Spectral Investigations on Cu<Superscript>2+</Superscript>-Doped ZnO Nanopowders

Cu²⁺-doped ZnO nanopowders, synthesized at room temperature by mild and simple solution method, are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), optical, electron paramagnetic resonance (EPR) and Fourier-transform infrared (FT-IR) techniques. From XRD and SEM, the crystal structure is identified as hexagonal, and the average crystallite size is around 53 nm. Lattice cell parameters are evaluated. The optical and EPR spectral investigations suggest that the Cu²⁺ ion enters the host lattice in two tetragonally distorted octahedral sites. Crystal field, tetragonal field, spin Hamiltonian and bonding parameters are estimated.     

EPR and Optical Absorption Study of Cu<Superscript>2+</Superscript> Ions Doped in Potassium Hexaaquazinc (II) Sulphate (PHZS) Single Crystals

Single-crystal electron paramagnetic resonance (EPR) studies at X-band have been done on Cu²⁺-doped potassium hexaaquazinc (II) sulphate (PHZS) at room temperature. The spin Hamiltonian parameters g, A and their direction cosines are evaluated using standard diagonalization procedure with the help of a computer program. The EPR spectrum is simulated using EasySpin program to justify the calculations. The ground-state wave function of the Cu²⁺ ion in this lattice is also determined, which is predominantly |x ² − y ²〉. The optical absorption spectrum of Cu²⁺ ions doped in PHZS single crystal at room temperature is also recorded and four main d–d transition bands in visible region are assigned. With the help of assigned bands, the crystal-field parameters (Dq, Ds and Dt) are evaluated. Finally, with the optical and EPR data, the nature of bonding in the complex is discussed.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Growth defect W<Superscript>3+</Superscript>-W<Superscript>2+</Superscript> of CdWO<Subscript>4</Subscript> crystals

CdWO₄ crystals grown by the Czochralski method at the low-temperature gradient were investigated with electron spin resonance (ESR) spectroscopy. ESR spectra did not contain the spectra of impurity ions typical for the CdWO₄ structure, i.e., Fe³⁺, Mn²⁺, and Cr³⁺. At the same time, in the studied crystals a complex ESR spectrum having the hyperfine structure due to two nonequivalent tungsten atoms was observed (W¹⁸³;I=1/2; natural abundance, 14.28%). Angular dependence analysis and simulation of ESR spectra have shown that this novel spectrum is described by a spin-Hamiltonian with the following parameters:D=839 G,E=80 G,g _xx=2.01,g _yy=1.97,g _zz=1.987 and electron spinS=7/2. There is one magnetically nonequivalent position of the center in the crystal structure and the direction ofD _zz andg _zz corresponds to the direction of W_n-W_n+2 (or Cd_n-Cd_n+2) in the crystal structure. Because of the fact that it is in principle impossible to achieve the electron stateS=7/2 for the d-shell of one transition metal ion and taking into account the fact that such electron state is realized for two nonequivalent tungsten atoms, we suppose the defect structure to be the chain W²⁺-M⁺-W³⁺. In the structure of this defect the ion M⁺ is diamagnetic, the ions W²⁺ and W³⁺ have electron spinS=2 andS=3/2, respectively. The necessary condition for such defect to exist is to place this chain of ions in cadmium positions for the charge compensation. the reason for such defects to form is supposed to be the incorporation of M⁺ ions into the CdWO₄ lattice. The presence of W²⁺ and W³⁺ in Cd positions in the defect structure provides the charge compensation and the lowering of the lattice stress.     

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

Electron paramagnetic resonance of Ce<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> impurity ions in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.13</Subscript>

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ and Nd³⁺ impurity ions in unoriented powders of the YBa₂Cu₃O_6.13 compound are observed and interpreted for the first time. It is demonstrated that, upon long-term storage of the samples at room temperature, the EPR signals of these ions are masked by the spectral line (with the g factor of approximately 2) associated with the intrinsic magnetic centers due to the significant increase in its intensity.     

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.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Enhanced blue emission and EPR study of LaMgAl11O19:Eu phosphors

Europium doped LaMgAl₁₁O₁₉ phosphor was prepared by the combustion method. The as-prepared and post-treated (1350 °C 10 h 5% H₂+95% N₂) phosphors were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques. XRD patterns show that LaMgAl₁₁O₁₉:Eu phosphors have hexagonal structure. FT-IR spectrum exhibits absorption bands corresponding to the stretching vibration of AlO₄ and AlO₆. Morphological studies reveal that this phosphor has faceted plates of varying sizes and shapes. The as-prepared LaMgAl₁₁O₁₉:Eu phosphor consists of both Eu³⁺ and Eu²⁺ ions. The phosphor exhibits a bright blue emission at 450 nm (4f⁶5d→4f⁷ transition of Eu²⁺). On post-treating the phosphor we are able to enhance the blue emission efficiency by 330%. The process was detected from the evolution of excitation, emission and EPR spectra and the results are discussed.     

