Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe₈₆Zr₇Cu₁B₆. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, -Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.     

On the Bond Length Change upon 4f 1 → 5d 1 Excitations in Eightfold Coordination: CaF2:Ce3+ Cubic Defects

The bond lengths of 4f ¹ and 5d ¹ electronic states of cubic (CeF₈)⁵⁻ defects in fluorite have been calculated using quantum mechanical embedding, spin-free relativistic Hamiltonian, and dynamic electron correlation through second-order perturbation theory. The results predict the bond length between Ce and the surrounding eight F to shorten upon 4f ¹ → 5d ¹ excitation. This result coincides with previous findings for lanthanide and actinide ions in sixfold octahedral complexes where the ligand field splitting of the 5d (6d) orbitals is inverted with respect to the eightfold cubic field splitting. Altogether, the results of sixfold and eightfold coordination indicate that the bond shrinkage experienced upon 4f ⁿ to 4f ^n-15d ¹ excitations (5f ⁿ to 5f ^n-16d ¹ in the actinides) seems to be a general result of f-element complexes. These theoretical results contradict a widespread assumption according to which the bond length increases upon f→ d excitation and, therefore, experimental measurements of the sign of the bond length distortion that either validate or refute the quantum chemical predictions are most desirable.     

Self-consistent embedded clusters: Building block equations for localized orthogonal orbitals

In order to be able to study a large electronic system following a building block approach, in which smaller tractable subsystems are handled at a time rather than the system as a whole, equations are proposed in this paper whose solutions are variational orthogonal orbitals localized on the subsystems. The equations for a given subsystem correspond to a molecular cluster embedded in the field created by the rest of the system, and are coupled to the corresponding equations for all subsystems under consideration, so that they must be solved self-consistently. While the localized nature of the solutions makes the equations appropriate for use in conjunction with local basis sets in practical implementations without significant loss of precision due to truncation errors, their orthogonality properties allow for the use of the advantages of the theory of separability of McWeeny in order to calculate total energies and (generalized product) wave functions. Since the building block equations proposed involve inter-subsystem interactions very cumbersome to calculate, an approximation is proposed in order to make their application to actual problems feasible: the representation of the cumbersome interaction operators by ab initio model potentials which are obtained directly from them, without resorting to any parametrization procedure based on a reference. This ab initio model potential approximation has been found to provide considerable computational savings without significant loss of accuracy in frozen-core calculations on molecules and frozen-lattice calculations on imperfect crystals.     

Theoretical study of the effects of F to Cl chemical substitution on the electronic structure and the luminescence properties of Cs2GeF6:Os4+ and Cs2ZrCl6:Os4+ materials

It has been experimentally determined that Cs2ZrCl6:Os4+ shows luminescence and up-converted luminescence from the highest t(2g) (4) excited level 2 A1g(1A1g), whereas Cs2GeF6:Os4+ 2 A1g(1A1g) does not luminescence at all. Ab initio quantum chemical calculations on these materials are presented here and show that the variation of the energy gap between the t2g 4 and t2g 3 eg 1 manifolds with F to Cl chemical substitution is a key factor to interpret the experimental findings. This energy gap is calculated to be some 1500 cm(-1) (approximately 2nua1g) in the fluoride host, whereas it is about 3300 cm(-1) (approximately 9nua1g) in the chloride host. The calculated values for the ground state totally symmetric vibrational frequency nu(a1g) are 626 cm(-1) (Cs2GeF6:Os4+) and 355 cm(-1) (Cs2ZrCl6:Os4+), in good agreement with the available experimental data. Geometrical structure of (OsX6)2- clusters (X=F,Cl) embedded in Cs2GeF6 and Cs2ZrCl6 lattices is calculated as well. New assignments for some spectral features based in the results of our calculations are proposed.     

