Paramagnetic Resonance Absorption Spectra Research Articles

Enhanced the energy storage performance in AgNbO3‑based antiferroelectric ceramics via manipulation of oxygen vacancy

The applications of silver niobate (AgNbO3)-based antiferroelectric (AFE) ceramics for potential energy storage are limited by the introduction of oxygen vacancies (OVs). The inevitable OVs narrow the band gap and promote grain growth, resulting in poor breakdown strength and low recoverable energy density (Wrec). Here, we report a significant energy density performance of (Ag1-2xSrx)(Nb0.78Ta0.22)O3 AFE ceramics designed by restraining OVs. Electron paramagnetic resonance (EPR) and UVvis absorption spectra experiments demonstrate that the OV content gradually decreases and the band gap increases with increasing Sr content. Donor doping of Sr leads to the generation of silver ion vacancies, thus, the OV concentration decreases to maintain the electrical neutrality of the system. As a result, a high Wrec of ~5.6 J/cm3 together with an energy efficiency of 70.1% at 300 kV/cm is achieved in the (Ag0.92Sr0.04)(Nb0.78Ta0.22)O3 ceramic. This work offers a novel strategy for improving the energy storage properties of AgNbO3-based AFE ceramics.

Spectroscopic properties of the V-doped borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra in borate glasses of Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5, were investigated. The EPR spectroscopy clearly shows that the V impurity is incorporated into the network of studied glasses, mainly, as isolated vanadyl (VO2+) molecular complex centres. Spin Hamiltonian parameters of axial symmetry (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) constants of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry) for all investigated glasses were determined from their experimental EPR spectra. Optical absorption bands, observed in the investigated glasses have been identified and interpreted. Crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the studied glasses were calculated and compared with referenced data for other V-doped borate glasses with similar compositions.

Structural flexibility of the heme cavity in the cold-adapted truncated hemoglobin from the Antarctic marine bacterium Pseudoalteromonas haloplanktis TAC125.

Truncated hemoglobins build one of the three branches of the globin protein superfamily. They display a characteristic two-on-two α-helical sandwich fold and are clustered into three groups (I, II and III) based on distinct structural features. Truncated hemoglobins are present in eubacteria, cyanobacteria, protozoa and plants. Here we present a structural, spectroscopic and molecular dynamics characterization of a group-II truncated hemoglobin, encoded by the PSHAa0030 gene from Pseudoalteromonas haloplanktis TAC125 (Ph-2/2HbO), a cold-adapted Antarctic marine bacterium hosting one flavohemoglobin and three distinct truncated hemoglobins. The Ph-2/2HbO aquo-met crystal structure (at 2.21 Å resolution) shows typical features of group-II truncated hemoglobins, namely the two-on-two α-helical sandwich fold, a helix Φ preceding the proximal helix F, and a heme distal-site hydrogen-bonded network that includes water molecules and several distal-site residues, including His(58)CD1. Analysis of Ph-2/2HbO by electron paramagnetic resonance, resonance Raman and electronic absorption spectra, under varied solution conditions, shows that Ph-2/2HbO can access diverse heme ligation states. Among these, detection of a low-spin heme hexa-coordinated species suggests that residue Tyr(42)B10 can undergo large conformational changes in order to act as the sixth heme-Fe ligand. Altogether, the results show that Ph-2/2HbO maintains the general structural features of group-II truncated hemoglobins but displays enhanced conformational flexibility in the proximity of the heme cavity, a property probably related to the functional challenges, such as low temperature, high O2 concentration and low kinetic energy of molecules, experienced by organisms living in the Antarctic environment.

Chromium point defects in hexagonalBaTiO3: A comparative study of first-principles calculations and experiments

Chromium-doped hexagonal barium titanate is studied with first-principles density functional theory. The results are compared with experimental data available from electron paramagnetic resonance, x-ray diffraction, and optical absorption spectra. The probable site for the impurity atom occupancy in the lattice, their probable charge states, and the role of oxygen vacancies in their stabilization are investigated. Defect formation energy is used to analyze the role of electronic- and ionic-compensation mechanisms in stabilizing the point defect. Various atomic positions for the oxygen vacancy surrounding the impurity atom are taken into consideration in order to compare with some of the conclusions derived from experiments. Our results on the substitutional site preference and the location of oxygen vacancy in the next-neighbor surrounding of the impurity Cr are in good agreement with experiments.

