High-frequency tunable EPR spectroscopy of non-Kramers ions in AgGaSe<Subscript>2</Subscript>: Cr,AgGaS<Subscript>2</Subscript>: Cr,and CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>: Cr crystals

Crystals of the chalcopyrite family, AgGaSe₂, AgGaS₂, and CdGa₂S₄, doped with chromium ions have been investigated using high-frequency broad-band EPR spectroscopy in the range 65–530 GHz at T = 4.2 K. It has been revealed that, in the AgGaSe₂ and AgGaS₂ crystals, the Cr²⁺ ions occupy positions with orthorhombic and tetragonal symmetry, whereas the previously investigated CdGaS₄ crystals contain only tetragonal centers. The observed spectra have been described in the framework of the spin-Hamiltonian formalism. Apart from the divalent chromium centers, the EPR lines attributed to non-Kramers ions are observed in the frequency range 300–450 GHz for all the crystals under investigation. The nature of these lines has been discussed.     

Electrical conductivity of pulverulent iron-cobalt molybdates Co1−xFexMoO4: evidence for hopping conduction

Different mixed iron-cobalt molybdates Co_1−xFe_xMoO₄ (0 < x ≤ 1) were prepared by means of a ceramic process. The influence of the isostructural substitution of Co²⁺ by Fe²⁺ and Fe³⁺ on the electrical conductivity of CoMoO₄ was studied in the temperature range (50–600°C). The results show that the iron substitution increases the electrical conductivity and changes the conduction mechanism of CoMoO₄. From a band conduction mechanism with an activation energy higher than 0.8 eV the conduction mode transforms into a hopping mechanism between the Fe²⁺ and Fe³⁺ ions in the octahedrally coordinated divalent cation sublattice. The activation energy is lower (0.4 eV) and does not alter around the polymorphic transition temperature. Owing to careful oxidations of the samples into cation deficient phases it was shown that the conductivity is proportional to the [Fe²⁺]/[Fe³⁺] ratio. These mild oxidations confirm the hopping mechanism. The presence of Co²⁺/Co³⁺ pairs has a minor contribution to the overall conductivity process. Paper presented at the 2nd Euroconference, Funchal, Madeira, Portugal, 10 – 16 Sept. 1995     

Nature of optical properties of GdFe3(BO3)4 and GdFe2.1Ga0.9(BO3)4 crystals and other 3d5 antiferromagnets

Influence of the partial substitution of paramagnetic Fe³⁺ ions by diamagnetic Ga³⁺ ions in the trigonal crystal GdFe₃ (BO₃)₄ on its optical and magnetic properties is studied and discussed in connection with problems common for all antiferromagnets containing 3d ⁵ ions. Polarized optical absorption spectra and linear birefringence of GdFe₃ (BO₃)₄ and GdFe_2.1Ga_0.9 (BO₃)₄ single crystals have been measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K) and structure of GdFe_2.1Ga_0.9 (BO₃)₄ crystal have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature diminishes from 38 till 16 K, the strong absorption band edge shifts on 860 cm^-1 (0.11 eV) to higher energy and the d-d transitions intensity decreases substantially larger than the Fe concentration does. Strong absorption band edge is shown to be due to Mott-Hubbard transitions. Correlation between position of the strong absorption band edge and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly forbidden d-d transitions in the studied crystals and in other antiferromagnets are explained within the framework of the model of the exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model permitted us to determine the connection between parameters of d-d absorption bands (intensity, width and their temperature dependences), on the one hand, and the exchange, spin-orbit and electron-lattice interactions, on the other hand.     

Optical and EPR spectroscopy of manganese ions in Sr3Ga2Ge4O14 crystals

Results of investigations of the spectroscopic properties of manganese-activated single crystals of Sr₃Ga₂Ge₄O₁₄ by the methods of optical and EPR spectroscopy are reported. It is shown that magnagese activator ions form substitutional centers Mn³⁺ and Mn²⁺ in 1a-octahedral positions of the Sr₃Ga₂Ge₄O₁₄ lattice. Changes in the opticla properties of Sr₃Ga₂Ge₄O₁₄: Mn after vacuum thermal annealing are attributed to charge transfer of some of the manganese ions (Mn³⁺→Mn²⁺). The relationship between the spectroscopical properties of Mn²⁺ ions and the crystallochemical structure of the system are discussed. I. Franko L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 779–783, November–December, 1997.     

