Czesław Rudowicz

Can the electron magnetic resonance (EMR) techniques measure the crystal (ligand) field parameters

In this paper, the question posed in the title is critically examined on the basis of the available literature evidence implying the positive answer. The distinction between, on the one hand, the actual crystal field (CF) or equivalently ligand field (LF) related quantities and, on the other hand, the actual zero-field splitting (ZFS) or equivalently fine structure (FS) quantities, is elucidated. The origin and possible roots of the incorrect terminology consisting in mixing up the two physically distinct quantities at different levels are examined. Aspects concerning Hamiltonians, parameters, energy level splitting, and nature of the operators involved are taken into account. Problems with the various notations for the operators and parameters used in the electron magnetic resonance (EMR) area are also identified and reviewed. A large number of cases of incorrect terminology and other inconsistencies identified in the course of a comprehensive literature survey are analyzed and systematically classified. Implications of the confusion in question, which go beyond the simple semantic issues, are discussed. The results of the survey reveal that the two most serious categories of this confusion lead to misinterpretation of the experimental EMR data. Several examples of serious misinterpretations found in the books, reviews, and original papers are discussed. The incorrect terminology contributes also to misleading keyword classifications of papers in journals as well as references in scientific literature databases. Thus, the database searches may produce unreliable outcomes. Examples of such outcomes are also shown. It is concluded that, in order to prevent further proliferation of the incorrect terminology and thus to increase reliability of the published EMR data, a concerted effort within the EMR community is indispensable. Various ways in this regard at the international level are suggested.

Crystal field analysis of the energy level structure of Cs2NaAlF6:Cr3+

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

Crystal-field energy level analysis for Nd 3+ ions at the low symmetry C 1 site in (Nd(hfa) 4 (H 2 O))(N(C 2 H 5 ) 4 ) single crystals

Optical absorption measurements of Nd(3+) ions in single crystals of [Nd(hfa)(4)(H(2)O)](N(C(2)H(5))(4)) (hfa = hexafluoroacetyloacetonate), denoted Nd(hfa) for short, have been carried out at 4.2 and 298xa0K. This compound crystallizes in the monoclinic system (space group P 2(1)/n). Each Nd ion is coordinated to eight oxygen atoms that originate from the hexafluoroacetylacetonate ligands and one oxygen atom from the water molecule. A total of 85 experimental crystal-field (CF) energy levels arising from the Nd(3+) (4f(3)) electronic configuration were identified in the optical spectra and assigned. A three-step CF analysis was carried out in terms of a parametric Hamiltonian for the actual C(1) symmetry at the Nd(3+) ion sites. In the first step, a total of 27 CF parameters (CFPs) in the Wybourne notation B(kq), admissible by group theory, were determined in a preliminary fitting constrained by the angular overlap model predictions. The resulting CFP set was reduced to 24 specific independent CFPs using appropriate standardization transformations. Optimizations of the second-rank CFPs and extended scanning of the parameter space were employed in the second step to improve reliability of the CFP sets, which is rather a difficult task in the case of no site symmetry. Finally, seven free-ion parameters and 24 CFPs were freely varied, yielding an rms deviation between the calculated energy levels and the 85 observed ones of 11.1xa0cm(-1). Our approach also allows prediction of the energy levels of Nd(3+) ions that are hidden in the spectral range overlapping with strong ligand absorption, which is essential for understanding the inter-ionic energy transfer. The orientation of the axis system associated with the fitted CF parameters w.r.t. the crystallographic axes is established. The procedure adopted in our calculations may be considered as a general framework for analysis of CF levels of lanthanide ions at low (triclinic) symmetry sites.

Low symmetry aspects in spectroscopic and magnetic susceptibility studies of Tb3+(4f8) in TbAlO3