X-Ray Diffraction, Electron Paramagnetic Resonance and Optical Absorption Study of Bauxite

The bauxite mineral obtained from Araku, Vishakapatnam district of Andhra Pradesh, India is used in the present work. Structural characterization was performed by X-ray diffraction (XRD). The mineral was found to be gibbsite in phase. The transitional metal ions present were investigated using electron paramagnetic resonance (EPR) and optical absorption spectra. The EPR results suggest that Fe3+ has replaced Al3+ in the unit cell of bauxite. The optical absorption spectrum is due to Fe3+ which indicates that it is in distorted octahedral environment. The near-infrared (NIR) spectrum is due to water fundamentals and combination overtones, which confirm the formula of the compound. The impurities in the mineral are identified using spectroscopic techniques.

SYNTHESIS AND SPECTROSCOPY OF SOME QUATERNARY OXYFLUORIDE GLASSES DOPPED WITH COPPER

Electron paramagnetic resonance (EPR) and optical absorption spectra of copper ions in xLiF-(50-x)Li2O-20SrO-30Bi2O3 glass system have been studied. MDSC studies showed that the glass transition temperature decreases with LiF content. Optical absorption spectra of the pure glasses reveled that the cut off wave length increased and optical band gap energy decreased with increase in LiF content. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu2+ ions. The Cu2+ ions are in well-defined axial sites but subjected to small distortion leading to the broadening of the spectra. The spin-Hamiltonian parameter values indicate that the ground state of Cu2+ is d x2 y2 and the site symmetry around Cu2+ ions is tetragonally distorted octahedral. The optical absorption spectra exhibited a broad band corresponding to the d-d transition bands of Cu2+ ion. By correlating EPR and optical absorption data, the bond parameters are evaluated from various techniques.

Spectral studies on Cu2+ ions in sodium–lead borophosphate glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cu 2+ ions in sodium–lead borophosphate glasses doped with different concentrations of Cu 2+ ions have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu 2+ ions. The values of spin-Hamiltonian parameters indicate that the Cu 2+ ions in sodium–lead borophosphate glasses are present in octahedral sites with tetragonal distortion. The optical absorption spectra of all the glass samples show a single broad band, which has been assigned to the 2 B 1g → 2 B 2g transition of Cu 2+ ions. The optical band gap energy ( E opt ) and Urbach energy (Δ E ) are calculated from their ultraviolet absorption edges. The emission bands observed in the ultraviolet and blue region are attributed to 3d 9 4s→3d 10 triplet transition in Cu + ion. The FT-IR spectra show that the glass system contains BO 3 , BO 4 and PO 4 structural units.

Gamma rays induced defect centers in phosphate glass for radio-photoluminescence dosimeter

Abstract Gamma induced defect centers of undoped phosphate glasses and the influence of these centers on the sensitivity and build-up time of silver-doped phosphate glasses were investigated. According to the ratio of oxygen and phosphorus ions (O/P) in phosphate glass, the basic glasses were polyphosphate glass (O/P = 3.07), metaphosphate glass (O/P = 3) and ultraphosphate glass (O/P = 2.93). Concentration of Ag + was 2 mol%. Defect centers induced by gamma rays of three base glasses were detected by Electron paramagnetic resonance (EPR) and optical absorption spectra. Three kinds of defect centers were observed and the characteristics of the hyperfine splitting doublets were quite similar in these glass samples. All the defect centers are phosphorus and oxygen related radiation-induced hole trapped defect centers (POHC) and electron centers PO 3 2 − and PO 4 4− . Based on EPR and optical absorption spectra, polyphosphate glasses have the highest relative POHC concentration. The fluorescence spectra and EPR spectra of silver-doped glasses were measured. The results indicate that polyphosphate glasses have the highest sensitivity and shortest build-up time of three silver-doped phosphate glasses which are related to the concentration of POHC.

All-Electron Scalar Relativistic Basis Sets for Third-Row Transition Metal Atoms.

