EPR of trivalent iron centers in a BaF2: Fe crystal

An orthorhombic paramagnetic Fe³⁺ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF₂: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F^? ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba²⁺ cation by a Fe²⁺ cation, which transforms into a Fe³⁺ (⁶ A _1g ) cation under x-ray irradiation and shifts into a neutral position along the C₂ axis of the cubic coordination shell of the replaced host cation.     

An EPR study of trivalent iron in wardite

Applied Magnetic Resonance - EPR of Fe3+ in a natural phosphate single crystal (wardite) revealed the presence of only one prominent center in orthorhombic symmetry. From the orientation of the...     

Initial stage of laser ablation of LiCaAlF6 single crystal under F2 laser irradiation

Initial stage of F₂ laser ablation of LiCaAlF₆ single crystal was investigated for clarifying the possibility of applying this wide bandgap fluoride crystal to vacuum ultraviolet (VUV) optical components. The ablation threshold, determined by the appearance of line emission from ablated species, was approximately 2 Jcm^-2pulse^-1, similar to that of VUV grade CaF₂ single crystal. The laser-induced damage on the front surface of LiCaAlF₆ was faint, though adhesion of aggregated particulates of several microns was observed. PACS 79.20.Ds; 42.70.–a; 78.20.-e; 61.80.Ba; 61.82.Ms     

Absorption and EPR spectra of Cr3+ ions in a LaBeAl11O19 crystal

Electron spectra of optical absorption and EPR of Cr³⁺ ions in a LaBeAl₁₁O₁₉ crystal are investigated. It is shown that the Cr³⁺ ions occupy, three different octahedral positions of Al³⁺ in the LaBeAl₁₁O₁₉ structure, namely, 12k, 2a, and 4f₂; the ratio of their intensitites is 1∶2∶30, respectively. Parameters of the Cr³⁺ centers are determined and its is shown that the optical absorption spectra in the visible region are practically determined by the Cr³⁺ (III) occupying the 4f₂-positions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 275–277, March–April, 1999.     

EPR and optical spectroscopy of Yb3+ ions in single crystal CsCaF3

CsCaF₃ crystals doped with Yb³⁺ were studied using EPR and optical spectroscopy methods. Several types of paramagnetic centers of Yb³⁺ were found including a paramagnetic center in an uncommon 12-coordinated position. The schemes of the energy levels of the observed centers are determined and the potentials of the respective crystalline fields are calculated.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Single crystal and powder EPR study of trivalent gadolinium in Cs2NaBiCl6 in the 30-300 K interval

A crystalline electric field cubic symmetry site has been reported for Gd³⁺ in Cs₂NaBiCl₆ at room temperature. This host exhibits an apparent structural transformation below 100 K that is completely reversible. However, an EPR examination for a powdered sample of Cs₂NaBiCl₆:Gd³⁺ clearly demonstrates that there are no new large crystalline electric field symmetry sites arising between the transition temperature (100 K) and 30 K, suggesting, therefore, that the site symmetry remains predominantly cubic even at temperatures close to 30 K. In order to substantiate this statement, a computer EPR powder simulation was performed using the single-crystal-spin-Hamiltonian parameters obtained from the three different sites that emerge from the original site while observed at 30 K. A remarkable agreement is observed while comparing the computer-simulated data with that of powdered experimental data. It is important to mention here that several attempts were done trying to fit the observed new spectra to lower crystalline field symmetries, however, our best analytical adjustment was obtained with the cubic spin-Hamiltonian.Below 30 K, new structural transitions are present and the lattice loses its original cubic nature. However, at 10 K the EPR spectrum of the crystal again shows only seven lines that are very broad. This new spectrum cannot be fitted with previously used cubic spin-Hamiltonian parameters.     

Cr~（3＋）：LiCaAlF_6晶体的缺陷

观察并分析了Ｃｒ３＋：ＬｉＣａＡｌＦ６晶体中的缺陷，发现其中主要包括组分过冷、不定形固体颗粒和异相微晶颗粒等三类包裹体。此外，缺陷的宏观分布与生长方向有关。最后，讨论了缺陷的产生机理和消除途径。     

EPR of a non-Kramers iron ion in KTaO3

The EPR spectrum of the non-Kramers iron ion Fe⁴⁺ (S=2) in a KTaO₃:Fe crystal appearing after illumination of the sample in the visible has been detected and studied. Because of the large initial splitting (|D|=4.15 cm⁻¹), only transitions within the |±1〈 and |±2〈 doublets are seen experimentally. Superhyperfine structure in the spectrum of a non-Kramers ion in perovskites has been detected for the first time. A structure is proposed for the center responsible for the new EPR spectrum, which represents a complex of a Fe₄₊ ion substituting for Ta with an oxygen vacancy at the nearest anion site. Fiz. Tverd. Tela (St. Petersburg) 41, 1424–1427 (August 1999)     

XRD and EPR structural investigation of some zinc borate glasses doped with iron ions

Glasses in the system xFe₂O₃·(100?x) [45ZnO·55B₂O₃] (0≤x≤10 mol%) have been prepared by melting at 1200 °C and rapidly cooling at room temperature. The obtained samples were submitted to an additional thermal treatment at 570 °C for 12 h in order to relax the glass structure as well as to improve the local order. The as cast and heat treated samples were investigated using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurements. The XRD patterns of all the studied samples show their vitreous nature. Structural modifications occurring in the heat treated samples compared to the untreated ones have been pointed out. EPR spectra of untreated and heat treated samples revealed resonance absorptions centered at g≈2.0, g≈4.3 and g≈6.4. The compositional variation of the line intensity and linewidth of the absorptions from g≈4.3 and g≈2.0 have been interpreted in terms of the variation in the concentration of the Fe³⁺ ions and the interaction between the iron ions. The EPR spectra of the untreated samples containing 5 mol% Fe₂O₃ have been studied at different temperatures (110–290 K). The line intensity of the resonance signals decreases with increase in temperature whereas the linewidth is found to be independent of temperature. It was also found that the temperature variation of reciprocal line intensity obeys the Boltzmann law.     

