Values Of Crystal Field Parameters Research Articles

Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

AbstractThe systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

Dynamic multicritical phase diagrams of the mixed spin (2, 5/2) Blume-Emery-Griffiths model with repulsive biquadratic coupling

We investigated the dynamic phase transitions (DPTs) in the mixed spin (2, 5/2) Blume-Emery Griffiths model with repulsive biquadratic interaction in the presence of a time-varying magnetic field. We used the path probability method to obtain the set of the dynamic equations. We numerically solved these dynamic equations to characterize the nature of first- and second-order phase transitions and to find the DPT temperatures as well as obtain the phases in the system. We constructed the dynamic phase diagrams (DPDs) in reduced temperature and amplitude of oscillating magnetic field plane. We observed that the DPDs display richer, complex and more topological various type of phase diagrams. In particular, DPDs exhibit the disordered phase, antiquadrupolar or staggered phase, six different ferrimagnetic phases, three different nonmagnetic phases, and numerous mixed phases. DPDs also display two dynamic tricritical points for only smaller values of crystal-field interactions, multiple critical end and double critical end points, one zero-temperature critical point, one inverse critical end point, and a quadruple point depending on interaction parameters. The system always shows the reentrant behaviors for the higher values of magnetic field amplitude, but it does not exhibit the dynamic tricritical behavior for higher values of crystal-field parameter.

Correlation of EMR and optical spectroscopy data for Cr3+ and Mn2+ ions doped into yttrium aluminum borate YAl3(BO3)4 crystal – Extracting low symmetry aspects

In this study, the crystal field analysis for Cr3+ and Mn2+ ions doped into yttrium aluminum borate YAl3(BO3)4, for short YAB, crystal has been carried out to complement earlier study of the zero-field splitting (ZFS) parameters (ZFSPs). This analysis utilizes data on the distortion models obtained from analysis of the ZFSPs obtained experimentally by EMR for Cr3+ and Mn2+ ions at the Y3+ and Al3+ sites in YAB. This approach enables to verify and enhance reliability of the ZFSP modeling based on superposition model (SPM) analysis and the distortion models predicted previously. Subsequently, modeling of the crystal field parameters (CFPs) based on SPM analysis is carried out for Cr3+ and Mn2+ ions located at possible cation sites in YAB. The SPM predicted CFP values serve as input for the Crystal Field Analysis (CFA) package to calculate the CF energy levels. The predicted physical ZFS of the ground spin state, i.e. the 4A2 state for Cr3+ ion and the 6S state Mn2+ ions, enable calculation of the theoretical ZFSP values, D and D & (a–F), respectively, using the microscopic spin Hamiltonian (MSH) module in the CFA package. In this way, data on the distortions around the Cr3+ centers in YAB (and to a certain extent also for Mn2+ centers) obtained using the ZFSP data from EMR measurements may be correlated with data on the CF energy levels measured by optical spectroscopy. This modeling approach uncovers certain incompatibilities in the existing data for Cr3+:YAB, which call for reanalysis of the previous assignments of the energy levels observed in optical spectra and more accurate experimental data.

Crystal field analysis of Dy and Tm implanted silicon for photonic and quantum technologies.

We report the lattice site and symmetry of optically active Dy3+ and Tm3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence.

First principles and crystal field calculations of the spectral, structural and electric properties of (Na, Li)VSi2O6 clinopyroxenes crystals

In the present paper we report on the results of theoretical modeling of the structural, spectral and electronic properties of (Na, Li)VSi2O6 clinopyroxenes crystals. At first, the semi-empirical model of crystal field theory, namely, the exchange charge model was employed for modeling the crystal field parameters (CFP), taking into account the effects of the covalent bond formation between the V3+ and O2– ions. The calculated CFP values were used for diagonalization of the crystal field Hamiltonian matrix in a complete basis set spanned by all 25 wave functions of the LS terms of the 3d2 electron configuration. The second approach, the ab initio density functional theory (DFT)-based calculations, was employed for the structural optimization, analysis of the band structure, density of the states and the pressure effects of the considered compounds. All obtained results are discussed and compared with available experimental data.

Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites in AB(WO4)2 crystals: II. Pr3+ and Nd3+ ions in KRE(WO4)2 (RE = Y or Gd), Pr3+ ions in M+Bi(XO4)2 (M+ = Li or Na and X = W or Mo), and Nd3+ ions in NaBi(WO4)2 and AgNd(WO4)2

In part I, the crystal-field (CF) parameter (CFP) sets for important potential solid state laser systems Tm3+, Ho3+, and Er3+ ions in KGd(WO4)2 and Tm3+ ions in KLu(WO4)2 were thoroughly revisited using a general framework for the analysis of CF levels and CFP modeling. In this part the non-standard CFP sets for Pr3+ and Nd3+ ions in KR(WO4)2 (R = Y or Gd) and the standard CFP sets for Pr3+ ions in M+Bi(XO4)2 (M+ = Li or Na and X = W or Mo) and Nd3+ ions in the related systems NaBi(WO4)2 and AgNd(WO4)2 are analyzed. Due to structural similarity of the hosts, the CFP values for a given trivalent rare-earth (RE3+) ion should be quite close in these systems. However, the fitted (and model) CFP sets appear disparate for the systems in question. The standardization criteria are utilized to ensure direct comparability of the apparently disparate CFP sets reported in the literature. The CFP sets standardized by us are compared with the originally standard CFP sets for Pr3+ and Nd3+ ions in related AB(XO4)2 systems. Following part I, we argue that meaningful analysis of the mixed CFP sets, i.e. standard and non-standard ones, must take into account the intrinsic features of CF Hamiltonians for orthorhombic and lower symmetry cases, which have not been fully recognized in the literature as yet. The model or fitted CFP sets that belong to disparate regions in the CFP space are intrinsically incompatible, i.e. such sets should not be directly compared. The correlated alternative CFP sets are calculated using monoclinic standardization transformations. The closeness of the standardized CFP sets is assessed in a quantitative way using the closeness factors and the norms ratios. Comparative analysis of the monoclinic CFP sets reported for the titled ion–host systems is carried out and several inconsistencies in the previous studies are clarified. The CFP sets determined by standardization are utilized as starting sets for applications of the multiple correlated fitting technique to independently obtain and additionally verify the fitted CFPs based on published energy levels data. Multiple correlated fittings offer an advantage over the single-fitting tactics by enabling an improved fine-tuning of the final fitted CFPs as well as their interpretation and comparability with the sets obtained by others. The present consistent methodology may enable better understanding of the intricate aspects inherent in the spectroscopic studies for other ion–host systems exhibiting orthorhombic, monoclinic, and triclinic site symmetry.

Study of the Defect Structure and Crystal-Field Parameters of α-Al2O3:Yb3+

The methods currently used for studying the defect structure of laser host crystals doped with transition metal or rare-earth ions have several drawbacks or limitations. This study proposes an alternative approach for obtaining optimized impurity structures using molecular dynamics calculation in conjunction with the superposition model. This new approach is specifically applied to a system named α-Al2O3:Yb3+, in which the simulated defect structure is used to fit the superposition model parameters directly onto the observed energy levels. Such an approach provides predicted values of crystal-field parameters, Zeeman splitting g-factor, and hyperfine structure constants. Moreover, the C3v site symmetry is found to be a good approximation for the actual C3 site of Yb3+, as doped in an α-Al2O3 crystal.

Analysis of optical spectra and SH parameters of V3+ centers in ZnO crystals

The optical spectra, spin-Hamiltonian parameters, and local structure of V3+ ion in ZnO are theoretical studied by using exchange charge model of crystal field. In the calculations, a large cluster around V3+ ion is considered. The local structure of V3+ in ZnO is obtained. The convergence of the crystal field parameter values is discussed.

A way of lowering the uncertainty in values of crystal field parameters derived from the spectra of Ln 3+ ions in cubic field with rhombic distortions

