EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Electron spin resonance in chromium doped VO2

Electron spin resonance has been studied in V_1?xCr_xO₂ crystals in the monoclinic insulating phase M₁(x<0.3%). Two distinct substitutional Cr³⁺ centers are observed. The two centers differ in their axial character which we interpret as due to different charge compensation mechanisms. One of the centers is preponderant at higher Cr concentrations (x>0.1%) and its spectrum can be interpreted by assuming charge compensation by a nearest neighbour V⁵⁺.     

Paramagnetic defects in manganese-doped lead tungstate

In manganese-doped PbWO₄ crystals, low-intensity signals of triclinic clusters Mn⁴⁺-V _O and Fe³⁺-V _Pb have been revealed in addition to signals of Mn²⁺ tetragonal centers. The Mn⁴⁺-V _O cluster is formed by a Mn⁴⁺ ion in the W⁶⁺ position, which is associated with a vacancy of the nearest neighbor O^2?ion, and the Fe³⁺-V _Pb cluster consists of a Fe³⁺ ion substituting for Pb²⁺ with a local compensation of by a lead vacancy. It has been shown that, in PbWO₄: Mn, there is also a small amount of Mn⁴⁺ tetragonal centers located in the Pb²⁺ position with a nonlocal compensation of an excess charge.     

V centers in single crystal Al2O3

From optical and thermal bleaching experiments it is concluded that the 400 nm absorption band which appears in Al₂O₃ after γ-irradiation is a composite V band. One of its components is attributed to the V^-_OH center which also is responsible for a localized vibrational band at 3316 cm^-11 analogous to the one observed for the V_OH center in MgO. The irradiation also results in electron trapping at Cr³⁺ impurity ions to produce a band at 227 nm. Annealing at 170°C destroys the V^-_OH center by releasing holes which convert the Cr²⁺ to Cr³⁺ with an attendant thermoluminescence.     

EPR parameters and defect structures of the off-center Ti ion on the Sr site in neutron-irradiated SrTiO3 crystal

The EPR parameters (g factors and hyperfine structure constants A) for the tetragonal Ti³⁺ center in cubic phase and the rhombic Ti³⁺ center in tetragonal phase in the neutron-irradiated SrTiO₃ crystals are calculated from the third-order perturbation formulas of EPR parameters for d¹ ions. These low-symmetry Ti³⁺ centers in both phases of SrTiO₃ are due to the Ti³⁺ ion at “off center” on the Sr²⁺ site. From the calculation, the defect models (including the direction and magnitude of the Ti³⁺ off-center displacement) of the two Ti³⁺ centers in SrTiO₃ are estimated and the EPR parameters of both Ti³⁺ centers are reasonably explained on the basis of the defect models. The results are discussed.     

Intrinsic V- and Vo centers in single crystal SrO

The intrinsic V^- center (a positive hole trapped at a cation vacancy) has been observed in single crystals of SrO following x-irradiation at 77 K. The ESR spectrum corresponds to a spin-$\text{1}\text{2}$ defect having <100> axial symmetry with g₆ = 2.0012(3) and g_⊥ = 2.0703(3). The concentration of V^- centers was enhanced significantly by heating crystals in oxygen at 1000°C and rapidly quenching. V^o centers (two holes trapped at a cation vacancy) were also observed after x-irradiation at low temperature, with g₆ = 2.0011(3), g_⊥ =2.0751(3), and D = 380.4(5) MHz.     

Cr5+ in SrTiO3: An example of a static Jahn-Teller effect in A d1 system

The effect of uniaxial stress on the EPR spectrum of Cr⁵⁺ in SrTiO₃ has been studied. It is concluded that SrTiO₃:Cr⁵⁺ is a static Jahn-Teller system. The strain-coupling coefficient V₂ is found to be 2 × 10⁴ cm^?1. Our results show that in the absence of external stress the intensity ratio of the EPR lines, at temperatures below the cubic-to-tetragonal phase transition, is related to the macroscopic strain, present in SrTiO₃ at these temperatures.     