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     

Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.     

Searching for the Exchange Shift: a Set of Test Systems

We suggest that intramolecular exchange coupling J in solutions can possibly be measured with just a ruler using an analogue of the nuclear magnetic resonance paramagnetic shift. If one of the two coupled electron spins is rapidly relaxing, the resulting electron paramagnetic resonance (EPR) spectrum of the system is the resolved spectrum for the second spin with a shifted g-factor. The amount and the sense of the shift are both linear in J, which thus can be obtained from the measured shift by simple algebra. We prepared a set of complexes of rare-earth ions with nitroxides to probe this idea, but the sought effect was not yet found indicating J < 0.1 cm⁻¹ for these systems. Either the radical centers should be brought closer to the central lanthanide ion, or the ion itself should be substituted for a rapidly relaxing transition-metal ion, e.g., Cr²⁺ or Co²⁺. Furthermore we report evolving EPR spectra that reflect the inner life of the complexes in solution. Authors' address: Dmitri Stass, Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, 630090 Novosibirsk, Russia     

EPR of Yb<Superscript>3+</Superscript> ions in a monoclinic KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystal

The electron paramagnetic resonance (EPR) of Yb³⁺ ions in a KY(WO₄)₂ single crystal was investigated at T=4.2 K and fixed frequency of 9.38 GHz. The resonance absorption observed on the lowest Kramers doublet represents the complex superposition of three spectra, corresponding to the ytterbium isotopes with different nuclear moments. The EPR spectrum is characterized by a strong anisotropy of the g-factors. The temperature dependence of the g-factors is shown to be caused by the strong spin-orbital and orbital-lattice coupling. The resonance lines broaden with increasing temperature due to the short spin-lattice relaxation times.     

Nanocrystalline GdAlO3: XPS,EPR and magnetic susceptibility studies

Nanocrystalline gadolinium monoaluminate (GdAlO₃) has been synthesized by sol–gel method after sintering the precursor gel at 950°C. The microstructural features have been proved by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray analysis (EDX). The XRD pattern confirms the formation of single-phase GdAlO₃ while EDX shows that this nanomaterial is stoichiometric; the average size of the nanoparticles is 40 nm. X-ray photoelectron spectroscopy (XPS) has been used to study the chemical composition and bonding in the as-prepared samples. The binding energies of core-level electrons in Gd, Al and O in GdAlO₃ nanopowder have been found slightly shifted compared to the corresponding values of the same elements. The electron paramagnetic resonance (EPR) spectra at 9.23 GHz (X-band) and different temperatures indicate the existence of magnetically concentrated solid containing Gd³⁺ ions. Nèel temperature, T _N =3.993 K, effective Bohr magneton number, μ _eff=8.18, and constant of magnetic exchange interaction, J _ex=−0.069 cm⁻¹, have been determined from DC magnetic susceptibilities measured in the range 2–300 K.     

Physical and Spectral Investigations of Cu<Superscript>2+</Superscript>-Doped Alkali Zinc Borate Glasses

Physical and spectral studies on 20ZnO + xLi₂O + (30-x)Na₂O + 50B₂O₃ (5 ≤ x ≥ 25) doped with 0.1 mol% of paramagnetic CuO impurity are carried out. Powder X-ray diffraction patterns of the glass samples confirm the amorphous nature. The physical parameters of all the glasses were also evaluated with respect to the composition. The electron paramagnetic resonance spectra of all these glasses exhibit resonance signals that are characteristic of Cu²⁺ ions. The optical absorption spectra also confirm the Cu²⁺ ion in tetragonally elongated octahedral site. Various crystal field, spin-Hamiltonian and bonding parameters are evaluated. It is observed that the mixed alkali effect is significant.     

EPR and optical spectroscopy of Yb<Superscript>3+</Superscript> ions in CaF<Subscript>2</Subscript>: an analysis of the structure of tetragonal centers

CaF₂ crystals doped with Yb³⁺ ions have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy. EPR spectra of paramagnetic centers (PCs) for cubic (T_c) and tetragonal (T_tet) symmetries were identified. Empirical energy level diagrams were established and crystal field parameters were determined. Information on the CaF₂∶Yb³⁺ phonon spectra was obtained from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyze the crystal lattice distortions in the vicinity of the Yb³⁺ ion. Within the framework of a superposition model, it is established that four F⁻ ions located symmetrically with respect to the fourfold axis from the side of the ion-compensator approach the impurity ion and deviate from the PC axis. The second set of four fluorine ions also approaches the Yb³⁺ ion and the PC axis. The ion-compensator F⁻ also approaches considerably the impurity ion.     

