EPR and Optical Absorption Study of VO2+ Ions Doped Potassium Hexaaquazinc (II) Sulfate Single Crystals

X-band single-crystal electron paramagnetic resonance (EPR) studies are done on VO²⁺ ions doped in potassium hexaaquazinc (II) sulfate, K₂[Zn (H₂O)₆] (SO₄)₂ (PHZS) at room temperature. The spin Hamiltonian parameters, i.e., g and A tensors and their direction cosines, are evaluated by the standard diagonalization procedure using angular variation of the EPR spectra in three planes (ab, bc* and c*a), with the help of a computer program. The EPR spectrum is simulated using the EasySpin program to verify the calculations. The detailed EPR analysis indicates the presence of two magnetically inequivalent VO²⁺ sites. Both the vanadyl complexes are found to take up the substitutional position in the host lattice. The optical absorption spectrum of VO²⁺ ions doped in PHZS single crystal at room temperature is also recorded and four main d–d transfer bands in the visible region are assigned. The theoretical band positions are obtained using energy expressions and a good agreement is found with the experimental values. 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.     

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.     

EPR and optical absorption study of Cr-doped tetramethyl ammonium cadmium chloride single crystals

EPR study of Cr³⁺-doped tetramethyl cadmium chloride (TMCC) single crystals is carried out at room temperature. The crystal field and spin-Hamiltonian parameters are evaluated from the resonance line positions of different lines observed in the EPR spectra. The g and D parameter values are found to be g=1.9741±0.0002 and D=553±2×10⁻⁴ cm⁻¹, respectively. EPR data indicate that the site symmetry of Cr³⁺ ion in the crystal is distorted octahedron. Cr³⁺ ions enter the lattice substitutionally replacing Cd²⁺ sites and bind to the neighboring extra Cd vacancies necessary for charge compensation. The optical absorption spectra are measured in 195–925 nm wavelength range at room temperature. From optical study the energy values of different orbital levels are estimated. Further, the bonding parameters are obtained by correlating optical and EPR data and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are obtained to be B=722, C=2845, Dq=2043 cm⁻¹, h=1.015 and k=0.21.     

EPR and optical absorption studies of VO doped l-alanine (C3H7NO2) single crystals

VO²⁺ doped l-alanine (C₃H₇NO₂) single crystals and powders are examined by electron paramagnetic resonance (EPR) and optical absorption spectroscopy. Three magnetically different sites are resolved from angular variations of l-alanine single crystal EPR spectra. In some specific orientations each VO²⁺ line splits into three superhyperfine lines with intensities of 1:2:1 and maximum splitting value of 2.23 mT. The local symmetries of VO²⁺ complex sites are nearly axial. The optical absorption spectra show three bands. Spin Hamiltonian parameters are measured and molecular orbital coefficients are calculated by correlating EPR and optical absorption data for the central vanadyl ion.     

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

Electron paramagnetic resonance (EPR), optical absorption, and luminescence spectral studies of Mn²⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xMnO₂ (x=1.0, 2.0, 3.0, 4.0, and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g value at g_eff≈2.02 with six line hyperfine structure. A weak resonance signal with effective g value at g_eff≈4.3 is also observed for higher concentrations of Mn²⁺ ions. The EPR spectra of x =3.0 mol% of Mn²⁺ in sodium-lead borophosphate glass sample have been studied at various temperatures. It is observed that the resonance signal intensity decreases with increase in temperature. The optical absorption spectrum exhibits bands characteristic of Mn²⁺ 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 exhibits single broad band in the green region.     

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe²⁺ and Fe³⁺ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe²⁺-Fe³⁺) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe³⁺ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123-297 K. The number of spins (N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility (χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/χ versus T graph. The infrared spectral studies reveal the formation of Fe³⁺-OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si-O-Si stretching and Si-O bending modes.     

EPR and optical absorption studies on VO ions in KZnClSO4·3H2O single crystals—an observation of superhyperfine structure

EPR spectra of VO²⁺ ions doped in KZnClSO₄·3H₂O single crystals have been studied at different temperatures. The EPR spectrum shows a well-resolved hyperfine and superhyperfine structure patterns. The angular variation of EPR spectra reveals the presence of more than three magnetic complexes, which correspond to distinct sites of VO²⁺ ion. From the angular variation EPR data, the spin-Hamiltonian parameters are evaluated and discussed. The optical absorption spectrum studied at room temperature shows bands corresponding to C_4v symmetry. From the EPR and optical data, the molecular-orbital bonding coefficient ε² and β² are evaluated and discussed. The observed five-line superhyperfine structure has been attributed to four protons (with I=1/2) from the surrounding water molecules of one of the vanadyl sites.     

