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)     

Local structure of the rhombic Fe3+ center in KTaO3

A study is reported of the effect of high-temperature annealing in oxygen, an inert gas, and water vapor on EPR spectra of the Fe³⁺ center of different local symmetries in the incipient ferroelectric KTaO₃. An analysis of the relations obtained permits one to propose and substantiate a model of the rhombic Fe³⁺ center, by which Fe³⁺ substitutes for Ta⁵⁺ near two oxygen vacancies (Fe³⁺-2V _O). Calculations of the crystal-field parameters performed within the Newman superposition model showed in the rhombic center the Fe³⁺ ion is displaced along [011] from the Ta⁵⁺ position it occupies within the tetrahedron formed by four oxygens, to a distance of about 0.25 Å. Some of the recent results obtained in second-harmonic light scattering in iron-doped KTaO₃ samples are interpreted. It is shown that, within the temperature range of 4.2–300 K, rhombic Fe³⁺ centers are static electric dipoles, and that they cannot therefore be a source of dielectric losses in KTaO₃ at T≈40 K, as suggested earlier in some publications.     

EPR of trivalent iron ions in a LiCaAlF6 crystal

The orientational dependences of the EPR spectra of Fe³⁺-doped LiCaAlF₆ single crystals (space group P31c, Z=2), grown at the Laboratory of Magnetic Radio Spectroscopy at Kazan’ State University, have been investigated in detail. The spectrum is described by a trigonal spin Hamiltonian with the following parameters: B ₂₀=40.072×10⁻⁴ cm⁻¹, B ₄₀=−5.799×10⁻⁴ cm⁻¹, B ₄₃=−4.281×10⁻⁴ cm⁻¹, A _s=24.33±1, A _p=6.13±1, g _∥=g _⊥=2.00217±0.0003. A theoretical calculation of the hyperfine structure parameters shows that they are described quite well when allowance is made for the overlapping of the wave functions of the paramagnetic center and the ligands (F⁻). Fiz. Tverd. Tela (St. Petersburg) 39, 488–490 (March 1997)     

Quasiclassical calculation of the fermi level and of the forbidden energy band narrowing in crystal semiconductors with heavy doping

In an effective-mass approximation it is shown that in a heavily doped slightly compensated crystal the narrowing of the band ΔE_g>0 on complete ionization of the impurity is equal to the sum of the exchange interaction ΔE_g ^(exc) of the majority charge carriers and of the energy of the correlation interaction ΔE_g ^(cor) of a nonequilibrium minority charge carrier with a screening cloud of majority ones. When the mean-square fluctuation of the potential energy of an electron (hole) is much higher than the thermal energy, the approximation ΔE_g/E_B=1.3(Na_B ³/v)^0.58+2.7(Na_B ³/v)^0.23 is obtained, where v is the number of equivalent energy minima (valleys) at different values of the quasimomentum of the majority charge carriers, are the Bohr energy and radius; ε is the dielectric permittivity of the crystal lattice; m is the effective mass of the state density in one valley; N is the concentration of the doping impurity. The values of ΔE_g and of the high-energy edge of the interband radiating recombination calculated by the model suggested agree with the data on low-temperature photoluminescence of n-Si, p-Si, p-GaAs, and p-GaSb for 3·10⁻³<Na_B ³<2. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 367–373, May–June, 1997.     

ODMR and EPR investigations of Fe centers in KTaO3

The analysis of EPR spectra obtained from iron doped KTaO₃ crystals in the as-grown state revealed three dominant iron centers: Fe³⁺-O_I, axial Fe-centers with spinS = 3/2 and rhombic Fe³⁺. By comparison with data from literature possible assignments for the center withS = 3/2 are discussed. For the rhombic species the temperature dependence of the main parameters of the Spin- Hamiltonian was measured. The result makes it most plausible that only one rhombic iron center exists in KTaO₃, in contrast with literature. The understanding of the EPR spectra allows us to assign transitions, observed at very low magnetic fields by optically detected magnetic resonance (ODMR), to this rhombic Fe center. On this basis, the magnetic circular dichroism (MCD) of this defect could be identified using the method of tagged-MCD. This spectrum is compared to the tagged-MCD of Fe³⁺-O₁ and of axial Fe⁴⁺ centers, which may be generated metastably by optical charge transfer. Considerably different structures in the MCD spectra of both Fe³⁺ centers indicate different local surroundings and electronic states.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

