Set Of Crystal Field Parameters Research Articles

Optical spectroscopy ofYb2Ti2O7andY2Ti2O7:Yb3+and crystal-field parameters in rare-earth titanate pyrochlores

Optical absorption spectra of the ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ single crystals and luminescence spectra of the ${\mathrm{Y}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}:\mathrm{Yb}$ (1%) polycrystalline samples were registered at temperatures $4.2--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These spectra and earlier published data on magnetic properties of ${\mathrm{Yb}}^{3+}$ ions and on the temperature dependence of the electric field gradient at Yb nuclei in ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ were used to analyze the crystal-field parameters in rare-earth titanates with the pyrochlore structure. The self-consistent sets of crystal-field parameters for rare-earth ions in the $16d$ sites with the ${D}_{3d}$ symmetry that describe satisfactory all known single-ion magnetic properties and low-energy excitations in ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ crystals ($R=\mathrm{Tb}$, Ho, Er, Tm, Yb) are presented.

Pr3+crystal-field excitation study of apical oxygen and reduction processes inPr2−xCexCuO4±δ

We present an infrared transmission ${\mathrm{Pr}}^{3+}$ crystal-field study of as-grown, reduced, and oxygenated ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ single crystals and thin films. Excitations from the ground-state multiplet ${}^{3}{\mathrm{H}}_{4}$ to the ${}^{3}{\mathrm{H}}_{5},$ ${}^{3}{\mathrm{H}}_{6},$ ${}^{3}{\mathrm{F}}_{2},$ and ${}^{3}{\mathrm{F}}_{3}$ excited multiplets are observed in all samples. In addition to the ${\mathrm{Pr}}^{3+}$ regular sites, which remain unperturbed following the cerium doping or the oxygen content modifications, ${\mathrm{Pr}}^{3+}$ sites are detected. A precise set of crystal-field parameters, which reproduces the energy and the symmetry of the levels, is determined. The reduction process, which drives the electron-doped cuprates superconducting, is discussed in detail and scenarios for the reduction mechanism and induced vacancies are proposed. In contrast to the common belief, the apical oxygen, which is clearly detected in all samples, is not removed in ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$ following reduction. This observation questions the role attributed to the apical oxygen removal in triggering the superconductivity.

Energy levels of Dy3+(4f 9) in orthoaluminate crystals

A detailed crystal-field splitting analysis is reported for the energy levels of Dy3+(4f9) in crystals of YAlO3 and DyAlO3 orthoaluminates. Experimental energy (Stark) levels for the 15 lowest-energy LJ2S+1 multiplet manifolds of Dy3+ are reported and supplemented with values found in the literature. A parameterized Hamiltonian, including Coulombic, spin-orbit, configuration interaction, and crystal-field terms in Cs symmetry, was diagonalized for the HJ6, FJ6, FJ4, IJ4, and GJ4 states. Initial sets of crystal-field parameters were determined from lattice-sum calculations and the three-parameter theory. Considerable J-mixing is found for states having nearly the same energy such as the manifolds F11/26, H9/26, and F9/26, H7/26. Even states that are well separated from each other in energy show sufficient J mixing to explain the polarized absorption and Zeeman effects that involve transitions from the nearly pure (less than 0.2% J mixing) ground state, H15/26 to excited states such as F5/26 and F3/26. The rms for 70 calculated-to-experimental Stark levels for Dy3+ in YAlO3 is 10 cm−1; the rms for 43 calculated-to-experimental levels for Dy3+ in DyAlO3 is 8 cm−1.

An appraisal of the crystal field in rare earth intermetallics: TmCu2Si2

AbstractA crystal field (CF) investigation of the properties of Tm3+ in TmCu2Si2 has been done for an appraisal of the CF proposed by Kosaka et al. [J. Phys. Soc. Jpn. 66, 2844 (1997)] from their magnetic studies and inelastic neutron scattering investigation. As these CF parameters were found to give only a qualitative explanation of the observed data, they were refined to have a closer fit to the observed magnetic susceptibility, Stark levels obtained from inelastic neutron scattering studies, and thermal variation of nuclear electric quadrupole splitting. It has been found that molecular field interactions play a dominant role at low temperatures. Other properties investigated theoretically with the help of the refined set of CF parameters are amenable to experimental check.

