Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

Spin-spin interactions of Ce3+ ions in the elpasolite crystal Cs2NaInCl6

The EPR method is used to study at T=4.2 K crystals of Cs₂NaInCl₆:Ce³⁺ (1 mol %) at which have elpasolite structure. A solitary cubic Ce³⁺ center and three types of Ce³⁺-Ce³⁺ pairs were observed, for which the spin-spin interaction parameters D and E were determined. It is established that deformation of the local structure results in a shift {Δg} of the pair relative to the g-factor of the solitary center. Models of the solitary center and the three types of Ce³⁺-Ce³⁺ pairs are discussed, as well as the nature of the spin-spin interactions in the pairs. The non-dipole nature of the interactions is established. Possible chains for the creation of an indirect exchange between the Ce³⁺ ions are considered. Fiz. Tverd. Tela (St. Petersburg) 39, 62–67 (July 1997)     

Identification of the La6F37 cubooctahedral clusters in mixed crystals (BaF2)1 ? x(LaF3)x by the electron paramagnetic resonance method

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF₂)_{1 − x} (LaF₃) _x (x = 0, 0.001, 0.002, 0.005, 0.010, 0.020) doped with Ce³⁺ ions (0.1%) are investigated at a frequency v ≈ 9.5 GHz in magnetic fields up to 1.45 T at temperatures T = 10 and 15 K. The EPR spectrum of “pure” barium fluoride BaF₂ (x = 0) is characterized by a single Ce³⁺-F⁻ center with tetragonal symmetry (i.e., the O center with g _‖ = 2.601 and g _⊥ = 1.555). For a lanthanum trifluoride concentration x ≠ 0, the spectrum exhibits new lines due to the presence of the clusters containing Ce³⁺ and La³⁺ ions. The intensity of EPR signals from the O centers decreases rapidly as the lanthanum trifluoride concentration x increases. The lines attributed to a paramagnetic center with tetragonal symmetry and strongly anisotropic g factors (i.e., the K center with g _‖ = 0.725 and g _⊥ = 2.52) are separated in the complex EPR spectrum with the use of the angular dependence of the EPR signal intensity measured for the samples with x ≥ 0.002. This center is identified as a cubooctahedral cluster of the La₆F₃₇ type in which one of the La³⁺ ions is replaced by the Ce³⁺ ion. Original Russian Text ? L.K. Aminov, I.N. Kurkin, S.P. Kurzin, I.A. Gromov, G.V. Mamin, R.M. Rakhmatullin, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 11, pp. 1990–1993.     

EPR study of exchange interactions in the nonmagnetic Kondo system La1−x CexCu6

The EPR of paramagnetic impurities Gd³⁺ and Mn²⁺ was studied in nonmagnetic Kondo system La_1−x Ce_xCu₆ containing in the 1.6–200 K range. The exchange interaction parameters of gadolinium and manganese ions with conduction electrons, of cerium ions with conduction electrons and with one another, the Kondo temperature of cerium ions, and the temperature behavior of cerium-ion spin-fluctuation rate have been determined. A pseudogap in the density of states at the Fermi level has been detected in the CeCu₆ regular system, which is apparently due to s-f hybridization. This pseudogap can be destroyed by introducing an aluminum impurity, which induces strong conduction-electron scattering. It was also found that RKKY interaction among manganese ions in CeCu_6−y Mn_y is considerably stronger than it is in LaCu_6−y Mn_y, which implies enhancement of nonlocal spin susceptibility due to an f band contribution to conduction-electron states. Fiz. Tverd. Tela (St. Petersburg) 40, 593–599 (April 1998)     

Electron paramagnetic resonance in mixed crystals (BaF<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(LaF<Subscript>3</Subscript>)<Subscript>x</Subscript> activated by Ce<Superscript>3+</Superscript> ions

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF₂)_{1?x? y}(LaF₃)_x(CeF₃)_y (y = 0.001 = 0.1%, x = 0–0.02) are investigated in a magnetic field H‖C₄ at a frequency of 9.5 GHz. The angular dependence of the EPR spectrum is measured for the sample with x = 0.02. The lines attributed to Ce³⁺ impurity centers with tetragonal symmetry and g factors (g_‖ = 0.75, g_⊥ = 2.4) close to those measured for the KY₃F₁₀: Ce³⁺ compound are separated in the complex EPR spectrum. The assumption is made that the aforementioned impurity centers are cubooctahedral clusters of the La₆F₃₇ type in which one of the La³⁺ ions is replaced by the Ce³⁺ ion.     