EPR of Fe3+ ion and symmetry of [AIF5·H2O]2? complex ion in (NH4)2AIF5·H2O single crystals

We used the spin-Hamiltonian method for the analysis of the electron paramagnetic resonance (EPR) spectrum of Fe³⁺ as a probe ion in (NH₄)₂AlF₅·H₂O single crystalline basic material. The theoretical expressions for the magnetic field (at which the fine structure transition lines appear) versus the angle between the magnetic field and the axis of symmetry of the magnetic complex are also given. These values were calculated by applying the perturbation theory to the second-order terms. From the experimental results (at 300 K and 9.21 GHz), the spin-Hamiltonian parameters were deduced:D=(668±10)·10⁻⁴ T,E=(−56±10)·10⁻⁴ T,a=(−54±10)·10⁻⁴ T,F=(30±10)·10⁻⁴ T. An isotropic superhyperfine structure was evidenced for the five fluorine ions. The obtained EPR data were used to determine the local symmetry of the Al³⁺ ion. A good agreement with X-ray diffraction measurements was found.     

EPR and Optical Study of Cu<Superscript>2+</Superscript> Ions Doped in Sodium Zinc Sulfate Tetrahydrate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺-doped sodium zinc sulfate tetrahydrate is done at liquid nitrogen temperature. Two magnetically equivalent sites for Cu²⁺ are observed. The spin-Hamiltonian parameters determined by fitting the EPR spectra to the rhombic-symmetry crystalline field are g _x  = 2.2356, g _y  = 2.0267, g _z  = 2.3472, A _x  = 27 × 10⁻⁴ cm⁻¹, A _y  = 54 × 10⁻⁴ cm⁻¹and A _z  = 88 × 10⁻⁴ cm⁻¹. The ground state wave function is also determined. The g-anisotropy is evaluated and compared with the experimental value. With the help of optical study, the nature of bonding in the complex is discussed.     

Specific features of the EPR spectra of KTaO<Subscript>3</Subscript>: Mn nanopowders

The electron paramagnetic resonance spectra of KTaO₃: Mn nanocrystalline powders in the temperature range from 77 to 620 K have been measured and studied for the first time. The change observed in the spectra has been investigated as a function of the doping level. The doping regions in which Mn²⁺ ions are individual paramagnetic impurities have been established, as well as the regions where the dipole-dipole and exchange interactions of these ions begin to occur. The spin-Hamiltonian constants for the spectrum of non-interacting individual Mn²⁺ ions have been determined as follows: g = 2.0022, D = 0.0170 cm⁻¹, and A = 85 × 10⁻⁴ cm⁻¹. A significant decrease in the axial constant D in the KTaO₃: Mn nanopowder, as compared to the single crystal, has been explained by the remoteness of the charge compensator from the paramagnetic ion and by the influence of the surface of the nanoparticle. It has been assumed that the Mn²⁺ ions are located near the surface and do not penetrate deep into the crystallites.     

EPR of hole centers in nonlinear LiBO3 crystals

Results are presented of an EPR study of theO⁻ hole center in LiB₃O₅ (lithium triborate) crystals. Analysis of angular dependences of EPR spectra is used to determine the principal values of the g tensor (g _XX=2.032, g _YY=2.020, g _ZZ=2.007), along with the modulus of the isotropic part of the A ² tensor (|A|=12.2 G) and the orientations of the principal axes of the paramagnetic O⁻ center. The most probable model for the O⁻ center in lithium borate crystals is the following: a hole trapped by the center is localized on a p-orbital of an oxygen ion linking two boron atoms with coordination numbers three and four near a negatively charged stabilizing structural defect. In this case the characteristic hyperfine splitting is due primarily to the interaction of the unpaired electron spin with the ¹¹B nucleus. Fiz. Tverd. Tela (St. Petersburg) 39, 1380–1383 (August 1996)     