Potentialities of increasing the reliability of crystal field parameters (CFPs) obtained from an analysis of Stark splitting of J-states of spin–orbital multiplets 2S+1L J of the Ln 3+ ions in cubic crystal field with rhombic distortions via a reduction of the region of their final fitting are examined. Suggested method of lowering the uncertainty of the CFP consists in determination of the relation of general cubic CFP of the fourth and sixth ranks ( B 4 and B 6) by comparison of a fragment of the experimental Ln 3+ spectrum with well known picture of dependences of Stark splitting of J-states on the relation mentioned. These dependences were calculated many years ago and published as a set of diagrams by K. Lea, M. Leask, W. Wolf, J. Phys. Chem. Solids, 23 (1962) 1381. Cubic parameters derived from the relation determined can be used for a narrowing of the fitting region for the rest of CFP. The method described is applied to the luminescence spectra of dimeric europium carboxylates with heterocyclic diimines. To justify application of this method to the spectra of Eu 3+ ions in crystal field of cubic symmetry with rhombic distortions it was shown that the influence of the second rank components of the crystal field on the relative Stark splitting of J-states of europium ions is minimal for J = 3, 4. In some cases, this peculiarity provides an approach of Stark splitting of the levels with J = 4 in spectra of compounds with rhombic distortions of cubic crystal field to the image of the splitting at an effective field of cubic symmetry. It was demonstrated that for europium complexes under consideration the relation of the general CFP B 4/ B 6 > 119. Consequently, the CFPs of the sixth rank are very small and the region of fitting of the rest of CFP can be appreciably reduced.

What Use Are Crystal Field Parameters? A Chemist’s Viewpoint

Although first principles methods are gaining interest, the crystal field model is at present the only practicable model to analyze and simulate the energy level structures of lanthanide ions (Ln(3+)) in crystal hosts at the accuracy level of approximately 10 cm(-1). Three criteria are suggested to assess the use of energy parameters, especially crystal field parameters, from the crystal field parametrization of 4f(N) energy level data sets for the entire lanthanide ion (Ln(3+)) series, except Pm(3+). Systematic analyses have been performed upon the most complete energy level data sets available for Ln(3+) situated at sites of high symmetry in crystals of Cs(2)NaLnCl(6). This presents a stringent test for theory because the number of energy parameters is considerably reduced, and the data sets are representative and fairly complete. The results from these data set fittings are shown to comply with the three criteria put forward. First, the fittings of data sets are accurate, and a predictive capability has been employed to calculate the energy levels of Pm(3+) and to elucidate and list all of the potentially luminescent levels of Ln(3+) in the hexachloroelpasolite hosts. Second, the systematic and smooth variations of parameter values over the lanthanide series have been described by simple equations and rationalized. Third, a physical insight of the crystal field parameter variation across this series of elements has been achieved by utilizing a simple semiquantitative model considering the distributions of the 4f radial wave functions at the edge of the Ln(3+) ions, where the ligand orbitals extend. The parameter trends for an individual Ln(3+) ion have been shown to be consistent also for the Cs(2)NaLnF(6) host lattice, and predictions of the individual crystal field parameter values are made.

1H NMR spectral analysis in series of heteroleptic triple-decker lanthanide phthalocyaninato complexes: Contact and dipolar contributions of lanthanide-induced shifts

Model-free separation of lanthanide-induced shifts to contact and dipolar contributions was performed for the series of heteroleptic REE(III) trisphthalocyaninates, namely [(15C5) 4Pc]M(Pc)M(Pc), where Pc 2− – phthalocyaninato-dianion, [(15C5) 4Pc] 2− – 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninato-dianion, M = Nd, Sm, Eu, Tb, Dy, Er, Tm and Y as a diamagnetic reference. Application of one nucleus technique evidenced of abrupt changes of contact F k and dipolar A 2 0 〈 r 2 〉 · G k terms between Tb and Dy. Not withstanding structural non-equivalence of lanthanide ions, they have virtually equal values of crystal-field parameters, which justified application of several crystal-field techniques for the further analysis of LIS data. Both two ( Δ δ k / 〈 S Z 〉 Ln ; Δ δ l / 〈 S Z 〉 Ln ) and three nuclei ( Δ δ k / 〈 S Z 〉 Ln ; Δ δ l / 〈 S Z 〉 Ln ; Δ δ m / 〈 S Z 〉 Ln ) crystal-field independent graphical analysis evidenced of isostructurality of the complete series, therefore variation of solely A 2 0 〈 r 2 〉 between Tb and Dy was responsible for changes of dipolar terms. To study the behavior of contact terms, another three nuclei technique was firstly applied to lanthanide phthalocyaninates. Plotting Δ δ k / Δ δ m versus Δ δ l / Δ δ m demonstrated that in studied complexes F k actually vary between Eu and Tb, therefore evidencing of non-simultaneous variations of contact and dipolar terms. Altogether, it let us refine the values of these terms, which would be subsequently applied for structural analysis of triple-decker crownphthalocyaninates and their supramolecular assemblies in solution.