Ferromagnetic Interaction between Copper and Terbium Ions in Pentanuclear Cluster [Cu3Tb2(ClCH2COO)12(H2O)8] · 2H2O

For the first time the exchange interaction between copper and non-Kramers Tb³⁺ ions was studied by means of electron paramagnetic resonance (EPR). Features of the manifestation of this interaction in the EPR spectra of dimer fragments Cu–Tb and pentanuclear fragments Cu–Tb–Cu–Tb–Cu are analyzed. The possibility to determine the sign and value of this interaction from EPR spectra for the case when the lowest states of Tb³⁺ are the states |0〉, | ± 1〉 is shown. The exchange interaction between copper and trivalent terbium ions in the studied pentanuclear complex is ferromagnetic. Authors' address: Violeta K. Voronkova, Kazan Physical-Technical Institute, Russian Academy of Sciences, Sibirsky trakt 10/7, Kazan 420029, Russian Federation     

Growth and the luminescence properties of a lutetium gadolinium garnet doped with Ce3+ and Pr3+ ions

Crystals of lutetium gadolinium garnet solid solutions (Lu_{1 − x} Gd _x )Al₅O₁₂ (0 ≤ x ≤ 0.6) doped with Ce³⁺ and Pr³⁺ ions have been prepared by the horizontal directional crystallization method, and their optical and luminescence properties have been investigated. It has been established that the introduction of gadolinium into the lutetium garnet lattice leads to a decrease in the antisite luminescence (Lu_Al centers) in the UV spectral range and to sensitization of the Ce³⁺ ion luminescence. By contrast, the presence of gadolinium results in the quenching of the Pr³⁺ luminescence due to the nonradiative excitation transfer from Pr³⁺ ions to Gd³⁺ ions.     

Magnetic susceptibility study of Ce in PbCeA (A=Te,Se, S)

The magnetic susceptibility of Pb_1-xCe_xA (A=S, Se and Te) crystals with Ce³⁺ concentrations 0.006≤x≤0.036 was investigated in the temperature range from 2 K to 300 K. The magnetic susceptibility data was found to be consistent with a ²F_5/2 lowest manifold for Ce³⁺ ions with a crystal-field splitting Δ=E(Γ₈)−E(Γ₇) of about 340 K, 440 K and 540 K for Pb_1-xCe_xTe, Pb_1-xCe_xSe, and Pb_1-xCe_xS, respectively. For all the three compounds the doublet Γ₇ lies below the Γ₈ quadruplet which confirms the substitution of Pb²⁺ by Ce³⁺ ions in the host crystals. The observed values for the crystal-field splitting are in good agreement with the calculated ones based on the point-charge model. Moreover, the effective Landé factors were determined by X-band (∼9.5 GHz), electron paramagnetic measurements (EPR) to be g=1.333, 1.364, and 1.402 for Ce ions in PbA, A = S, Se and Te, respectively. The small difference with the predicted Landé factor g of 10/7 for the Γ₇ (J=5/2) ground state was attributed to crystal-field admixture.     

Study of the Structures of the Tetragonal Paramagnetic Centers in the Mixed Fluorite Crystals with Rare-Earth Ions by EPR

Electron paramagnetic resonance (EPR) of the mixed fluorite crystals with the general formula (MeF₂)_1?x?y (REF₃) _x (RF₃) _y (Me = Ca, Sr, Ba; R = Y, La, Lu; RE—paramagnetic trivalent rare-earth ions) were studied comprehensively by different authors and several structural models of paramagnetic centers were considered. However, a lot of details of EPR spectra still remain unexplained. In this work some modifications of the simplest models are proposed which allow explaining adequately the variety of the tetragonal centers in crystals grown under the different conditions. The calculated from the proposed models components of g-factors for Ce³⁺, Nd³⁺, Sm^3+, Er³⁺ ions are in a good agreement with the experimental values.     