Paramagnetic defects in neutron-irradiated phenakite crystals

Radiation-disordered crystals of phenakite Be₂SiO₄ (at fast neutron fluences of (0.75, 1.0, 1.2, 6.6, 8.5) × 10¹⁸ cm⁻²) have been investigated using EPR spectroscopy. It has been established that the main radiation damages predominantly occur in the silicon-oxygen sublattice of the crystal. Vacancy centers (E′ centers, g = 2.0015), defects of the displaced oxygen type, and O⁻ hole-type centers have been revealed. The paramagnetic absorption signal with the parameters g _x = 2.0218, g _y = 2.0124, and g _z = 2.0027 has been identified with the [SiO₄]³⁻ complex (a variety of O⁻ centers). The intense EPR signals with the parameters g _x = 2.0526, g _y = 2.0020, g _z = 2.0066 and g _x = 2.0290, g _y = 2.0030, g _z = 2.0099 have been assigned to two types of O₂⁻ molecular ions with different local environments. The theoretical models of radiation-induced centers have been discussed by comparing the EPR and optical spectroscopic data.     

Spectroscopic studies on Fe and Mn doped SrB4O7 glasses

EPR and optical absorption studies on Fe³⁺ and Mn²⁺ doped strontium tetraborate (SrB₄O₇) glasses are carried out at room temperature. The EPR spectrum of the Fe³⁺ doped glass consists of signals with g-values 9.04, 4.22 and 2.04, whereas the EPR spectrum of Mn²⁺ doped glass exhibits a characteristic hyperfine sextet around g=2.0. The spectroscopic analyses of the obtained results confirmed distorted octahedral site symmetry for the Fe³⁺ and Mn²⁺ impurity ions. Crystal field and Racah parameters evaluated from optical absorption spectra are: Dq=790, B=700 and C=3000 cm⁻¹ for Fe³⁺doped glass and Dq=880, B=700 and C=2975 cm⁻¹ for Mn²⁺ doped glass.     

Paramagnetische Resonanz an Einkristallen einiger Selten-Erd-Oxide

The paramagnetic resonance of Nd³⁺ in Y₂O₃ has been measured at 4.2°K and 9.25 kMe/s. The values of theg-tensors are: ions onC _3i -sites:g _∥=2.434±0.007;g _⊥=0.702±0.005; ions onC ₂-sites:g _x =4.395±0.012;g _y =0.433±0.009;g _z =1.648±0.006. Further measurements have been performed on La₂O₃ crystals doped with Ce³⁺, Dy³⁺, and Er³⁺; the results are (C _3v -sites only): .      

Study of EPR parameters and defect structure for two tetragonal impurity centers in MgO:Cr<Superscript>3+</Superscript> and MgO:Mn<Superscript>4+</Superscript> crystals

The electron paramagnetic resonance (EPR) parameters (g-factors g _‖, g _⊥ and zero-field splitting D) of two tetragonal 3d³ impurity centers M_3d-V_Mg and M_3d-Li⁺ (where M_3d = Cr³⁺ or Mn⁴⁺, V_Mg is the Mg²⁺ vacancy) in M_3d-doped MgO crystals are calculated from the high-order perturbation formulas including both the crystal-field (CF) and the charge-transfer (CT) mechanisms for 3d³ ions in the tetragonal symmetry. The calculated results are in reasonable agreement with the experimental values. From the calculations, it can be found that the relative importance of the CT mechanism for EPR parameters increases with increasing valence state of the 3d³ ion. So, for the high-valence 3d ⁿ ions in crystals, a reasonable explanation of EPR parameters should take into account both CF and CT mechanisms. The defect structures (characterized by the displacement ΔR of O²⁻ in the intervening M_3d and V_Mg or Li⁺ at the Mg²⁺ site) for these tetragonal impurity centers are obtained from the calculations. The results are consistent with the expectations based on the electrostatic interactions.     

