EPR and NMR in powders of doped and undoped IV-VI crystals

The results of the extensive investigations of the variation of the EPR and NMR spectra of active centers due to the existence of the native defects generated by disorder in the IV-VI semiconductor matrices are presented. Both undoped and doped with Gd impurity powder samples of different grain sizes, made from Pb(1-x)Sn(x)Te crystals with the composition in the range 0 < or = x < or = 0.2 were studied. Impurity Gd ions were used as the paramagnetic EPR probe, whereas the 207 Pb nuclei as the NMR probe. The following aspects have been ascertained. (i) Grinding of the initial single crystals into powders leads to an additional component line appearing in the NMR spectra of the 207 Pb nuclei and also to a significant increase in the intensity of EPR spectra of the impurity Gd ions. (ii) Both the Gd EPR spectra as well as the 207 Pb NMR spectra undergo modifications due to isothermal annealing, whereas the character of these modifications is determined by both the temperature and duration of the thermal treatment applied. (iii) Some characteristic correlation between the variation of the EPR spectra of impurity Gd ions and that of the NMR spectra of 207 Pb nuclei, which results from the annealing of the samples, has been observed. Experimental results are interpreted based on the prevailing models of the behavior of the doped impurities and the native defects in the lead and tin telluride crystals.     

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 ( , ) associated with the Stevens operators ( X = S , J , or L ), which belong to the class of the tesseral‐tensor operators, are contrasted with those of the CFPs B_kq associated with the Wybourne operators , 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. © 2014 Wiley Periodicals, Inc.     

Analysis of the temperature dependence of the high-frequency EMR spectra of Mn ions in the lithium-ion battery material LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn³⁺ and Mn⁴⁺ ions in the lithium manganese spinel LiMn₂O₄. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis of the temperature dependence of the HF-EMR spectra due to Mn³⁺ and Mn⁴⁺ ions in LiMn₂O₄ is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn³⁺ and Mn⁴⁺ ions in accounting for the characteristics most suitable for application of LiMn₂O₄ as a cathode material may be gained.     

A method for determination of higher-order magnetic anisotropy constants — Importance of the cubic K3 and K4 for certain energy levels models

A method for determination of magnetocrystalline anisotropy constants of arbitrary order is proposed. The method is based on a least squares fitting of a phenomenological anisotropy energy for a given symmetry truncated at an arbitrary order term to a theoretical anisotropy energy computed exactly for a given energy level model. Several applications of the method to cubic systems are considered. The study reveals that the widely used expressions in the literature for the cubic constants K₁ and K₂ in terms of free energy for the three symmetry direction are of rather limitedvalidity only. The higher-order cubic constants K₃, K₄ and K₅ are determined besides the usual K₁ and K₂ in temperature range 0 to 300 K. The importance of the higher- order terms with respect to the first term in the cubic anisotropy energy is discussed. The results show that the cubic constants K₃ and K₄ cannot be neglected for most of the energy level models studied at certain temperatures.     

Electron paramagnetic resonance study of Fe<Superscript>3+</Superscript> ions at octahedral and tetrahedral mirror symmetry sites in the LiScGeO<Subscript>4</Subscript> crystal

An electron paramagnetic resonance (EPR) study of a synthetic single crystal of LiScGeO₄ doped with Cr ions carried out earlier at the X- and Q-bands at 300, K has indicated additional weak lines. A detailed analysis of these EPR lines, which were tentatively attributed to the Fe³⁺ ions at two different mirror symmetry sites, is presented in this paper. The angular dependences in the three crystallographic planes were resolved by fitting the two distinct spectra denoted Fe³⁺(I) and Fe³⁺(II) with a spin Hamiltonian (S=5/2) of monoclinic symmetry. The rank-4 crystal field tensors at tetrahedral and octahedral sites were calculated with the point-charge model to determine the principal axis orientations of their cubic, tetragonal and trigonal components. A comparative analysis of the zero-field splitting tensors and the crystal field ones indicates that Fe³⁺(I) ions substitute for Sc³⁺ at octahedral sites and Fe³⁺(II) ions substitute for Ge⁴⁺ at tetrahedral sites with no significant distorition of the coordination polyhedra in the structure of LiScGeO₄.     

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.     

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 Bayrak?eken 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.     

Further EPR study of paramagnetic Cr3+ centers in KTiOPO4

Careful reanalysis of EPR spectra of Cr³⁺ ions in KTiOPO₄ has been carried out in order to resolve the weaker lines not considered in our previous study. Two additional centers denoted as C and D have now been identified and the respective spin Hamiltonian parameters have been determined. The reanalysis of the results for the centers A and B identified previously and those for the new centers C and D indicates that the centers A and C originate from Cr³⁺ ions at Ti(2) sites, whereas the centers B and D originate from Cr³⁺ ions at Ti(1) sites. Sixteen magnetically inequivalent Cr³⁺ sites are assigned to two crystallographically different titanium sites: Ti(1) and Ti(2). Possible charge compensation models for the two different centers arising from Cr³⁺ ions at each of the two Ti sites are also suggested.     

Crystal field and microscopic spin Hamiltonians approach including spin-spin and spin-other-orbit interactions for d and d ions at low symmetry C3 symmetry sites: V in Al2O3

A new module has been developed within the CFA/MSH computer package, which is applicable for d² and d⁸ ions at sites of trigonal symmetry type I (C_3v,D₃,D_3d) and type II (C₃,C_3i), including the ‘imaginary’ CF term. For the first time the spin-spin (SS) and spin-other-orbit (SOO) interactions have also been included in the Hamiltonian. This module enables to study the contributions to the energy levels and the spin Hamiltonian parameters, i.e. zero-field splitting D and g-factors: g_∥ and g_⊥. The contributions arising from the spin-orbit (SO), SS, and SOO interaction as well as those due to the low symmetry CF effects induced by the distortion angle ?, which describes the difference between C₃ and C_3v symmetry, can be studied. As an application of the new module, calculations have been carried out for V³⁺(3d²) ions in α-Al₂O₃ crystal, taking into account for the first time the SS and SOO interactions, and the low symmetry CF effects. The results show that (i) the contributions from the SS and SOO interactions to the energy levels are larger for free V³⁺ ions than those for V³⁺ ions in α-Al₂O₃ crystal, (ii) both the contributions to the SH parameters and the energy levels arising from the SOO interaction are larger than those arising from the SS interaction, (iii) the contributions due to the low symmetry CF effects induced by the distortion angle ? are in general significant, (iv) D and g_⊥ are sensitive to the distortion angle ?, whereas g_∥ is insensitive to ?, and (v) the influence of the lattice distortions on the spectroscopic properties of V³⁺ ion in α-Al₂O₃ is pronounced. It appears important for similar ion-crystal cases to consider the lattice distortions in detailed calculations, which take into account the relevant contributions from the SO, SS and SOO interactions. A good agreement between the theoretical and experimental results has been obtained.