Infrared transmission study of crystal-field excitations inLa2−x−yNdxSryCuO4

We report an infrared crystal field study of ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Nd}}_{x}{\mathrm{Sr}}_{y}{\mathrm{CuO}}_{4}$ single crystals for $(x=0.35,$ $y=0),$ $(x=0.34,y=0.14),$ and $(x=0.42,y=0.19).$ ${\mathrm{Nd}}^{3+}$ crystal field excitations from the ground state multiplet ${}^{4}{I}_{9/2}$ to the ${}^{4}{I}_{11/2},$ ${}^{4}{I}_{13/2},$ ${}^{4}{I}_{15/2},$ and ${}^{4}{F}_{3/2}$ excited multiplets are observed. A precise set of crystal field parameters, that reproduces the ${\mathrm{Nd}}^{3+}$ electronic levels in all the samples as well as the ${\mathrm{La}}_{1.65}{\mathrm{Nd}}_{0.35}{\mathrm{CuO}}_{4}$ susceptibility measurements as a function of temperature, is determined. A very strong Ising-like anisotropy of the low-temperature ${\mathrm{Nd}}^{3+}$ magnetic moment is related to a large value of the second rank crystal field parameter ${B}_{20}.$ In addition to the ${\mathrm{Nd}}^{3+}$ ${C}_{4v}$ symmetry regular site a second one, of lower symmetry, is detected and related to possible stripe formation.

Optical study of praseodymium dicarboxylate [Pr(H 2O)] 2[O 2C(CH 2) 3CO 2] 3.4H 2O

Praseodymium dicarboxylate, [Pr(H 2O)] 2[O 2C(CH 2) 3CO 2] 3.4H 2O]–glutarate, Pr[glut], is synthesized by hydrothermal techniques. The title compound crystallizes in the monoclinic space group C2/c (No. 15). The rare earth cation is coordinated by nine oxygen atoms, eight oxygen atoms from the carboxylate groups and one from the water molecule. The local symmetry of Pr site is low, C s. The absorption spectra of Pr[glut] are recorded from the visible to the far IR domain at 300, 77 and 9 K. Under various Ar + laser excitations no emission is detected from 3P 0 and 1D 2 excited levels of Pr 3+ ion. In the low temperature absorption spectra only one electronic line is recorded for 3H 4→ 3P 0 transition. It confirms a unique local environment for the rare earth ion in Pr[glut]. The utility of the ‘barycenter curves’ in the attribution of electronic lines is demonstrated. Energy level scheme of 36 Stark components is deduced from the absorption spectra. The parametric calculation was performed on the whole 4f 2 (Pr 3+) configuration with the starting set of crystal field parameters obtained previously for the Eu 3+ ion in the isostructural compound. Eight free ion and nine phenomenological crystal field parameters in C 2v symmetry reproduce quite well several electronic levels of Pr 3+ ion experimentally observed in Pr[glut]. A good r.m.s. standard deviation of 14.8 cm −1 is obtained.

Infrared transmission study of Pr 2 CuO 4 crystal-field excitations

We present an infrared crystal-field study of Pr 2 CuO 4 single crystals and thin films. Excitations from the ground state multiplet 3 H 4 to the 3 H 5 , 3 H 6 , 3 F 2 and 3 F 3 excited multiplets are observed in both single crystals and thin films. A precise set of crystal-field parameters, that reproduces the energy and the symmetry of the levels, is determined.