Radiation-induced paramagnetic centers in Ca3Ga2Ge4O14 crystals

Studies of EPR spectra, additional optical absorption, and the kinetics of buildup and decay of radiation-induced paramagnetic centers in compositionally disordered crystals of trigonal Ca-gallogermanate are reported. The parameters of the spin-Hamiltonian describing the observed room-temperature EPR spectra are determined. It is established that the radiation-induced paramagnetic centers in Ca₃Ga₂Ge₄O₁₄ crystals are O⁻ centers in common 6g sites stabilized by the Ge⁴⁺ vacancy in tetrahedral 2d sites. Fiz. Tverd. Tela (St. Petersburg) 39, 1044–1049 (June 1997)     

Improved tau polarisation measurement

Using 22 pb⁻¹ of data collected at LEP in 1992 on the peak of the Z resonance, the ALEPH collaboration has measured the polarisation of the tau leptons decaying into ,πν, ρν and a₁ ν from their individual decay product distributions. The measurement of the tau polarisation as a function of the production polar angle yields the two parametersN _τ andN _e, where, in terms of the axial and vector couplingsg _Al andg _Vl,N _l=2g _Vl g_Al/(g _Vl ² +g _Al ² ). This analysis follows to a large extent the methods devised for the 1990 and 1991 data but with improvements which bring a better understanding of the systematic uncertainties. Combining the 1992 measurements with our previously published results yieldsN _τ=0.136±0.012±0.009 andN _e=0.129±0.016±0.005. Assuminge−τ universality, the measurements imply an effective weak mixing angle of sin ² θ _W ^eff =0.2332±0.0014. Deceased     

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)     

Critical concentrations in Ba-doped incipient ferroelectric SrTiO3

Temperature-induced variations of light refraction and dielectric permittivity in single-crystal Sr_1−x Ba_xTiO₃ (x=0.02, 0.05, 0.07, and 0.14), Sr_1−x Ca_xTiO₃ (x=0.014), and in nominally pure strontium titanate have been studied within the 17–300 K temperature range. The spontaneous polar contribution to the refractive index has been isolated. It was used to calculate the temperature and concentration dependences of the polarization autocorrelation function 〈P _s ^2〉 in the Sr_1−x Ba_xTiO₃ system. For x⩽0.07, the polarization P _s=〈P _s ² 〉^1/2 varies proportional to (x−x _g)^1/2, where x _g=0.0027 is the new critical concentration in Sr_1−x Ba_xTiO₃, below which short-range polar order vanishes. Fiz. Tverd. Tela (St. Petersburg) 39, 704–710 (April 1997)     

Zeeman splitting factor of the Er3+ ion in a crystal field

Numerical computations are presented on the energy levels of the Er³⁺ ion in crystalline fields of cubic, trigonal, tetragonal and orthorhombic symmetry. Zeeman splitting factors were obtained from the level splitting in an additional magnetic field. For the quartet Γ₈ states in cubic symmetry the Zeeman effect is described by an effective Hamiltonian ℋ= gμ_BBJ+ uμ_BBJ³ with the parametersg andu calculated for mixed fourth- and sixth-order potentials. For the eight doublets in the lower symmetry of an axial trigonal or tetragonal crystal field the principalg tensor components g_∥ and g_⊥ were calculated. The results of such calculations for a ground-state doublet can exactly account for the experimental data obtained on around 70 erbium centers in various crystalline hosts. However, sometimes different sets of parameters give comparably good results. An empirical rule of constant trace g_∥ + 2g_⊥ is supported by the calculations. In contrast to analytical treatments the effect of the crystalline field can be followed over a continuous range of the crystal field parameters. This allows one to establish relations on the relative signs of tensor components. It is found that the measured trace of tensors |g_∥| + 2|g_⊥| is not always equal to their real trace g_∥ + 2g_⊥. In an exploratory calculation a nonaxial center was simulated in an orthorhombic field, with calculation of the three principal values g_x, g_y and g_z. A good agreement is obtained for the recently reportedg values of an erbium center in silicon.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)₂, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g _‖=2.1025,g ₊=2.031,A=75.1×10⁻⁴ cm⁻¹,B=14.8×10⁻⁴,D=276.0×10⁻⁴ cm⁻¹ andE=46.7×10⁻⁴ cm⁻¹. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm⁻¹. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)₂—direct exchange and superexchange through the bridging sulphurs—are discussed.