Characterization, EPR and luminescence studies of ZnAl2O4:Mn phosphors

ZnAl₂O₄:Mn green light emitting powder phosphors have been prepared by urea combustion technique involving furnace temperatures about 500 °C in a short time (<5 min). The prepared powders were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectrometry and the surface area measurements by a Brunauer-Emmet-Teller (BET) adsorption isotherms. The EPR spectrum exhibits a resonance signal at g≈2.0, which shows a six-line hyperfine structure (hfs). From the EPR spectra the spin-Hamiltonian parameters have been evaluated at room temperature as well as at 110 K. EPR and photoluminescence (PL) studies revealed that manganese ions were present in divalent state and the site symmetry around Mn²⁺ ions is distorted tetrahedral. The spin concentration (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed. The green emission at 511 nm in ZnAl₂O₄:Mn phosphor is assigned to a transition from the upper ⁴T₁→⁶A₁ ground state of Mn²⁺ ions.     

EPR and optical absorption studies on VO ions in zinc lactate trihydrate

EPR and optical absorption studies of VO²⁺-doped zinc lactate trihydrate single crystals are done at room temperature. The EPR spectra of VO²⁺ are characteristic of tetragonally compressed octahedral site. The angular variation of the EPR spectra shows single site occupying interstitial position in the lattice. The spin Hamiltonian parameters are evaluated as g_x=1.9771, g_y=2.0229, g_z=1.9236 and A_x=76, A_y=104, A_z=197 (×10⁻⁴) cm⁻¹. Using these parameters and optical absorption data various bonding parameters are determined and the nature of bonding in the complex is discussed.     

Structural Properties of Cr<Superscript>3+</Superscript>-Doped Cadmium Oxide Nanopowders

The Cr³⁺-doped cadmium oxide nanopowder is prepared at room temperature by a mild and simple solution method. The prepared powder is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical, electron paramagnetic resonance (EPR) and Fourier transform infrared (FT-IR) techniques. The XRD powder pattern reveals the lattice structure and cell parameters are evaluated. The SEM image shows the stone-like morphology of the nanopowder. The optical absorption spectrum indicates the distorted octahedral site symmetry of Cr³⁺ ions. The crystal field Dq and interelectronic repulsion parameters B and C are evaluated. The EPR spectrum gives a resonance signal at g = 1.973 for Cr³⁺ ions. The FT-IR spectrum reveals the characteristic vibrations of cadmium oxide.     

Effect of Li2O content on physical and structural properties of vanadyl doped alkali zinc borate glasses

The effect of Li₂O content in vanadyl doped 20ZnO+xLi₂O+(30−x)Na₂O+50B₂O₃ (5≤x≥25) glasses has been studied with respect to their physical and structural properties. The absence of sharp peaks in XRD spectra of these glass samples confirms the amorphous nature. The physical parameters like density, refractive index, ionic concentration and electronic polarizability vary non-linearly with x mol% depending on the diffusivities of alkali ions. EPR and optical absorption spectra reveal that the resonance signals are characteristics of VO²⁺ ions in tetragonally compressed octahedral site. Spin-Hamiltonian, crystal field, tetragonal field and bonding parameters are found to be in good agreement with the other reported glass systems. The tetragonal distortion (g_⊥-g_∥) and Dt reveals that their values vary non-linearly with Li₂O content and reaches a minimum at x=10 mol%. An anomaly of character has been observed in all the properties of vanadyl doped glass systems, which gives a clear indication of mixed alkali effect.     

EPR and photoluminescence properties of green light emitting LaAl11O18:Mn phosphors

The LaAl₁₁O₁₈:Mn²⁺ powder phosphor has been prepared using a self-propagating synthesis. Formation and homogeneity of the LaAl₁₁O₁₈:Mn²⁺ phosphor has been verified by X-ray diffraction and energy dispersive X-ray analysis respectively. The EPR spectra of Mn²⁺ ions exhibit resonance signals with effective g values at g≈4.8 and g≈1.978. The signal at g≈1.978 exhibits six-line hyperfine structure and is due to Mn²⁺ ions in an environment close to tetrahedral symmetry, whereas the resonance at g≈4.8 is attributed to the rhombic surroundings of the Mn²⁺ ions. It is observed that the number of spins participating in resonance for g≈1.978 increases with decreasing temperature obeying the Boltzmann law. Upon 451 nm excitation, the photoluminescence spectrum exhibits a green emission peak at 514 nm due to ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. The crystal field parameter Dq and Racah inter-electronic repulsion parameters B and C have been evaluated from the excitation spectrum.     

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.     