Competing magnetic anisotropies in the mixed rare-earth Laves hydrides

We have studied the temperature evolution of magnetic structure in mixed Laves hydrides ${(R}^{1}{,R}^{2}){\mathrm{Mn}}_{2}{\mathrm{H}}_{4},$ where R is a rare-earth ion, by powder neutron diffraction. The spin orientation results from a compromise between the competing anisotropies of ${R}^{1},$ ${R}^{2},$ and Mn ions. The spin orientation at low temperature was analyzed using a single-ion cubic crystal-field model. We found quite an original behavior in the hydrides: the spin orientations differ from nonhydrogenated compounds, and cannot be explained by a unique set of crystal-field parameters. This suggests that the hydrogen superstructure strongly influences the anisotropy.

Simulation of optical and magnetic properties of neodymium (4f 3) in the solid state: extreme sensitivity to the wave vector’s composition

The optical absorption spectra of neodymium at 4 K are reproduced with a very good agreement between calculation and experiment (15<rms<20 cm −1). The wavefunctions which are obtained as a result of the energy calculation can be used to compute other physical properties. Among them we consider the paramagnetic susceptibility and the electron paramagnetic resonance. We report the results of these calculations concerning six compounds (Nd 2O 2S, A-Nd 2O 3, NdOCl, NdOF, Nd 3+:LaCl 3 and NdF 3) for which the optical absorption spectra were thoroughly interpreted. We notice that these properties, depending on the point symmetry of the neodymium, are very sensitive to the values of the coefficients of the kets in the wavefunctions. These wavefunctions are extremely dependent on the crystal field parameters (cfps). A regard on the magnetic properties is very useful to determine if a set of crystal field parameters which fits the overall Stark levels is also convenient for the ground state ( 4I 9/2). A comparison is also made between cfps determined from atomic positions in the structure (Simple Overlap Model) and phenomenological cfps. We also consider the effect of the spin-correlated crystal field (SCCF) through the action of its radial contribution represented by the c k parameters ( k=2,4,6).

Interstitial-fluoride and substitutional-oxygen charge compensations of Er3+doped in CaF2crystal

A detailed crystal-field analysis, based on the Racah' theory, was carried out for the so-called A, B and G1 isolated charge-compensation centers of Er 3+ ion doped in CaF 2 crystal. Three sets of crystal-field parameters were obtained by a least-squares fitting of the optical data of Er 3+ ion diluted in epitaxial Ca 1− x Er x F 2+ x thin film. This theoretical analysis confirms the expected site symmetry for the A center and the site symmetry for the G1 center. For the B center, however, the site symmetry is not exactly in contrast to what is believed.

Crystal-Field Analysis of Er3+ Emission Spectrum in Epitaxial Ca1-xErxF2+x Thin Films

A detailed crystal-field analysis, based on the Racah theory, was carried out for the two isolated interstitial-fluoride charge-compensation centers of Er 3+ ion doped in CaF 2 crystal, termed A and B centers. Assuming the C 4v and C 3v site symmetry, respectively, two sets of crystal-field parameters were obtained by a least-squares fitting of the optical data of Er 3+ ion diluted in epitaxial Ca 1-x Er x F2 +x thin film. This theoretical analysis confirms the expected C 4v site symmetry for the A center. For the B center, however, the site symmetry is not exactly C 3v in contrast to what is believed.

Magnetic ordering in single crystal CeNi 0.82Cu 0.18Sn

We report on the magnetic, transport and thermal measurements for a single-crystal sample of CeNi 0.82Cu 0.18Sn. An antiferromagnetic order below T N=3.5 K is observed in magnetic susceptibility χ, specific heat C and electrical resistivity. The specific heat jump at T N is only 0.7 J/mol K and a large value of C/T=380 mJ/mol K 2 remains at 0.5 K. The strong magnetic anisotropy, χ a>χ b>χ c, is similar to that for the host compound CeNiSn. From the analysis of χ( T), we derived a set of crystal-field parameters, which yield the ground state consisting mainly of |J Z〉=|±5/2〉.