Low-frequency detected EPR: Principles and applications

The experimental works described are performed by the authors over last two decades by means of the LFD EPR technique. The essence of this method is low-frequency detection of the longitudinal spin magnetization while the magnetic resonance is excited by a strong microwave field. The first kind of LFD EPR is the enchanced longitudinal susceptibility effect (ELSE) which has been elaborated and applied to study spin thermodynamics in solids since 1972. Various applications of ELSE are described such as direct measuring of the spin-spin interaction temperatureT _ss in the course of resonance saturation, spin-lattice and cross relaxation, dynamic nuclear polarization etc. Another version of LFD EPR was employed to study electron spin-lattice relaxation of paramagnetic centers in high-temperature superconductors (HTSC). Recent experimental data are presented on the electron spin-lattice relaxation of Cu²⁺ ions in YBa₂Cu₃O_6+x at various temperatures andx values.     

EPR and magnetic susceptibility studies of iron ions in ZnO-Fe2O3-SiO2-CaO-P2O5-Na2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe₂O₃)(65−x)SiO₂20(CaO, P₂O₅)15Na₂O (6≤x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe²⁺ and Fe³⁺ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe²⁺ and Fe³⁺ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.     

Electron paramagnetic resonance of scandium in silicon carbide

The EPR spectra of scandium acceptors and Sc²⁺(3d) ions are observed in 6H-SiC crystals containing a scandium impurity. The EPR spectra of scandium acceptors are characterized by comparatively small hyperfine interaction constants, whose values are consistent with the constants for other group III elements in SiC: boron, aluminum, and gallium acceptors. The EPR spectra of scandium acceptors undergo major changes in the temperature interval 20–30 K. In the low-temperature phase the EPR spectra are characterized by orthorhombic symmetry, whereas the high-temperature phase has higher axial symmetry. The EPR spectra observed at temperatures above 35 K and ascribed by the authors to Sc²⁺(3d) ions, or to the A ²⁻ state of scandium, have significantly larger hyperfine structure constants and narrower lines in comparison with the EPR spectra of scandium acceptors. The parameters of these EPR spectra are close to those of Sc²⁺(3d) in ionic crystals and ZnS, whereas the parameters of the EPR spectra of scandium acceptors correspond more closely to the parameters of holes localized at group III atoms, in particular, at scandium atoms in GeO₂. It is concluded that in all centers the scandium atoms occupy silicon sites. Fiz. Tverd. Tela (St. Petersburg) 39, 52–57 (January 1997)     

Structural and magnetic studies of CuO-TeO2 and CuO-TeO2-B2O3 glasses

The glass systems xCuO ^. (1- x)TeO₂ and xCuO ^. (1- x)[75TeO₂ ^. 25B₂O₃] with 0 < x ? 50 mol% were investigated by means of X-ray diffraction, electron paramagnetic resonance (EPR) and a.c. magnetic susceptibility ( ) measurements, the principal aim of the investigation being the study of the structural modifications in the tellurite glasses introduced by the addition of boron oxide. In the case of first glass system, i.e. without B₂O₃, EPR spectra of Cu²⁺ ions undergo changes with the increasing concentration of CuO. At very low concentrations, spectra are due to isolated Cu²⁺ ions in axially distorted octahedral sites. The EPR signal for samples with 3 ? x ? 20 mol% can be explained as being the superposition of two EPR absorptions, one showing the hyperfine structure typical for isolated Cu²⁺ ions and the other consisting of a symmetric line typical for clustered ions. The broadening of the absorption band is due to dipolar as well exchange interaction. The susceptibility data show that for x > 20 mol%, the Cu²⁺ ions are predominantly clustered and are coupled through antiferromagnetic exchange interaction. A comparative study of amorphous X-ray diffraction pattern of the glasses indicates a structural modification in the TeO₂ network with increasing CuO concentration; the effect is quite visible in the samples with CuO concentrations higher than 20 mole percent. Measurements of density corroborate the conclusions drawn from the X-ray diffraction. Additionally, we show that our data validates a model in which CuO rich regions are surrounded by a buffer boundary which separates them from the tellurite glassy network; effect of introducing B₂O₃ can be best described as breaking these regions into smaller size regions. Received 23 October 2000 and Received in final form 1st February 2001     

Dispersion of the Voigt effect in the magnetic semiconductors Cd1−x MnxTe

Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd_1−x Mn _x Te (0≤x≤0.52) in order to investigate how the exchange interaction of Mn²⁺ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x ² is independent of x and the magnetic field direction, i.e., the effect is due to the Mn²⁺ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(E _g −ℏω) ^−3.5. This can be explained by a decrease of the exchange interaction of Mn²⁺ ions with itinerant electrons with increasing distance from the center of the Brillouin zone. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 569–573 (25 April 1998)     