Optical spectra and energy levels of the Cr 3+ ions in MWO 4 (M=Mg, Zn, Cd) and MgMoO 4 crystals

Single crystals of MWO 4 (M=Mg, Zn, Cd) and MgMoO 4 doped with Cr 3+ have been grown by the flux growth method. Their optical spectra have been systematically measured and assigned on the basis of the classical Ligand Field Theory. The exchange charge model of the crystal field has then been applied to calculate the crystal field parameters (CFPs) and the energy levels of the Cr 3+ ion in all studied crystals. These are in reasonable agreement with the experimental data. Systematic trends in the CFPs values, crystal field splittings and Racah parameters have been evidenced and their relation with sizes and symmetry properties of the host cavities occupied by Cr 3+ has been pointed out.

Spectroscopic and crystal field studies of (NH 4) 2BeF 4 : Co 2+

Detailed studies of the optical spectra of new synthesized (NH 4) 2BeF 4:Co 2+crystals are presented. First consistent theoretical simulations of the absorption spectra were performed using exchange charge model of crystal field (CF). As a result, we obtained the values of crystal field parameters (CFPs) and Co 2+ energy levels; these results were used for absorption bands assignments up to 25 000 cm −1 in the experimental spectrum. Good agreement between calculated and observed energy levels supports reliability of the obtained CFPs values and Racah parameters. The method used in the present paper may be readily applied for the prediction and analysis of the spectroscopic features for new synthesized nonoxide materials doped with transition metal ions.

Exchange charge model calculations of crystal field parameters and crystal field energy levels for [N(CH 3) 4] 2CoCl 4 and [N(CH 3) 4] 2MnCl 4 single crystals

Complex study of [N(CH 3) 4] 2MCl 4 (M = Co, Mn) crystals (crystal growth, optical spectroscopy, crystal field analysis) is presented. Exchange charge model of crystal field was used to calculate the crystal field parameters (CFPs) and energy levels for both Co 2+ and Mn 2+ ions. Effects of the covalent bond formation between Co 2+(Mn 2+) and Cl − ions are treated from the point of view of overlap of the corresponding wave functions. Dependence of the CFPs values on the number of ions included into the crystal lattice summation was analyzed. Comparison between the calculated energy levels and corresponding absorption spectra is discussed; on the basis of the calculations, assignment of the absorption bands in the experimental spectra was done.

Ground and excited state absorption of Ni 2+ ions in MgAl 2O 4: Crystal field analysis

The exchange charge model (ECM) of crystal field is utilized to provide the theoretical explanation of the ground state absorption and the excited state absorption observed for the octahedrally coordinated Ni 2+ ions in the spinel MgAl 2O 4. The ECM enables modeling of the crystal field parameters (CFPs) for the impurity ions based on the crystal structure data of the host lattice. To ensure the reliability of the CFPs, the convergence of the CFP values with the increasing number of the coordination spheres taken into account in the ECM calculations is considered. The trigonal CFPs B 2 0 , B 4 0 and B 4 − 3 determined by the ECM, together with the appropriate Racah parameters B and C, serve as input to two crystal field analysis computer packages, which compute the energy level schemes within the whole 3d 8 configuration. The cubic approximation utilizing only one CFP Dq is also discussed. Basic features of the ground and excited state absorption spectra observed for MgAl 2O 4:Ni 2+ are satisfactorily explained by our crystal field analysis. In order to model the pressure dependence of the CFPs (and thus of the absorption spectra when relevant experimental data become available), the variation of the CFPs induced by possible distortions of the lattice due to, e.g. overall relaxation of the ions or accommodation of the impurity ions in the lattice, is studied. Analysis of the experimental absorption spectra enables us to evaluate also the Huang–Rhys parameter, the effective phonon energy, and the zero-phonon line position.