A family of segmented all-electron relativistically contracted (SARC) basis sets for the elements Hf-Hg is constructed for use in conjunction with the Douglas-Kroll-Hess (DKH) and zeroth-order regular approximation (ZORA) scalar relativistic Hamiltonians. The SARC basis sets are loosely contracted and thus offer computational advantages compared to generally contracted relativistic basis sets, while their sufficiently small size allows them to be used in place of effective core potentials (ECPs) for routine studies of molecules. Practical assessments of the SARC basis sets in DFT calculations of atomic (ionization energies) as well as molecular properties (geometries and bond dissociation energies for MHn complexes) confirm that the basis sets yield accurate and reliable results, providing a balanced description of core and valence electron densities. CCSD(T) calculations on a series of gold diatomic compounds also demonstrate the applicability of the basis sets to correlated methods. The SARC basis sets will be of most utility in calculating molecular properties for which the core electrons cannot be neglected, such as studies of electron paramagnetic resonance, Mössbauer and X-ray absorption spectra, and topological analysis of electron densities.

Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu 2+ ions

We investigated the crystal growth, electron paramagnetic resonance (EPR) and optical absorption spectra of l-threonine doped with Cu 2+. The quality, size and habit of the single crystals grown from aqueous solution by the slow solvent evaporation and by the cooling methods vary when the impurities are introduced during the growth process. The variations with the magnetic field orientation of the EPR spectra of single-crystal samples at room temperature and 9.77 GHz in three crystal planes ( ab, bc and ac) show the presence of copper impurities in four symmetry-related sites of the unit cell. These spectra display well resolved hyperfine couplings of the S = 1 2 of Cu 2+ with the I Cu= 3 2 of the copper nuclei. Additional hyperfine splittings, well-resolved only for specific orientations of the magnetic field, indicate that the copper impurity ions in the interstitial sites have two N ligands with similar hyperfine couplings. The principal values of the g and A Cu tensors calculated from the EPR data are g 1=2.051(1), g 2=2.062(2), g 3=2.260(2), A Cu,1=16.9(5)×10 −4 cm −1, A Cu,2=21.8(6)×10 −4 cm −1 , A Cu,3=180.0(5)×10 −4 cm −1. The principal directions corresponding to g 3 and to A Cu,3 are coincident within the experimental errors, reflecting the orientation of the bonding planes of the copper ions in the crystal. The values of the crystal field energies are evaluated from the optical absorption spectrum, and the crystal field and bonding parameters of the Cu impurities in the crystal are calculated and analyzed. The EPR and optical absorption results are discussed in terms of the crystal structure of l-threonine and the electronic structure of the Cu 2+ ions, and compared with data reported for other systems. The effects of the impurities in the growth and habit of the crystals are also discussed.

Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing CuO – An EPR and optical study

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5mol% CuO doped xLi2O–(30−x)Na2O–69.5B2O3 (5⩽x⩽25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Cu2+ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of Cu2+ ions participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of paramagnetic susceptibility (χ) obeys Curie–Weiss law. The paramagnetic Curie temperature (θp) is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition 2B1g→2B2g in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients α2 and β12 have been evaluated. It is interesting to observe that the optical band gap (Eopt) and Urbach energies (ΔE) exhibit the mixed alkali effect. The theoretical values of optical basicity (Λth) have also been evaluated.

Growth and EPR and optical properties of Li 2B 4O 7 single crystals doped with Co 2+ ions

Li 2B 4O 7 single crystals doped with Co ions were grown by the Czochralski method. The electron paramagnetic resonance (EPR) and optical absorption spectra of Li 2B 4O 7 (LBO) single crystals doped with Co 2+ ions were measured. The EPR spectra could be described by the spin-Hamiltonian with an effective spin of S = 1 2 in octahedral symmetry. The values of g and A tensors and direction cosines characterizing Co 2+ centers in the LBO crystal were determined for the first time also. The EPR data reveals at least two types of Co 2+ centers. Optical absorption was measured in the range of 200 to 3200 nm for “as-grown” and γ-irradiated samples of Co:Li 2B 4O 7 crystals confirming octahedral coordination of Co 2+ ions and cation vacancies arising in the “as-grown” crystal.

EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cr3+ ions in Calcium alumino borate (CaAB) glasses have been studied. The EPR spectra exhibit weak resonance signal at g ≈ 4.50 and intense resonance signal at g ≈ 1.98. A sharp resonance signal at g ≈ 1.97 was also observed at lower concentrations of chromium. The concentration dependence of the linewidth of the resonance signal at g ≈ 1.98 suggests the formation of Cr3+ ion clusters by magnetic superexchange interactions. The temperature dependence of the peak to peak intensity and the linewidth of the resonance signal at g ≈ 1.98 suggests that the exchange interactions between Cr3+ ions in the present sample were antiferromagnetic in nature with Neel temperature, TN = 233 K. From the number of spins participating in the resonance at g ≈ 1.98, the paramagnetic susceptibility (χ) was calculated at different temperatures (233–295 K). A plot of 1/χ and T was found to obey Curie-Weiss law with negative Curie temperature. By measuring the relative intensities of the resonance signal at g ≈ 1.98, at different temperatures, the value of antiferromagnetic coupling constant (J) has been estimated. The optical absorption spectrum of chromium doped CaAB glass exhibits four bands, characteristic of Cr3+ ions, in nearly octahedral symmetry. From the band positions, the crystal field splitting parameter, Dq and the Racah interelectronic repulsion parameters, B and C were evaluated. The optical band gap (Eopt) and the Urbach energy (ΔE) were calculated from the ultraviolet absorption edges.

EPR AND OPTICAL STUDIES OF VANADYL IONS IN ALKALI LEAD BOROTELLURITE GLASSES

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of VO 2+ ions in different alkali lead borotellurite glasses have been studied. The spin-Hamiltonian parameters (g and A), bonding parameter [Formula: see text] and Fermi contact interaction parameter k have been calculated. The values of spin-Hamiltonian parameters confirm that the vanadyl ions are present in the glasses as VO 2+ molecular ions in an octahedral site with a tetragonal compression. The number of spins (N) participating in resonance is calculated as a function of temperature (123–393 K) for 9 mol% of VO 2+ ions in lithium lead borotellurite glass sample. It is observed that N obeys the Boltzmann law. From EPR data, the paramagnetic susceptibility (χ) is calculated at various temperatures and the Curie constant has been evaluated from 1/χ–T graph. The optical absorption spectrum exhibits two bands characteristic of VO 2+ ions in tetragonal symmetry. The band gap (E opt ) and the Urbach energies (ΔE) have been determined from the ultraviolet absorption edges and are found to be dependant on the size of the alkali ion. The theoretical values of optical basicity (Λ th ) of these glasses have also been evaluated.

EPR and optical absorption studies of Cu 2+ ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu 2+ ions in alkaline earth alumino borate glasses doped with different concentrations of CuO have been studied. The EPR spectra of all the glasses exhibit the resonance signals, characteristic of Cu 2+ ions present in axially elongated octahedral sites. The number of spins participating in the resonance has been calculated as a function of temperature for calcium alumino borate (CaAB) glass doped with 0.1 mol% of CuO. From the EPR data, the paramagnetic susceptibility ( χ) was calculated at different temperatures ( T) and from the 1/ χ– T graph, the Curie temperature of the glass has been evaluated. The optical absorption spectra of all the glasses show a single broad band, which has been assigned to the 2B 1g → 2B 2g transition of the Cu 2+ ions. The variation in the intensity of optical absorption with the ionic radius of the alkaline earth ion has been explained based on the Coulombic forces. By correlating the EPR and optical absorption spectral data, the nature of the in-plane σ bonding between Cu 2+ ion and the ligands is estimated. From the fundamental ultraviolet absorption edges of the glasses, the optical energy gap ( E opt) and the Urbach energy (Δ E) are evaluated. The variation in E opt and Δ E is explained based on the number of defect centers in the glass.

EPR and optical investigations of manganese ions in alkali lead tetraborate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Mn 2+ ions in different alkali lead tetraborate glasses 90R 2B 4O 7 + 9.25PbO + 0.75MnSO 4 ( R = Li, Na and K) and 90Li 2B 4O 7 + (10 − x)PbO + xMnSO 4 ( x = 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5 and 2 mol%) have been studied. The EPR spectrum of all the glass samples exhibit three resonance signals at g = 2.0, 3.3 and 4.3. The resonance signal at g = 2.0 is attributed to the Mn 2+ ions in an environment close to an octahedral symmetry. The resonance signals at g = 3.3 and 4.3 have been attributed to the rhombic symmetry of the Mn 2+ ions. The effect of temperature (123–433 K) and the composition dependence of EPR signals have been studied for Mn 2+ ions in lithium lead tetraborate glasses. It is interesting to observe that the variation of paramagnetic susceptibility ( χ) with temperature obeys Curie–Weiss law. From the slope of 1/ χ versus T graph, the Curie constant ( C) has been evaluated. The zero-field splitting (zfs) parameter D has been calculated for different alkali lead tetraborate glasses from the intensities of the allowed hyperfine lines. The optical absorption spectrum exhibits three bands. An intense and broad band at lower energy side has been assigned to the spin-allowed ( 5E g → 5T 2g) transition of Mn 3+ ions in an octahedral symmetry. The intense and sharp band and a broad band at higher energy side have been assigned to charge transfer bands. A red shift is observed with increase of alkali ion size. The optical band gap energy ( E opt) decreases, whereas the Urbach energies (Δ E) increases with increase of Mn content. The theoretical values of optical basicity ( Λ th) of the glasses have also been evaluated.