On the standardization of crystal-field parameters and the multiple correlated fitting technique: Applications to rare-earth compounds

This work investigates the crystal-field parameter (CFP) sets for rare-earth (RE) ions doped at orthorhombic and/or lower symmetry, i.e. monoclinic or triclinic sites. Two important questions are addressed: (1) How do you compare CFP sets reported by different authors when there can be as many as six numerically different equivalent sets? and (2) How do you distinguish between global and local minima in the multi-parameter fittings? To answer the first question we propose to adopt the standardization of CFP sets. The orthorhombic and/or monoclinic standardization has been carried out for several illustrative CFP sets, including the contributions from various mechanisms, for NdF 3, RE 2Te 4O 11, and RE 3+ in garnet materials. It is shown that adopting a well-defined standardization convention enables meaningful comparison of CFP sets taken from various sources. The analysis of literature data reveals also cases when the CF parameters may be misinterpreted due to the choice of a different representation in the multi-parameter fittings. To answer the second question we propose the multiple correlated fitting technique (MCFT). Using experimental data for Nd 3+ in YAG and LaF 3, MCFT is employed to probe for more reliable CFP sets.

Measurement and simulation of the energy levels of R=Pr 3+ and Nd 3+ in GaRGe 2O 7

The optical absorption spectra of GaRGe 2O 7, R=Pr, Nd, have been measured, on polycrystalline powders, at 9 K. These compounds are monoclinic, space group P2 1/c , and in them R 3+ occupies a single crystallographic position with C 1 point symmetry. From the crystal field analysis of the spectra, energy level schemes and an expression of the associated wavefunctions for the 4f 2 and 4f 3 configurations have been derived. The semi-empirical simple overlap model is used to calculate an initial set of crystal-field parameters from the crystal structure. The fitting of experimental Stark level energies and the phenomenological calculation of crystal-field parameters were performed for the approximate C s(C 2) symmetry. Very satisfactory correlations were obtained between calculated and experimental levels, with root mean square deviations σ=14.1 and 15.7, respectively, and despite the very low symmetry of R 3+ in the matrix, the similarity of both sets of crystal-field parameters indicates the consistence of these calculations.

Spectroscopic studies ofEu3+andDy3+centers inThO2

Luminescence and excitation spectra of ${\mathcal{R}}^{3+}$-doped thorium oxide $({\mathcal{R}}^{3+}={\mathrm{Eu}}^{3+}{,\mathrm{}\mathrm{Dy}}^{3+})$ are reported and analyzed. ${\mathrm{ThO}}_{2}$ was synthesized and grown as crystal using the flux technique. Usually, ${\mathcal{R}}^{3+}$ ions substitute ${\mathrm{Th}}^{4+}$ as a cubic site, but charge compensation often results in the presence of other centers. Doping concentration also has an important effect on the formation of the crystallographic sites. Using selective dye laser excitation at 12 K and selection rules, site symmetries of ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Dy}}^{3+}$ ions in ${\mathrm{ThO}}_{2}$ are determined and fluorescence from different centers is isolated. Results show that under conditions investigated, ${\mathrm{Eu}}^{3+}$ ions have ${\mathrm{O}}_{h}$ and ${\mathrm{C}}_{3v}$ site symmetry in ${\mathrm{ThO}}_{2}$ while for ${\mathrm{Dy}}^{3+}$ only ${\mathrm{C}}_{3v}$ exists. Lifetimes were measured and discussed. A crystal-field (cf) calculation has been performed on the reduced ${}^{7}{F}_{J}$ basis of the ground ${}^{7}F$ term of the ${\mathrm{Eu}}^{3+}$ ion. The crystal-field parameter set is then used to calculate ${\mathrm{Dy}}^{3+}$ energy levels in ${\mathrm{ThO}}_{2}$ as a test of its consistency.

Studies of local structure of Cm3+ in borosilicate glass using laser and x-ray spectroscopic methods and computational modeling

The local environment of Cm3+ in a borosilicate glass has been probed by a combination of laser spectroscopy, structural modeling, and extended x-ray absorption fine structure (EXAFS) spectroscopy. The Stark splitting for the Cm f–f state transitions is significantly larger than the inhomogeneous line broadening that results from the disordered environment. As a result, the Cm optical spectrum can be fit using an effective operator Hamiltonian to obtain a set of crystal-field parameters. The fitting procedure, which requires the use of a descent-in-symmetry approach, provides a set of parameters for a best fit within tetragonal symmetry. These parameters are then linked to the local environment of Cm through exchange-charge modeling (ECM) of crystal field interactions. Cm in our borosilicate glass is best modeled with six oxygen ions with approximately tetragonal symmetry, and at an average distance of 2.31 (3) Å. The results of crystal-field modeling are supported by EXAFS results.