Luminescence and EPR studies of Eu doped BaAl12O19 blue light emitting phosphors

Europium doped BaAl₁₂O₁₉ powder phosphors have been synthesized by combustion process within few minutes. The phosphors have been characterized by XRD, SEM, FT-IR, EPR and PL techniques. The EPR spectrum exhibits an intense resonance signal at g=1.96 characteristic of Eu²⁺ ions. In addition to this two weak resonance signals have been observed at g=2.28 and g=4.86. The population of the spin levels (N) for the resonance signal at g=1.96 is calculated as a function of temperature. By post-treating the phosphor at 1350 °C under a reducing atmosphere, it is observed that the population of spin levels has been increased five times. The excitation spectrum shows a peak at 326 nm with a shoulder at 290 nm. Upon excitation at 326 nm, the emission spectrum exhibits a well defined broad band with maximum at 444 nm emitting a blue light corresponding to 4f⁶5d→4f⁷ transition. The luminescence intensity also has been enhanced to 60% by post-treating the phosphor at 1350 °C under a reducing atmosphere.     

Characterization and photoluminescence studies of Eu<Superscript>2+</Superscript>-doped BaSO<Subscript>4</Subscript> phosphor prepared by the recrystallization method

Eu doped BaSO₄ was prepared by the recrystallization method and characterization of the material was done by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) techniques. From the XRD pattern of Eu doped BaSO₄ compound, it was found that the prominent phase formed was BaSO₄ and traces of other phases were very weak and the result of FTIR spectrum of BaSO₄:Eu shows that the sulfur-oxygen stretch was found at around 1100 cm⁻¹. The room-temperature PL spectra of the Eu doped BaSO₄ sample showed one peak centered at 374 nm, which is the characteristic emission of Eu²⁺ ion. This emission band at 374 nm corresponds to the 4f⁶ 5d→4f⁷ (⁸S_7/2) transitions of Eu²⁺ ions. The excitation spectrum taken at the wavelength 374 nm extends over a wide range of wavelengths from 220–350 nm with a strong peak at around 260 nm. Furthermore, the present sample shows good crystal quality and high photoluminescence sensitivity. Hence our results suggest possible potential applications of Eu doped BaSO₄ phosphor in optoelectronic devices.     

EPR studies of highly pure,pure and doped chromium oxide powders

Abstract

EPR absorption measurements on ‘pure’, highly pure and A1₂O₃ doped Cr₂O₃ powder have been made. The EPR absorption in the ‘pure’ powders obtained below Ntel temperature is shown to be due to background magnetic impurities present in the powders and not due to superparamagnetism as suggested by earlier authors. No EPR absorption could be observed below Nkl temperature in highly pure powders (total background impurity concentration less than 5 ppm). ‘Pure’ powders or highly pure powders mixed with A1₂O₃ powder and annealed at high temperatures showed a symmetrical EPR absorption line at room temperature. The shape and the g value of this line are practically the same as those obtained for Cr³⁺ ions in Cr₂O₃ above Nee1 temperature or in other nonmagnetic crystals. It is concluded from these results that the impurities diffuse into Cr₂O₃ powder, the antiferro-magnetic coupling between some of the Cr³⁺ ions is broken and these Cr³⁺ ions become paramagnetic, even when the bulk of the material is in antiferromagnetic state. The variation of half-width of EPR lines with impurity concentration shows that the dipolar coupling between Cr³⁺ ions decreases with the increase in impurity concentration and when the impurity concentration is high the Néel temperature seems to shift to lower temperatures. A longer heat treatment of the ‘pure’ B powder resulted in the production of shining metal particles in the powder. The EPR of this powder showed excessive increase in the intensity of EPR absorption when the temperature of the powder was raised to a value just above the Néel temperature. A comparison of these reuslts with the work of earlier authors suggests that the shinning metal particles are those of chromium metal and are responsible for this increase in EPR absorption.     

Jahn-teller chromium ions in CdF<Subscript>2</Subscript> and CaF<Subscript>2</Subscript> crystals: An EPR spectroscopic study in the frequency range 9.3–300 GHz

The bivalent chromium impurity centers in CdF₂ and CaF₂ crystals are investigated using electron paramagnetic resonance (EPR) in the frequency range 9.3–300 GHz. It is found that Cr²⁺ ions in the lattices of these crystals occupy cation positions and form [CrF₄F₄]^6? clusters whose magnetic properties at low temperatures are characterized by orthorhombic symmetry. The parameters of the electron Zeeman and ligand interactions of the Cr²⁺ ion with four fluorine ions in the nearest environment are determined. The initial splittings in the system of spin energy levels of the cluster are measured.