Absorption and emission spectra of Ce3+ in elpasolite lattices

The experimental determination of the electronic energy levels for Ce(3+) in some chloroelpasolite hosts for both the ground 4f(1) and the excited 5d(1) configurations is described. High-resolution f-f absorption and f-(2)T(2g) d absorption and emission spectra have been recorded at low temperatures for Ce(3+) diluted into various hexachloroelpasolite lattices. A fluorescence spectrum at approximately 50 000 cm(-1) is tentatively assigned to the emission from the highest 5d crystal field level, (2)E(g), of a Ce(3+) impurity in Cs(2)NaErCl(6), enabling the values of all the energy levels of both the 4f(1) and 5d(1) configurations to be given for Ce(3+) in elpasolite hosts. Vibronic structure superimposed on the electronic transitions is analyzed in terms of a simple configurational coordinate model involving the ground and excited configurations. It is found that the difference in the Ce-Cl bond length between the 4f(1) and 5d(1) configurations is approximately 0.04 A. Ab initio model potential calculations on the (CeCl(6))(3-) cluster embedded in a reliable representation of the Cs(2)NaYCl(6) host support these conclusions.     

Spin relaxation in nanophased manganites

We present recent results obtained by DC and AC susceptibility, muon spin relaxation and neutron spin echo experiments in magnetically ordered La_0.7Pb_0.3MnO₃ and disordered La_0.7Pb_0.3Mn_0.8Fe_0.2O₃. These results aim to cover an extraordinary equivalent frequency range (from quasi-static experiments to almost the THz region) and give an overall picture of the processes within. The different relaxations observed are related either to the macroscopic long-ranged interactions among magnetic clusters (susceptibility techniques) and to short-ranged intracluster spin diffusion processes (muon and neutron techniques).     

5f-->5f transitions of U4+ ions in high-field, octahedral fluoride coordination: the Cs2GeF6:U4+ crystal

The U-F bond length, totally symmetric vibrational frequency, and 5f(2) energy levels of the Cs(2)GeF(6):U(4+) crystal are predicted through quantum-chemical calculations on the embedded (UF(6))(2-) cluster. The U(4+) ions substitute for much smaller Ge(4+) retaining octahedral site symmetry, which is useful to interpret the electronic transitions. The structure of the 5f(2) manifold: its energy range, the crystal splitting of the 5f(2) levels, their parentage with free-ion levels, and the energy gaps appearing within the manifold, is presented and discussed, which allows to suggest which are the possible 5f(2) luminescent levels. The effects of Cl-to-F chemical substitution are discussed by comparison with isostructural Cs(2)ZrCl(6):U(4+). The energy range of the 5f(2) manifold increases by some 6000 cm(-1) and all levels shift to higher energies, but the shift is not uniform, so that noticeable changes of order are observed from Cs(2)ZrCl(6):U(4+) to Cs(2)GeF(6):U(4+). The comparison also reveals that the green-to-blue up-conversion luminescence, which has been experimentally detected and theoretically discussed on Cs(2)ZrCl(6):U(4+), is quenched in the fluoride host. The results of the Cs(2)GeF(6):U(4+) are used as a high-symmetry model to try to understand why efficient radiative cascade emissions in the visible do not occur for charged U(4+) defects in low-symmetry YF(3) crystals. The results presented here suggest that theoretical and experimental investigations of 4f5f ions doped in octahedral, high-symmetry fluoride crystals may be conducted even when the mismatch of ionic radii between the lanthanide/actinide ions and the substituted cations of the host is considerably large. Investigations of these new materials should reveal interesting spectroscopic features without the difficulties associated with more commonly used low-symmetry fluoride hosts.     

Short range order in (Fe,Co, Ni)75Si15B10 amorphous alloys determined from magnetic anisotropy

Some previously reported experimental results dealing with magnetostriction and induced magnetic anisotropy are analyzed with respect to the alloy composition. It is found that the properties which are affected by directional order vary with the composition as the probability for finding Bernal tetrahedral holes surround by 3Co1Fe or 3Fe1Co and not as the probability for finding metal—metal pairs as predicted by diatomic ordering theory for ideal solutions.     

Detailed interpretation of the 5f-6d absorption spectrum of U3+ in Cs2NaYCl6 and high pressure effects based on an ab initio simulation

The 5f-->6d(t(2g)) absorption spectrum of U(3+)-doped Cs(2)NaYCl(6) is simulated with a quantum chemical ab initio embedded-cluster approach applied to U(3+) substitutional defects of O(h) local symmetry. The first-principles results help to provide a detailed interpretation of the very rich experimental absorption spectrum of this material between 14 000 and 25 000 cm(-1). Also, the effects of high pressures up to 26 kbars on the absorption spectrum are predicted, the most relevant feature being a redshift of around 21 cm(-1)/kbar, which is the fingerprint of a bond length shortening upon 5f-->6d(t(2g)) excitation.