The three sets of crystal field parameters (CFPs) obtained from spectroscopic and magnetic susceptibility studies of Tb3+(4i*) ions in TbA1O3 by Gruber et al. (J. Lumin. 128 (2008) 1271) were reanalyzed. These sets, fitted from experimental energy levels, are physically equivalent and correspond to specific choices of the axis system. Proper interpretation of experimental data for Tb3+ ions at monoclimc Cs symmetry sites in TbA1O3 crystal requires clarification of several intricate low symmetry aspects, namely, (a) three equivalent forms of monoclinic CF Hamiltonian, (b) relative orientation of the crystallographic axis system w.r.t. the symmetry-adapted axis system, (c) mono-clinic standardization of CFPs, (d) distinction between the actual and apparent low symmetry effects exhibited by CFPs, and (e) nominal nature of all fitted CFP sets. For this purpose, modeling of CFPs for Tb3+ in TbA1O3 was carried out using at the first stage only the Coulomb, i.e. point charge, contributions in the exchange charge model. The point charge model calculated CFPs disagree with the experimental CFPs, especially the rank k=6 CFPs. To explain this discrepancy and to verify the correctness of the theoretical CFP calculations additionally the superposition model was employed. The methods of analysis and modeling of CFP sets for monoclinic symmetry cases proposed here proved useful for the studied case as well as might be used for other ion-host systems exhibiting monoclinic or triclinic local site symmetry. Partial results for Tb3+ ions in TbA1O3 were presented here, whereas detailed results were given in a follow-up paper.

Electron paramagnetic resonance studies of cobalt and rare-earth impurity ions in YAlO3

The results of X-band electron paramagnetic resonance (EPR) measurements of Co2+ ions in YAlO3 (YAP) crystals in the temperature range 1.8?40?K are presented. The temperature and angular dependences of EPR spectra have been analysed using a triclinic spin Hamiltonian (SH) consisting of the electronic Zeeman and hyperfine terms. Two distinct positions ? and ? are identified for Co2+ complexes and ascribed to the substitutional Co2+ ions at the Al3+ and Y3+ sites, respectively. The values of the SH parameters are obtained by least squares fitting the ?-?and ?-type Co2+ spectra yielding the principal (orthorhombic-like) values of the tensors g and A as well as the orientation of their principal axes. The additionally observed EPR spectra of the unintentional impurities Nd3+ and Er3+ in YAP crystals are also analysed.

Software package SIMPRE—Revisited

This article elucidates the pitfalls identified in the software package SIMPRE recently developed by Baldoví et al. (J. Comput. Chem. 2013, 34, 1961) for modeling the spectroscopic and magnetic properties of single ion magnets as well as single‐molecule magnets. Analysis of the methodology used therein reveals that the crystal field parameters (CFPs), expressed nominally in the Stevens formalism, exhibit features characteristic for the CFPs expressed in the Wybourne notation. The resemblance of the two types of CFPs introduces a serious confusion that may lead to wrong comparisons of the CFPs taken from various sources. To clarify this confusion, the properties of the CFPs Bkq ( Akq , Ckq ) associated with the Stevens operators Okq (Xu2009=u2009S, J, or L), which belong to the class of the tesseral‐tensor operators, are contrasted with those of the CFPs Bkq associated with the Wybourne operators Cq(k) , which belong to the class of the spherical‐tensor operators. Importantly, the confused properties of Stevens and Wybourne operators may bear on reliability of SIMPRE calculations. To consider this question independent calculations are carried out using the complete approach and compared with those of the restricted approach utilized earlier. It appears that the numerical results of the package SIMPRE are formally acceptable, however, the meaning of the CFPs must be properly reformulated. Several other conceptual problems arising from misinterpretations of the crucial notions and the CFP notations identified therein are also discussed and clarified.

Thermally Induced Changes in the Structure, Composition, and Chemical Properties of LiMn2O4 ±x Spinel Prepared by Sol–Gel Method

Sol–gel method followed by calcination at temperatures of 300–900 °C was used to obtain a series of LiMn2O4 samples with varying amounts of chemical and structural defects while preserving a constant Li:Mn atomic ratio. The physicochemical and structural properties of the samples were characterized by X-ray diffraction (XRD), thermal gravimetry analysis–mass spectrometry (TGA–MS), differential scanning calorimetry (DSC), and Raman spectroscopy techniques. These results were correlated with separately performed high-frequency EMR measurements on the same samples. The oxidation number of Mn ions changes as compared with the stoichiometric spinel composition LiMn2O4. These changes, induced by the removal or incorporation of oxygen, were analyzed and correlated with the appearance or disappearance of the cubic to orthorhombic phase transition at around room temperature. The phase transition occurs only if the concentration of vacancies (cationic or anionic) is below the limit necessary to form sufficient majority of perfect MnO6 octahedra of the high symmetry, Oh7. Disturbance of this high local symmetry, being a condition for Jahn–Teller distortion, seems to also be the decisive factor in suppressing the phase transition that is regarded to be responsible for electrical capacity fading during cell cycling.