Tetragonal distortions of the octahedral environments of Cr3+, Fe3+ and Gd3+ ions in A2CdF4 (A = Rb,Cs) and ACdF3 crystals

Abstract

The tetragonal distortions of local octahedral environments of Cr³⁺, Fe³⁺ and Gd³⁺ ions in Rb₂CdF₄, Cs₂CdF₄, RbCdF₃ and CsCdF₃ crystals have been studied by analyzing their EPR spectra. From the studies, it is found that the tetragonal distortions for Cr³⁺ and Fe³⁺ impurity ions, which substitute Cd²⁺ and have nearly the same ionic radius, are close to each other, whereas that for Gd³⁺ impurity ion, having a larger ionic radius, is larger than those for Cr³⁺ and Fe³⁺ ions in the same crystal. It appears that not only the impurity-ligand distance, but also the tetragonal distortions of impurity centres in crystals are closely related to the size of impurity.     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

Spectroscopy of Sm3+ activator centers in Ca3Ga2Ge4O14 single crystals

Results of investigations of spectroscopic properties of Ca₃Ga₂Ge₄O₁₄ single crystals activated with Sm³⁺ ions are reported. It is shown that Sm³⁺ ions in Ca₃Ga₂Ge₄O₁₄ form a type of activator quasicenter whose nature is associated with disordering of the matrix. I. Franko Lvov State University, 50, Dragomanov St., Lvov, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 296–298, March–April, 1998.     

Photocontrolled magnetic resonance plasticity of γ-irradiated KCl:Fe crystals

A photocontrolled resonance decrease in microhardness, which is due to the application of mutually perpendicular static and microwave fields, in γ-irradiated KCl:Fe crystals has been revealed. It has been found that the magnetic plasticity of unirradiated γ-KCl:Fe crystals is due to the resonance effect of magnetic fields on two types of impurity centers: first, centers containing Fe²⁺-ν _c ion-vacancy pairs and, second, centers containing Fe⁺ ions. The illumination of γ-KCl:Fe crystals with F-light (with a wavelength of λ = 500–600 nm) is accompanied by rearrangement of the spectrum of electron paramagnetic resonance detected by a change in microhardness. The effect of F-light on the spectrum of magnetic resonance plasticity is manifested as the suppression of the spectra of Fe²⁺-ν _c ions with effective g-factors of 7.0 and 3.5 due to their recombination with F-electrons and reconstruction to Fe⁺ centers with g-factors of 2.2 and 4.1.     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.     

Thermo-electric method for nearly complete oxidization of highly iron-doped lithium niobate crystals

A new method is presented, allowing the nearly complete oxidization of lithium niobate crystals (LiNbO₃), doped with large amounts of iron oxide (0.05–3 wt. % Fe₂O₃) utilizing annealing at 700 °C in the presence of externally applied electric fields. The treatment results in a concentration ratio of Fe²⁺ and Fe³⁺ ions of less than 2×10^-3. Strong oxidization of iron in LiNbO₃ reduces the photorefractive effect and is therefore of particular interest for nonlinear-optical applications. PACS 42.65.-k; 66.30.Hs; 71.55.-i     

HgSe:Fe—a mixed-valency system and the problem of the ground state

The dependence of the electron mobility on the iron impurity content N _Fe and temperature is studied for three variants of the ordering of Fe³⁺ ions in crystalline HgSe:Fe, a weakly correlated gas, states with near ordering like that in a strongly correlated Coulomb liquid, and long-range ordering. The electron mobilities owing to scattering on the correlated system of Fe³⁺ ions are determined. The temperature dependence of the mobility is analyzed for electron scattering on fluctuations in the charge density in a system of Fe²⁺-Fe³⁺ iron ions with mixed valency, and the correlation length is determined. It is shown that the ordering region for the Fe³⁺ ions encompasses only the first coordination sphere, i.e., near ordering in the position of the Fe³⁺ ions is established, as in a liquid. The coupling between the ordering of the Fe³⁺ ions and the formation of a correlation gap in the density of impurity d-states and its effect on the low-temperature behavior of the electron mobility in HgSe:Fe crystals are examined. Fiz. Tverd. Tela (St. Petersburg) 40, 425–432 (March 1998)     

Saturable absorbers based on tetrahedrally coordinated transition-metal ions in crystals (Review)

Published data on spectroscopic characteristics of saturable absorbers based on crystals doped with tetrahedrally coordinated transition-metal ions (Cr²⁺, Cr⁴⁺, Cr⁵⁺, V³⁺, Cr²⁺, Fe²⁺) in addition to their use as passive Q-switches for solid-state lasers emitting in the visible and near-infrared regions are reviewed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 5–50, January–February, 2009.     