Crystal field in the heavy fermion compound CeAl3

We present the results of an analysis of inelastic neutron scattering data on CeAl3. By combining the neutron scattering results with measurements of the single-crystal magnetic susceptibility, we have obtained an unambiguous set of crystal field parameters in CeAl3. A comparison of the crystal field parameters along the RAl3 (R=Ce, Pr, and Nd) series of isostructural compounds shows that in CeAl3, A20〈r2〉 is an order of magnitude larger, and A40〈r4〉 is three times larger, than in PrAl3 and NdAl3. These increases cannot be explained by changes in lattice parameters but are more likely to result from the enhancement of the hybridization of the cerium 4f electrons with the conduction electrons in CeAl3. This conclusion is in agreement with the results of our study of the RCu2Si2 series.

Magnetism and phase-diagram of Ce(Ni 1− xPd x) 2Ge 2

Electrical resistivity and magnetic susceptibility have been measured on Ce(Ni 1− x Pd x ) 2Ge 2 single crystals ( x=0.0, 0.12 and 1.0). Ce(Ni 0.88Pd 0.12) 2Ge 2 is paramagnetic at least down to 0.4 K. The set of crystal field parameters of CePd 2Ge 2 are determined based on the fitting of the magnetic susceptibility.

Crystal field analysis of Pr 3+ and Nd 3+ ions in KR(WO 4) 2 (R=Y or Gd) potassium rare-earth tungstates

Absorption and emission spectra of KY(WO 4) 2:Pr 3+ and KGd(WO 4) 2:Pr 3+ crystals are recorded at 77 K, and the experimental energy level schemes of Pr 3+ ions are determined. Crystal field analysis for Pr 3+ ions is carried out using the true C 2 point symmetry. The initial set of crystal field parameters was obtained from superposition model calculations. Experimental crystal field energies of KY(WO 4) 2:Pr 3+ and KGd(WO 4) 2:Pr 3+ crystals are fitted with the root mean squares deviation σ=16.4 and 17.5 cm −1, respectively. The consistency of these sets of crystal field parameter for Pr 3+ ions is confirmed from comparative crystal field calculations for Nd 3+ ions doped in KY(WO 4) 2 crystal resulting in σ=8.2 cm −1.

The structure of the azide coordinated superoxide dismutase of Propionibacterium shermanii investigated by X-ray structure analysis, extended X-ray absorption fine structure, Mössbauer and electron paramagnetic resonance spectroscopy

Azide is bound to the cambialistic Fe-SOD (SOD = superoxide dismutase) of Propionibacterium shermanii in the range pH 6 up to pH 9.5. We describe experiments at pH 6.15 and pH 7.8. At low ionic strength all the iron centres are complexed by azide. The symmetry of the coordination sphere of the iron is hexacoordinated approximately octahedrally. The electron paramagnetic resonance (EPR) and Mössbauer spectra can be described within this symmetry by a consistent set of crystal field parameters of only one iron species. The models of the azide coordinated SOD from extended X-ray absorption fine structure (EXAFS) at high pH and the X-ray structure analysis at low pH are compatible. At pH 6.15 the SOD is pentacoordinated. If azide is added it occupies the sixth coordination site. At pH 7.8 the SOD is partly hexacoordinated with an H 2O or an OH −. Azide replaces this solvent molecule and occupies this position in addition in the pentacoordinated molecules. The decrease in the activity is explained by a competitive inhibition by the molecule at the sixth coordination site. This work shows that in the presence of azide the central iron of the SOD is hexacoordinated in the entire pH range.