A model of the capture volume of free carriers in fluorophosphate glasses doped with terbium

The induced electron paramagnetic resonance (EPR) and optical absorption spectra of Tb2O3-doped glasses in the LiF-MgF2-Ba(PO3)2 system are investigated. The intensities of the doublet signals from PO 4 2− and PO 3 2− centers in regions with different contents of the fluoride component are determined as functions of the terbium concentration. It is shown that fluorophosphate glasses can be characterized by two values of the capture volume parameter, which differ by a factor of 5.0–6.6 depending on the dopant concentration. In the range of minimum dopant concentrations at which the capture volume is maximum, the dopant ions have a hypothetically phosphate environment. It is suggested that the capture volume parameter defined by the expression lnc/c 0 = −VC 3 can be treated as a characteristic of the distribution of dopant ions in fluorophosphate glasses.

VO 2+ ions in zinc lead borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of vanadyl ions in zinc lead borate (ZnO–PbO–B 2O 3) glass system have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of VO 2+ ions. The values of spin-Hamiltonian parameters indicate that the VO 2+ ions in zinc lead borate glasses were present in octahedral sites with tetragonal compression and belong to C 4V symmetry. The spin-Hamiltonian parameters g and A are found to be independent of V 2O 5 content and temperature but changing with ZnO content. The decrease in Δ g ∥/Δ g ⊥ value with increase in ZnO content indicates that the symmetry around VO 2+ ions is more octahedral. The decrease in intensity of EPR signal above 10 mol% of V 2O 5 is attributed to a fall in the ratio of the number of V 4+ ions ( N 4) to the number of V 5+ ions ( N 5). The number of spins ( N) participating in resonance was calculated as a function of temperature for VO 2+ doped zinc lead borate glass sample and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility was calculated at various temperatures and the Curie constant was evaluated from the 1/ χ– T graph. The optical absorption spectra show single absorption band due to VO 2+ ions in tetragonally distorted octahedral sites.

High-dose neutron irradiation of MgAl 2O 4 spinel: effects of post-irradiation thermal annealing on EPR and optical absorption

Electron paramagnetic resonance (EPR) and optical absorption spectra were measured during thermal annealing of stoichiometric MgAl 2O 4 spinel that was previously irradiated in the Materials Open Test Assembly in the Fast Flux Test Facility (FFTF/MOTA) at ≈680 K to ≈50 dpa. Both F and F + centres are to persist up to very high temperatures (over 1000 K) suggesting the operation of an annealing mechanism controlled by the thermal stability of extended defects. Using X-ray irradiation following the different annealing steps it was shown that an optical absorption band at 37 000 cm −1 is related to a sharp EPR band at g = 2.0005 and that the defect causing these effects is the F + centre.

EPR and optical absorption spectral studies of Cu 2+ ions doped in alkali lead tetraborate glasses

Electron paramagnetic resonance and optical absorption spectra of Cu 2+ ions in 90R 2B 4O 7+(10− x) PbO + xCuO (where R=Li, Na and K; 0.25 ⩽ x ⩽ 1.5 mol% of CuO) glasses have been studied. EPR spectra of all glasses show resonance peaks characteristic of Cu 2+ ions. From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated. The values of spin-Hamiltonian parameters indicate that Cu 2+ ions in alkali lead tetraborate glasses have octahedral coordination with a strong tetragonal distortion. The number of spins participating in resonance is measured as a function of temperature and the activation energy is calculated. The paramagnetic susceptibility is also calculated from the EPR data at various temperatures. From the plot of temperature versus 1/ χ, the Curie constant and the Curie temperature values have been evaluated. The optical absorption spectra of all the glass samples show two bands. One broad band at low-energy side is assigned to 2B 1g→ 2B 2g transition of Cu 2+ ion and the intense band at high-energy side is assigned to charge transfer band. By correlating EPR and optical absorption data, the molecular orbital coefficients have been evaluated.