Electronic structure and term energy calculations for cubic and axial iron defects in SrTiO3

The spin-unrestricted Xα method is used to obtain the molecular orbital diagrams relevant to cubic Fe²⁺, Fe³⁺ and axial Fe⁺ ?V₀ to Fe³⁺ ?V₀ defects in SrTiO₃, where V₀ is an oxygen vacancy. The relative energies of the lower spin multiplets are determined through the transition-state model of Slater. The electronic structures and the term patterns allow to discuss the stability and the ground term of each center trapped in cubic SrTiO₃. The mechanisms of ligand-metal and intervalence charge transfers are examined and compared to the abundant experimental data provided by EPR and optical measurements. The different behaviours of ferroelectrics like BaTiO₃: Fe and LiNbO₃: Fe in regard to the mechanisms implied in the photorefractive effects are reviewed according to the theoretical electronic structures with a special emphasis to the Fe²⁺ ? Fe³⁺ charge transfer.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.     

Investigations of EPR Parameters of MgTiO3:Cr3+, SrTiO3:Cr3+, and SrTiO3:Mn4+ Crystals

Complete high-order perturbation formulas are established based on charge-transfer (CT) and crystal-field (CF) mechanisms. The electron paramagnetic resonance (EPR) g-factors of MgTiO₃:Cr³⁺, SrTiO₃:Cr³⁺, and SrTiO₃:Mn⁴⁺ crystals are calculated from the formulas. The calculations of the EPR g-factors agree well with the experimental values. The contribution rate of the CT mechanism to EPR parameters increases with increasing valence state of the 3d ⁿ ions in the crystals. For the higher-valence state 3d ³ Mn⁴⁺ ion in the crystals, the elucidation of the EPR parameters rationally involves both CF and CT mechanisms.     

Radiation-induced electrical and optical processes in materials based on Al2O3

The irradiation of dielectrics induces electric charging of microscopic regions in the bulk, which is associated with concentration inhomogeneities in the system of traps and the differences in characteristic diffusion lengths of free electrons and holes that are produced in ion tracks and collision cascades. Experimental data on radiation-induced luminescence (RIL) give evidence of the existence of three states of oxygen vacancies in Al₂O₃: an optically inactive (electrically neutral) vacancy, its excited state (known as F ⁺ center), and a negatively charged vacancy (F center). The formation of negatively charged regions under irradiation increases the intensity of the 415-nm band of F centers of RIL of Al₂O₃ single crystals. In Al₂O₃:Cr³⁺ ceramics, a radiation-induced negative charging of grain boundaries with respect to the bulk of grains takes place, which manifests itself as an increase in the intensity of the 690-nm band of RIL of Cr³⁺ ions, whereas the intensity of this band in Al₂O₃:Cr³⁺ single crystals remains unchanged. Using data on RIL, the local-charge density in grains of Al₂O₃:Cr³⁺ ceramics and the field produced by this charge are estimated.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

High-frequency tunable EPR spectroscopy of Cr3+ in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr³⁺ ion Δ_s = 66.7 GHz and between spin sublevels of the Cr³⁺-Cr³⁺ dimer Δ_d1 = 71.5 GHz and Δ_d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr³⁺ ions with an Mg²⁺ vacancy between them replacing three Mg²⁺ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J_z = 0.47 GHz and J_t = 0.79 GHz.     

Structural studies of Cr-doped mullite derived from single-phase precursors

Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr₂O₃ using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr₂O₃). A second phase, namely θ-(Al,Cr)₂O₃, was revealed by XRD in the sample containing 9.60 wt% Cr₂O₃. The structure of mullites was refined by the Rietveld method, location of Cr³⁺ was performed by the EPR spectroscopy. At low chromium doping level (Cr₂O₃ content less than ∼5 wt%) Cr³⁺ ions were substituted for Al³⁺ in the AlO₆ octahedra of the mullite structure (M1 site). For higher doping level, Cr³⁺ ions were additionally substituted for Al³⁺ in the AlO₆ octahedra of the second phase [θ-(Al,Cr)₂O₃ at 1400 °C, or α-(Al,Cr)₂O₃ at 1600 °C] which segregated in the system. Substitution of Cr³⁺ for Al³⁺ on M1 site in the mullite structure resulted in increase of average distances in (M1)O₆ octahedron and decrease of average distances in T*O₄ tetrahedron, while average distances in TO₄ tetrahedron stayed almost constant.     