Electron spectra of Me7Eu2UO2(PO4)5 crystals

Absorption and luminescence spectra of Me₇Eu₂UO₂(PO₄)₅ crystals (Me represents Na, Rb, or Cs) are investigated. It is established that the luminescence of Eu³⁺ ions in these crystals is sensitized by uranyl. The effect of the outer-sphere cation Me⁺ on the spectroscopic properties and structural features of the compounds under study is considered. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 89–94, January–February, 1999.     

Influence of 50 MeV Lithium Ion Irradiation on Hyperfine Interactions of Cr0.5 Li0.5 Fe2O4

Spinel oxide Cr_0.5 Li_0.5 Fe₂O₄ has been irradiated at Nuclear Science Centre, New Delhi, by 50 MeV lithium ions of fluence 5*10¹³ ions/cm² and irradiation effect on hyperfine interactions has been investigated by Mossbauer spectroscopy. The Mossbauer spectrum of irradiated sample shows no paramagnetic doublet contribution and the hyperfine fields corresponding to the Fe³⁺ in the octahedral (B) and the tetrahedral (A) sites are very well separated. That is the observed superimposed A and B sites in unirradiated sample are split into separate lines after Li irradiation. Further an increase of the intensity of the lines (2)–(5) with respect to (1)–(6) signals an orientation of the hyperfine magnetic field towards a direction perpendicular to the ion path due to the irradiation induced strain by the latent tracks. The computer simulation of Mossbauer spectra indicated that the irradiated Fe³⁺-site occupancy of the A-site hyperfine field increased from 43% to 55% whereas the B-site hyperfine field decreased from 57% to 45% compared to unirradiated sample.     

Luminescence of europium and ytterbium ions in strontium hexaborate

We have studied the luminescent properties of Eu^2+/3+ and Yb²⁺ ions in strontium hexaborate SrB₆O₁₀ for excitation in the 120–400 nm region. The luminescence spectra of Ln²⁺ ions in SrB₆O₁₀ consist of overlapping bands in the 370–520 nm region, due to 5d → 4f transitions at several nonequivalent centers. In the excitation spectra, besides the bands associated with 4f → 5d transitions in the Ln²⁺ ions, we also observe a band in the 135–160 nm region due to the transitions O(2p) → B(2s,2p) within the borate anions. The luminescence of the Eu³⁺ ions is excited most efficiently in the region of the Eu³⁺ charge transfer band (λ_max = 226 nm). The results obtained are compared with data for Ln in other strontium borates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 770–774, November–December, 2006.     

Effects of the interaction between R and Fe modes of the magnetic resonance in RFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> rare-earth iron borates

Resonance modes that are due to magnetic excitations in the exchange-coupled subsystems of rare-earth ions (R = Nd³⁺, Sm³⁺, and Gd³⁺) and Fe³⁺ ions have been detected in submillimeter transmission spectra (0.1–0.6 THz) of RFe₃(BO₃)₄ iron borate-multiferroic single crystals. The strong interaction between spin oscillations of the Fe and R subsystems has been revealed, which determines the behavior of the modes depending on the anisotropy of the exchange splitting of the ground doublet of the R ion. It has been shown that the intensities of coupled modes (contributions to the magnetic permeability) depend strongly on the difference between the g factors of Fe and R ions. This dependence makes it possible to determine the sign of the latter g factor. In particular, a noticeable intensity of exchange Nd modes in NdFe₃(BO₃)₄ is due to an increase in their contribution at g _{⊥, ‖}^Nd < 0, while in GdFe₃(BO₃)₄ with g _Gd ≈ g _Fe ≈ 2, the Fe and Gd contributions compensate each other and the exchange (Gd) mode is not observed. In spite of the weak interaction of Sm ions with the magnetic field, SmFe₃(BO₃)₄ exhibits resonance modes, which are attributed to the excitation of Sm ions through the Fe subsystem.     