Extracting structural information from low symmetry crystal field parameters-case study: Er3+ and Nd3+ ions in YAlO3

Abstract The experimental monoclinic CF parameter (CFP) sets obtained by Duan et al. (Phys. Rev. B 75 (2007) 195130) for Er3+ and Nd3+ ions in YAlO3 were reanalyzed. These CFPs fitted using R-approach, i.e. with the monoclinic second-rank CFP set to zero, and additionally with one six-rank CFP fixed to zero, turned out to be non-standard. In order to understand better the low symmetry aspects involved in the fitted CFPs and extract useful structural information inherent in monoclinic CFPs, an approach comprising four methods was utilized. First, superposition model (SPM) was applied to calculate CFPs in the crystallographic axis system. Second, the principal values for the SPM determined CFPs and the orientation of the principal axis system w.r.t. the crystallographic axis system were obtained using the procedure 3DD for diagonalization of the 2nd-rank CFPs. Third, analysis of higher symmetry approximations, i.e. orthorhombic and tetragonal, was carried out using the pseudosymmetry axes method. Fourth, the closeness factors and norm ratios were employed for quantitative comparisons of various CFP sets. Partial results for Er3+ ions in YAlO3 were presented here, whereas detailed results would be given in a follow-up paper.

Clarification of terminological confusion concerning the crystal field quantities vs. the effective spin Hamiltonian and zero-field splitting quantities in the papers by Bayrakçeken et al. [Spectrochim. Acta Part A 66 (2007) 462 and 1291].

The physically distinct notions: crystal field (CF) [or equivalently ligand field (LF)] and effective spin Hamiltonian, which comprises zero-field splitting (ZFS) [or equivalently fine structure (FS)], are often confused each with other in literature. Confusion of the type X=Y consists in referral to the quantity Y by the name X of another well-defined quantity. Most prevailing is the CF=ZFS confusion, i.e. labeling the actual ZFS/FS quantities as purportedly the CF/LF ones. Unique cases of the inverse ZFS=CF confusion, identified in recent papers by Bayrakçeken et al. [Spectrochim. Acta A 66 (2007) 462 and 1291], is discussed here. To clarify this confusion, clear distinction between operators of various nature used in electron magnetic resonance (EMR), optical spectroscopy, and related studies is provided. Other deficiencies in the two papers in question, which overlap to a large extent, and misinterpretations therein are critically commented on.

Low symmetry aspects inherent in EMR studies of the orthorhombic to monoclinic structural phase transition in the hexagonal form of barium titanate BaTiO3 doped by Fe3+ ions

Electron magnetic resonance (EMR) studies reveal different spectroscopic properties of transition ions doped in the two crystallographically different forms of barium titanate: cubic (normal) c-BaTiO3 and hexagonal polymorph h-BaTiO3. Recent comparative analysis of EMR data helped to solve the controversy concerning the disparate zero-field splitting (ZFS) parameters for Fe3+ ions in c-BaTiO3. This paper deals with the low symmetry aspects inherent in EMR studies of the orthorhombic to monoclinic structural phase transition in h-BaTiO3 doped by Fe3+ ions. Pertinent spin Hamiltonian notations and choices of axis systems are clarified. The second- and fourth-rank ZFS parameters determined by EMR and the second-rank ones computed using a superposition model for the Fe3+ ions in h-BaTiO3 are reanalyzed. The available ZFS parameters are presented in a well-defined axis system and in a unified way to ensure meaningful comparison. Pertinent transformations of ZFS parameters are carried out using the package CST. Simulations of the low symmetry ZFS parameters are carried out to assess the role of monoclinic and triclinic ZFS terms and to investigate the low symmetry aspects arising with lowering of temperature during the orthorhombic to monoclinic structural phase transition in Fe3+:h-BaTiO3. The procedure for analyzing experimental and theoretical ZFS parameters for transition ions at monoclinic and triclinic symmetry sites proposed here enables a better understanding of the low symmetry aspects involved. This study suggests the need to extend superposition model analysis to the fourth-rank ZFS terms for Fe3+ centers in h-BaTiO3.