Crystal field analysis of the energy level structure ofCs2NaAlF6:Cr3+

An analysis of the energy level structure ofCr3+ ionsin Cs2NaAlF6 crystal is performed using the exchange charge model (ECM) together with thecrystal field analysis/microscopic spin Hamiltonian (CFA/MSH) computerpackage. Utilizing the crystal structure data, our approach enables modellingof the crystal field parameters (CFPs) and thus the energy level structure forCr3+ ions at the two crystallographically inequivalent sites inCs2NaAlF6. Usingthe ECM initial adjustment procedure, the CFPs are calculated in the crystallographic axis system centredat the Cr3+ ion at each site. Additionally the CFPs are also calculated using the superpositionmodel (SPM). The ECM and SPM predicted CFP values match very well.Consideration of the symmetry aspects for the so-obtained CFP datasets revealsthat the latter axis system matches the symmetry-adapted axis system relateddirectly to the six Cr–F bonds well. Using the ECM predicted CFPs as an input forthe CFA/MSH package, the complete energy level schemes are calculated forCr3+ ions at the two sites. Comparison of the theoretical results with theexperimental spectroscopic data yields satisfactory agreement. Our resultsconfirm that the actual symmetry at both impurity sites I and II in theCs2NaAlF6:Cr3+ systemis trigonal D3d. The ECM predicted CFPs may be used as the initial (starting)parameters for simulations and fittings of the energy levels forCr3+ ions in structurally similar hosts.

Calculations for 4f–5d excitation spectra of C 4v site-selective F − charge-compensated CaF 2:Pr 3+ and CaF 2:Ce 3+

Reported 4f–5d excitation spectra of C 4v site-selective F − charge-compensated CaF 2:Pr 3+ and CaF 2:Ce 3+ have been fitted using the extended semi-experiential parameterized Hamiltonian. Values of crystal-field parameters of d electron in C 4v site are finally determined according to their band locations and intensities. The peaks in the measured spectra are reassigned with new interpretations. Large energy splitting of E g and T 2g bands due to non-cubic potential is found, and the distortion from cubic coordination geometry is semi-quantitatively discussed using the fitting values of crystal-field parameters for the d electron and the superposition model.

Resonant optical spectroscopy of Er3+ centers in hexagonal GaN-analysis of symmetry

Abstract In this paper the site symmetry and site location of Er 3+ ions in hexagonal GaN determined basing on high resolution resonant optical spectroscopy is reported. A single center is separated from the photoluminescence excitation spectra to the 4 I 11/2 state of Er 3+ and the energy levels of this center were fitted in point charge approximation. From the values of crystal field parameters we conclude that Er occupies a substitutional lattice site in the crystal. This work is a continuation of our previous results for the 4 I 9/2 state.

ESR Study on YBiPt compound doped by Er 3+ and Gd 3+

The compound YBiPt doped by Er and Gd has been studied by ESR technique in the X and Q-bands between 4 and 300 K. Two anisotropic lines with hyperfine peaks due to the interaction between electronic and nuclear spins of 167Er 3+ (I= 7 2 ) isotope have been observed below 30 K. Dysonian line shape of the Er and Gd ESR signals shows a metallic character of the conductivity in studied samples as well. Computer modeling has shown that these lines belong to the two allowed transitions in the quadruplet ground state of Er +3 ions in the ligand crystal field of cubic symmetry. The values of crystal field parameters B 4=−5.68×10 −4 cm −1 and B 6=−6.64×10 −6 cm −1 have been extracted. An additional isotropic line with g=1.99 observed in both frequency bands has been attributed to the Gd impurities. The temperature dependence of peak-to-peak linewidth of Gd 3+ signal has been well described by the Korringa relaxation of Gd magnetic moments to the itinerant electrons, d Hpp= a+ bT. The value of exchange constant J sf have been estimated from the Korringa slope of b=0.12 Oe/K. The intensity of Gd 3+ signal, which obeys the Curie's law for paramagnetic ions at lowest temperatures, deviates from this law at temperatures higher than 140 K.

Spectral shape of the 5D0–7F0 line of Eu3+ and Sm2+ in glass

In glass, the 5D0–7F0 line of Eu3+ is usually asymmetric in shape with a longer tail in the high-energy side, while that of Sm2+ is almost symmetric. This difference is discussed, and simulations of the line shape made by taking into account the transition mechanism and the broad distribution of the crystal-field parameter values are compared with the observed spectra. It is shown that the main cause of the different spectral shape is the difference in the dependence of the transition energy on the crystal-field parameter B20. The 5D0→7F0 fluorescence line narrows and its peak energy shows a blue shift with increasing temperature in Sm2+-doped fluoride glass. The origin of this phenomenon is also discussed.