Electronic structure and spectrum of Cr3+ in LiCaAlF6

The electronic structure and spectrum of Cr³⁺ in LiCaAlF₆ are investigated by using the discrete variatitional-local density functional (DV-LDF) method with embedded cluster model. The clusters (CrF₆)^3– withC ₃,D _3d andO _h point group symmetries embedded in the crystal are treated. The one-electron energy levels, densities of states, orbital populations, spin polarization splittings and energies of some terms are calculated. The results show that the relaxation of F^– ions around the Cr³⁺ impurity is inevitable, and that theD _3d andO _h (CrF₆)^3– clusters, with an extended bond-lengthR(Cr–F) chosen to be equal to 1.88 Å can represent this relaxation in a much better way. All the ligand-field transition energies, which are obtained from the transition-state energy and the Griffith parameters, as yielded by a restricted one-electron DV-LDF calculation, compare well with the experimental ones.     

Optical properties of trivalent chromium ions in the LiNbO<Subscript>3</Subscript> crystal

The formulas of the crystal-field theory have been adapted to a system with the symmetry group C _3v. A simple method has been proposed for including the polarization of the local environment of the Cr³⁺ impurity ion in LiNbO₃. A model dependent on one parameter has been proposed for a distortion of the niobium octahedron due to the incorporation of the trivalent chromium ion. This parameter has been determined from experimental data. The parameters of the intraionic and interionic interactions have been obtained for the Cr³⁺ ion in the lithium and niobium positions of the crystal lattice of lithium niobate.     

EPR sites of molybdenum ions in single crystals of Cs2HfCl6 and Cs2ZrCl6

Five different spectra of Molybdenum ions in single crystals of Cs₂ZrCl₆ and Cs₂HfCl₆ are reported. Two of them have a Γ₈ ground state and were assigned to the Mo³⁺ ion. The third is fitted to an axial spin Hamiltonian with S = 1. The two others have a Kramers' doublet ground state of S = 1/2 and due to Mo⁵⁺.     

EPR of trivalent chromium centers with monoclinic symmetry in SrF2 crystals

Cr³⁺ centers with monoclinic symmetry in SrF₂:Cr crystals are studied by EPR. Based on an analysis of the EPR data and a study of the conditions under which the observed samples were formed, a model is proposed which explains the low symmetry of the magnetic properties in terms of the simultaneous effect on the impurity ion state of interactions with the interstitial F⁻ ion and with normal vibrations of the crystal lattice in a trigonal symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 302–305 (February 1997)     

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr²⁺ and Cr³⁺ replacing magnesium ions at octahedral positions in Mg₂SiO₄: Cr and Mg₂SiO₄: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \(P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr²⁺ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \(P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \(P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr²⁺ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr³⁺ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr³⁺ and Li⁺ ions. Doping with lithium increases the concentration of Cr³⁺ ions and, thus, decreases the fraction of Cr²⁺ and Cr⁴⁺ ions in the total content of chromium centers.     

Van Vleck paramagnetism of the trivalent Eu ions

Magnetic susceptibilities of Eu₂O₃, EuF₃ and EuBO₃ have been measured over the wide temperature range 5-650 K. The Van Vleck paramagnetism, with the ground state of ⁷F₀ (S=3, L=3), has been investigated comprehensively. The temperature independent paramagnetism emerges manifestly below approximately 100 K. The variation of the susceptibility with temperature for EuBO₃ is in satisfactory agreement with the coupling constant , where the spin-orbit interaction is λL·S for the Russell-Saunders coupling on the basis of Van Vleck theory with one parameter λ. The value of can fit the susceptibility data of EuF₃. The deviation from the theory arises in Eu₂O₃. This discrepancy originates mainly from the influence of the crystalline field. Susceptibility of Gd₂O₃, having the ground state of ⁸S_7/2 (S=7/2, L=0), is also presented as a magnetic standard compound in comparison with these results.     

EPR of iron centres in silicon dioxide

A review is presented of the present status of EPR-based understanding of Fe in SiO₂. As a primary goal, a guide to the literature is given. Quantitative evaluation of all relevant spin-Hamiltonian parameters is discussed, as is computer-based spectral simulation, for single crystals (alpha-quartz), powders and glasses (vitreous quartz). Identification of the numerous centres now known (e.g., for Fe³⁺:[FeO₄/M]⁰ with M=H, Li, Na, …, as well as [FeO₄]^?) is featured, discussing atomic positions, surroundings and dynamic effects. Some general chemical considerations, including comparison between Fe in SiO₂ crystals and glasses, are covered, as are future needs.     

EPR investigation of the trivalent chromium complexes in SrTiO3

The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO₃ single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr³⁺-V_O dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr³⁺-V_O centers have been performed as function of temperature. The analysis of the trigonal Cr³⁺ centers found in SrTiO₃ indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr³⁺ in SrTiO₃ has been analyzed from the angular variation of the total electron spin of S=2 resonance lines.