EPR of Ce<Superscript>3+</Superscript> ions in mixed yttrium-lutecium orthoaluminates

The results of EPR studies of Ce³⁺ ions incorporated into single crystals of mixed yttrium-lutecium orthoaluminates Y_1?xLu_xAlO₃ (YLuAP, x=0.1, 0.3) are reported. In compositionally disordered YLuAP compounds, in comparison to YAlO₃, new paramagnetic Ce³⁺ centers are found. These centers are caused by the changes in symmetry and in the crystal field magnitude due to the isomorphic substitution of Y³⁺ ions by Lu³⁺ in the yttrium sublattice of orthoaluminates. It is shown that the formation of 27 different types of centers is possible in YLuAP with variation of the Lu content. The probabilities of formation of new paramagnetic centers are calculated.     

Structure and phase stability of nanocrystalline Ce1−x Ln x O2−x/2−δ (Ln = Yb, Lu) in oxidizing and reducing atmosphere

The structure and phase evolution of nanocrystalline Ce_{1− x} Ln _x O_{2− x/2−δ} (Ln = Yb, Lu, x = 0 − 1) oxides upon heating in H₂ was studied for the first time. Up to 950 °C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 °C in the air and H₂, phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu³⁺ and Yb³⁺ strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C in both atmospheres. Valency of Ce and Yb in Ce_0.1Lu_0.9O_1.55−δ and Ce_0.95Yb_0.05O_1.975−δ samples heated at 1100 °C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce⁴⁺, with small contribution of Ce³⁺ after heating in H₂. In the latter, Yb existed exclusively as 3+ in both O₂ and H₂.     

EPR of Yb3+ ions in Ba1−xLaxF2+x mixed crystals

Electron paramagnetic resonance (EPR) spectra of impurity Yb³⁺ ions (about 0.1 at.%) in mixed crystals BaF₂(1-x) plus LaF₃(x) have been investigated for different values of the concentrationx at a frequency of about 9.5 GHz by both continuous-wave (CW) EPR and electron spin echo methods. A spectrum of trigonal symmetry with a complex hyperfine structure is observed in “pure” BaF₂:Yb³⁺ (x=0). Upon admixture of small amounts of LaF₃ (x=0.001), additional EPR lines arise with intensities increasing with the increase ofx up to 0.005. These lines are attributed to trigonal centers including two rare-earth ions and two compensating fluorine ions. A further increase ofx results in a decrease of the total EPR spectrum intensity, and atx≥0.05 the CW resonance becomes practically unobservable. This may be due to the formation of rare-earth ion clusters with paramagnetic Yb³⁺ ions occurring in domains with a disordered structure of surroundings resulting in very broad EPR lines, which cannot be registered by CW EPR. Indeed, very broad (not less than 1 KG) EPR lines were observed by the electron spin echo method for concentrationsx<-0.02.     

The valence states of manganese ion in potassium-borate oxide glasses

EPR and magnetic susceptibility measurements have been performed on xMnO·(1-x) [2B₂O₃·K₂O] with 0?x?50 mol %. The X-ray diffraction analysis showed that in this glass system homogeneous glasses are formed up to x = 70 mol %.EPR and magnetic susceptibility data have shown that in the glasses with x ? 5 mol % only Mn²⁺ ions are present as magnetically isolated species. EPR spectra are modified with the increasing of manganese ions content. In the concentration range 0.5 ? x ? 5 mol %, the spectra are characterized by appearance of three resonance absorptions at g ? 4.3 and g ? 3.3 without hyperfine structure, and at g ? 2.0 with hyperfine structure. For the glasses with x >62; 5 mol %, the resonance spectra are characterized by the appearance of the broad line at g ? 2, characteristic for clustered ions. The magnetic susceptibility data suggest the appearance of superexchange interactions for x >62; 5 mol %. From Curie constant values and qualitative chemical analysis we have established that in the glasses with x ? 10 mol % both, Mn²⁺ and Mn³⁺ ions are present.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Crystal structure and photoluminescence of (Y1−xCex)2Si3O3N4

Oxonitridosilicate phosphors with compositions of (Y_1−xCe_x)₂Si₃O₃N₄ (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce³⁺ ions have substituted for Y³⁺ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce³⁺ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce³⁺ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.