Theoretical studies of the optical and EPR spectra for V in Y2O3 crystal

In this paper, we give an alternative suggestion that both the observed optical and electron paramagnetic resonance (EPR) spectra of Yttrium oxide (Y₂O₃):V³⁺ are attributed to V³⁺ ions at the S₆ site of Y₂O₃. This suggestion is different from the opinion in the previous paper that the optical and EPR spectra are attributed to V³⁺ ions at the C₂ and S₆ sites, respectively. From the suggestion, the optical band positions and spin-Hamiltonian parameters are calculated by diagonalizing the complete energy matrix for 3d² ions in trigonal symmetry. The results are in good agreement with the experimental values, suggesting that both the observed optical and EPR spectra in Y₂O₃:V³⁺ may be due to V³⁺ at S₆ site of Y₂O₃ crystal.     

Effect of interface states associated with transitional nanophase precipitates in the enhancement of red emission from SrAl12O19:Pr by Ti incorporation

SrAl₁₂O₁₉:Pr³⁺, Ti⁴⁺ phosphor suitable for field emission displays is prepared by the wet chemical gel-carbonate method and the mechanism of enhancement in red photoluminescence (PL) intensity with Ti⁴⁺ therein has been investigated. The PL spectra of Pr³⁺ show both ¹D₂-³H₄ and ³P₀-³H₆ emission in the red region with very weak intensity when excited at 355 nm. The emission intensity has increased by about 100 times at room temperature in the compositional range SrAl_12−xTi_xO_19+x/2:Pr³⁺, with 0.1≤x≤0.3 in comparison to Ti-free SrAl₁₂O₁₉:Pr³⁺. TEM investigations show the presence of exsolved nanophase of SrAl₈Ti₃O₁₉, the precipitation of which is preceded by the presence of defect centers at the interfacial regions between the semicoherent transient phase and the parent SrAl₁₂O₁₉ matrix. The presence of transitional nanophase and the associated defects modify the excitation-emission process by way of formation of electronic sub-levels at lower energy (3.5 eV) than the band gap of SrAl₁₂O₁₉ (∼7 eV) followed by non-resonance energy transfer to Pr³⁺ level, leading to magnetic-dipole related red emission with enhanced intensity. The PL intensity of Pr³⁺ decreases at high Ti⁴⁺ concentrations (x>0.3) due to higher extent of segregation of non-emissive SrAl₈Ti₃O₁₉:Pr³⁺ phase.     

Spectral and generation properties of a new laser crystal (Cr<Superscript>3+</Superscript>,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

Based on the results of a comparative analysis of luminescence spectroscopy and EPR spectroscopy data, it was found for the first time that the wide-band luminescence of Cr³⁺ ions in a forsterite crystal is due to the Cr³⁺-V_Mg center or, in a crystal additionally doped with lithium, to a Cr³⁺-Li⁺ center. For the first time, tunable laser action was obtained with Cr³⁺-Li⁺ centers responsible for the wide-band luminescence.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Electron paramagnetic resonance, optical and electrical properties of vanadyl doped alkali germanoborate glasses

Glasses with composition xGeO₂.(0.30−x)M₂O.0.70B₂O₃ (M=Li, K) containing 2.0 mol% of V₂O₅ have been prepared in the range 0.00≤x≤0.15 by normal melt quenching method. Electron paramagnetic resonance (EPR), optical transmission and absorption spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHPs) of VO²⁺ ions, dipolar hyperfine coupling parameter, P, Fermi contact interaction parameter, K and molecular orbital coefficients (α² and γ²) have been calculated. In GeO₂·Li₂O·B₂O₃ glasses there is no change in the tetragonality of the V⁴⁺O₆ complex and the size of 3d_xy orbit also remains unchanged with increase in GeO₂ content. In GeO₂·K₂O·B₂O₃ glasses, there is an increase in the tetragonality of the V⁴⁺O₆ complex and the 3d_xy orbit expands with increase in GeO₂ content. Values of the theoretical optical basicity, Λ_th, have also been reported. Optical band gap decreases with increase in GeO₂ content. The dc conductivity of these glasses decreases and the activation energy increases with increase in GeO₂:M₂O ratio.