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mössbauer measurements on nanocrystalline perovskite structured manganite La_0.8Sr_0.2Mn_0.8Fe_0.19 ⁵⁷Fe_0.01O₃ as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 Å. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe³⁺ as well as Fe⁴⁺ ions are distributed in two different environments – Fe³⁺ in low symmetric site surrounded by Mn³⁺ ions only while Fe⁴⁺ in high symmetric site with at least one Mn³⁺ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe⁴⁺ to Fe³⁺. Another transition at 3.7 GPa, represents the presence of Fe³⁺ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.     

Antiferromagnetic resonance and magnetic anisotropy in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system

The antiferromagnetic resonance in single crystals of the YFe₃(BO₃)₄-GdFe₃(BO₃)₄ system is studied in the frequency range 25–140 GHz and the temperature range 4.2–50.0 K. It is established that the YFe₃(BO₃)₄ crystal containing only the magnetic subsystem of Fe³⁺ ions is an antiferromagnet with an easy anisotropy plane. The temperature dependences of the gaps in the antiferromagnetic resonance spectra of GdFe₃(BO₃)₄ and Y _x Gd_{1 ? x} Fe₃(BO₃)₄ are used to calculate the contributions of the Fe³⁺ and Gd³⁺ subsystems to the magnetic anisotropy of these crystals. The contributions are found to be close in magnitude and have opposite signs. This leads to a relatively weak uniaxial anisotropy field in the crystals under investigation. Since the exchange interaction between the Gd³⁺ and Fe³⁺ ions magnetizes the magnetic subsystem of gadolinium, both subsystems start to contribute simultaneously at the Néel temperature of the iron subsystem.     

Investigating the impurity structure for Fe3+ ions doped into rutile titanium dioxide single crystal

The local environment around Fe³⁺ centers in rutile TiO₂ crystals is studied by employing fourth-order perturbation theory formula based on the dominant spin–orbit coupling mechanism. The zero-field splitting parameters (ZFSPs) D and E and crystal field parameters are modeled for the Fe³⁺ ions not only at the substitution Ti⁴⁺ site, but also at the interstitial site with local symmetry D_2h. In order to acquire the best agreement between the calculated ZFSPs and those measured by electron magnetic resonance, the model parameters are adjusted on the basis of several approaches. This enables us to determine the feasible values of the structural distortions resulting from dopant Fe³⁺ ions. Consequently, it is confirmed that Fe³⁺ ions substitute for Ti⁴⁺ sites in rutile TiO₂ crystals.     

Spectroscopic properties of Fe ions at tetragonal sites—Crystal field effects and microscopic modeling of spin Hamiltonian parameters for Fe (S=2) ions in K2FeF4 and K2ZnF4

Magnetic and spectroscopic properties of the planar antiferromagnet K₂FeF₄ are determined by the Fe²⁺ ions at tetragonal sites. The two-dimensional easy-plane anisotropy exhibited by K₂FeF₄ is due to the zero field splitting (ZFS) terms arising from the orbital singlet ground state of Fe²⁺ ions with the spin S=2. To provide insight into the single-ion magnetic anisotropy of K₂FeF₄, the crystal field theory and the microscopic spin Hamiltonian (MSH) approach based on the tensor method is adopted. Survey of available experimental data on the crystal field energy levels and free-ion parameters for Fe²⁺ ions in K₂FeF₄ and related compounds is carried out to provide input for microscopic modeling of the ZFS parameters and the Zeeman electronic ones. The ZFS parameters are expressed in the extended Stevens notation and include contributions up to the fourth-order using as perturbation the spin-orbit and electronic spin-spin couplings within the tetragonal crystal field states of the ground ⁵D multiplet. Modeling of the ZFS parameters and the Zeeman electronic ones is carried out. Variation of these parameters is studied taking into account reasonable ranges of the microscopic ones, i.e. the spin-orbit and spin-spin coupling constants, and the energy level splittings, suitable for Fe²⁺ ions in K₂FeF₄ and Fe²⁺:K₂ZnF₄. Conversions between the ZFS parameters in the extended Stevens notation and the conventional ones are considered to enable comparison with the data of others. Comparative analysis of the MSH formulas derived earlier and our more complete ones indicates the importance of terms omitted earlier as well as the fourth-order ZFS parameters and the spin-spin coupling related contributions. The results may be useful also for Fe²⁺ ions at axial symmetry sites in related systems, i.e. Fe:K₂MnF₄, Rb₂Co_1−xFe_xF₄, Fe²⁺:Rb₂CrCl₄, and Fe²⁺:Rb₂ZnCl₄.