCl 3

The spectra of three type Gd 3+ centres with cubic, 〈100〉 tetragonal and 〈111〉 trigonal symmetry have been investigated in the cubic phase of CsCaCl 3( T = 300 K) and CsPbCl 3( T = 330 K). The Gd 3+ ion is substituted for Ca 2+ or Pb 2+ions. The hexadecapolar crystal field parameter b 4 is discussed within the framework of Newman's superposition model for cubic centres. The trigonal centre of CsCaCl 3 results from a slight 〈111〉 distortion of the cubic local symmetry at Gd 3+ sites connected with a Cs vacancy. The tetragonal centre corresponds to charge compensation by S 2− impurities substituted to Cl − ions which results in a strong 〈001〉 axial crystal field. The spin Hamiltonian parameter b 2 0 which reflects this essential feature of the centre is tentatively discussed in the light of some existing theoretical models.

ESR and Mössbauer studies of Fe 3+ ion in calcium boro-aluminate glasses

A correlated ESR and Mössbauer spectroscopic study of the glass system 2Ca O3B 2 O 3.Al 2 O 3.xFe 2 O 3 (0.1⩽ x⩽1) is reported. At low Fe contents ( x⩽0.1) Fe 3+ ions are found only in tetrahedral environments, giving a single, sharp ESR peak at g=4.13. For higher x, occupation by Fe 3+ of other-symmetry sites become possible, as seen by the evolution of g=2.0 and g=6.0 ESR features. A careful examination of Mössbauer isomer shift data has yielded an average value of 1.96 Å for the Fe 3+−O 2− distance, besides confirming the tetrahedral site symmetry. Using this value, and quadrupole splitting data the extent of angular distortion (α) from perfect tetrahedral symmetry has been deduced as 30° from a calculation based on Newman's superposition model.

Superposition Model Analysis of the Spin-Hamiltonian Parameters for Fe3+ Ions in Trigonal Symmetry Sites of KMgF3 and KZnF3 Single Crystals

A calculation of the trigonal spin-Hamiltonian parameters for Fe 3+ in KZnF 3 was performed using Newman's superposition model and the knowledge of the local arrangement of the fluorine coordinated ligands surrounding the iron ion. It was found that both the sign and the magnitude of the calculated parameters are in good agreement with those previously determined experimentally. Also, an Electron Paramagnetic Resonance (EPR) investigation of Fe 3+ in four inequivalent axial sites (trigonal) of KMgF 3 was carried out and the spin-Hamiltonian parameters describing the site symmetry were determined for the first time. From this information, an analysis was made, using the superposition model, of the local arrangement around the impurity Fe 3+ ion responsible for the observed trigonal EPR spectra.

On the microscopic mechanism of the ferroelectric phase transition of TSCC: Mn2+

AbstractIn ferroelectric TSCC doped by Mn2+ ions Windsch et al. observed a large rotation of some EPR lines, the limiting rotation angle being 30°. Using Newman's superposition model, the EPR fine structure tensor Dij is calculated via the field gradient tensor Vij in a point charge model. It turns out that rather small (some 10−9 cm) shifts together with a small change in the effective charge of the oxygen ions at the sites of the distorted octahedron around the manganese ions are sufficient to explain the large rotation of the EPR lines in a good agreement with the experimental results.