EPR and optical absorption studies of vanadyl impurity in zinc potassium phosphate hexahydrate single crystal

Electron paramagnetic resonance (EPR) study of VO²⁺ doped zinc potassium phosphate hexahydrate single crystal is carried out. The angular variation of the spectra is studied in the three crystallographic planes. The principal value of spin Hamiltonian parameters g and A and the direction cosines which principal axes make with the crystallographic axes are determined. The observed values are site I: g_∥=1.9664±0.0002, g_⊥=1.9973±0.0002, A_∥=150±2×10⁻⁴, A_⊥=60±2×10⁻⁴ cm⁻¹; site II: g_∥=1.9276±0.0002, g_⊥=1.9921±0.0002, A_∥=155±2×10⁻⁴ and A_⊥=62±2×10⁻⁴ cm⁻¹. By comparison of direction cosines of g from EPR with the direction cosines of different bonds obtained from crystal structure data it is ascertained that the VO²⁺ ion occupies Zn²⁺ substitutional sites. The optical absorption study of the crystal at room temperature is also carried out. The bands observed in the optical absorption spectrum are attributed to d-d transitions. The EPR results together with the optical data are employed to estimate the molecular orbital (MO) coefficients. These MO coefficients (also called bonding coefficients) are further used to discuss the nature of bonding of VO²⁺ ion with different ligands in the crystal.     

Synthesis and photoluminescent and nonlinear optical properties of manganese doped ZnS nanoparticles

In this work we synthesized ZnS:Mn²⁺ nanoparticles by chemical method using PVP (polyvinylpyrrolidone) as a capping agent in aqueous solution. The structure and optical properties of the resultant product were characterized using UV-vis optical spectroscopy, X-ray diffraction (XRD), photoluminescence (PL) and z-scan techniques. UV-vis spectra for all samples showed an excitonic peak at around 292 nm, indicating that concentration of Mn²⁺ ions does not alter the band gap of nanoparticles. XRD patterns showed that the ZnS:Mn²⁺ nanoparticles have zinc blende structure with the average crystalline sizes of about 2 nm. The room temperature photoluminescence (PL) spectrum of ZnS:Mn²⁺ exhibited an orange-red emission at 594 nm due to the ⁴T₁-⁶A₁ transition in Mn²⁺. The PL intensity increased with increase in the Mn²⁺ ion concentration. The second-order nonlinear optical properties of nanoparticles were studied using a continuous-wave (CW) He-Ne laser by z-scan technique. The nonlinear refractive indices of nanoparticles were in the order of 10⁻⁸ cm²/W with negative sign and the nonlinear absorption indices of these nanoparticles were obtained to be about 10⁻³ cm/W with positive sign.     

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.     

EPR and optical studies of Cu doped ammonium dihydrogen phosphate single crystals

Electron paramagnetic resonance (EPR) spectra of Cu²⁺ ion in ammonium dihydrogen phosphate are studied at liquid nitrogen temperature (77 K). Four magnetically inequivalent Cu²⁺ sites in the lattice are identified. The angular variation spectra of the crystal in the three orthogonal planes indicate that the paramagnetic impurity, Cu²⁺ enters the lattice substitutionally in place of NH₄⁺ ions. The spin Hamiltonian parameters are determined with the fitting of spectra to rhombic symmetry crystalline field. The ground state wave function of Cu²⁺ ion is constructed and found to be predominantly |x²-y²〉. The cubic field parameter (Dq) and tetragonal parameters (Ds and Dt) are determined from optical spectra at room temperature. By correlating EPR and optical absorption spectra, the bonding coefficients are calculated and nature of bonding of metal ion with different ligands in the crystal is discussed.     

Absorption spectra of vanadyl ion doped in MgNH4PO4·6H2O (struvite) crystal

Results of Electron Paramagnetic Resonance (EPR) and optical absorption studies of VO²⁺ ion doped in struvite at room liquid nitrogen temperatures are reported. Three preferential V=O bond directions in the crystal have been identified. The optical and EPR data have shown the formation of NH₄(PO₄VO(H₂O)₅ complex in the crystal as a result of VO²⁺ doping. Correlating the optical and EPR data the molecular orbital coefficients are also obtained and discussed.     

EPR and Optical Absorption Studies of VO2+-Doped Diammonium Hexaaqua Magnesium(II) Sulfate

Electron paramagnetic resonance (EPR) studies of VO²⁺ impurity ion in single crystals of diammonium hexaaqua magnesium(II) sulfate are carried out at 9.5 GHz (X-band) at room temperature. Different spin-Hamiltonian parameters are determined. VO²⁺ is expected to enter the lattice substitutionally. Superhyperfine splitting is also observed. An EPR study of a powder sample is done that supports the data obtained from single crystal studies. Optical absorption studies are also performed at room temperature. The crystal field parameter (D_q), tetragonal parameters (D_s and D_t), and various bonding parameters are evaluated to estimate the covalency and nature of bonding of VO²⁺ with its different ligands.