Electron Spin Resonance Investigation Research Articles

Surface acidity and basicity of Mg/Al hydrotalcite for 2, 4-dichlorophenoxyacetic acid degradation with ozone: Mineralization, mechanism, and implications to practical water treatment

The Mg/Al hydrotalcite (Mg/Al HT) was firstly used as a heterogeneous ozonation catalyst and 2,4-dichlorophenoxyacetic acid (2,4-D) was efficiently degraded by Mg3/Al HT with a COD removal of 68 %. It was higher than that of α-FeOOH with a COD removal of 50 %. The effects of Mg/Al atomic ratio, phosphate and pyrrole on the ozonation performance of Mg/Al HTs were also investigated. The X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption experiment and temperature programmed desorption of adsorbed CO2 or NH3 were used to characterize the surface properties of Mg/Al HT. The surface acidity and basity was proven to be responsible to the excellent ozonation activity of Mg/Al HT. The results of electron spin resonance (ESR) analysis and probe experiments confirmed that OH, O2− and 1O2 were involved in the 2,4-D degradation process and their contributions are as followed: OH > O2- >1O2. The synergistic effect of surface acid (ozone adsorption center) and base sites (catalytic center) determines Mg/Al HT in the enhanced catalytic ozone decomposition into reactive species. More important, the transition metal free based Mg/Al HTs is steady, non-toxic, naturally abundant and environment friendly, which provided a promising alternative in practical water treatment by catalytic ozonation.

On the incorporation of iron into hexagonal barium titanate: II. Magnetic moment, electron paramagnetic resonance (EPR) and optical transmission

Systematic measurements of the magnetic moment in dependence on temperature and magnetic field of hexagonal 6H-BaTiO3 + 0.04 BaO + x/2 Fe2O3 (0.005 ⩽ x ⩽ 0.05) ceramics were performed to study the influence of Fe ions on the magnetic properties. While the samples show Curie–Weiss paramagnetism for Fe concentrations ⩽1.0 mol%, antiferromagnetic interactions become manifest for 2.0 and 5.0 mol% iron. With increasing Fe content the antiferromagnetic interaction, which is assumed to be caused by a superexchange mechanism , becomes stronger. At external magnetic fields smaller than 1 T a further, ferromagnetic interaction between Fe3+ ions is detected below 200 K. The interactions between Fe3+ ions in the samples with 2.0 and 5.0 mol% iron are also manifest in the EPR spectra by numerous lines with low intensity. Q-band EPR investigations of 5.0 mol% Fe doped single crystals confirm the existence of only one type of Fe3+–VO associates in the samples.

State of Charge Effects on Vanadium Crossover in Vanadium Redox Flow Batteries

Large-scale energy storage technologies play a pivotal role in the global clean energy transition, enabling intermittent renewable energy sources such as solar and wind to serve as feasible replacements for fossil fuels. The vanadium redox flow battery (VRFB) is a promising candidate for renewable energy storage applications due to its high energy efficiency, low toxicity, and long lifespan. [1] The half-cells of the battery are separated by a membrane through which ions migrate in order to maintain charge balance. Nafion, a perfluorinated polymer with sulfuric acid functional groups that facilitate proton transport, is the most widely used cation-exchange membrane due to its high proton conductivity and chemical and thermal stability. However, the low ion selectivity of Nafion permits positively charged vanadium ions to also cross through the membrane into the opposite electrolyte resulting in self-discharge of the battery, reducing efficiency.The characterization of vanadium crossover should consider the correlation between electrolyte composition and membrane properties. Vanadium concentration, sulfuric acid concentration, and state of charge are the three main factors that determine electrolyte composition. Electrolyte solutions with higher H2SO4 concentration corresponded to lower VO2+ membrane permeability.[2] A higher initial VO2+ concentration also corresponded to lower VO2+ membrane permeability. The low permeability at high H2SO4 and VO2+ concentrations could only be partially attributed to increased viscosity; other contributing factors may include dehydration of the membrane from interactions with sulfuric acid.In previous crossover-diffusion experiments,[3] the permeability of VO2+ with respect to the counter ion followed the trend H+ > VO2 + > VO2+, while the permeability of V3+ with respect to the counter ion was found to be VO2 + > VO2+. Uptake of vanadium species in the membrane followed the order V3+ ≈ VO2+> VO2 + at all concentrations of H2SO4 (0.5 M to 5 M), suggesting that permeability behavior of each vanadium species depends on the presence or absence of competitive partitioning resulting from particular vanadium counter ions. For all vanadium species, increased acid concentration decreased both the permeability and uptake of vanadium ions, likely due to the dehydration of the membrane in more acidic conditions (Lawton, JES, 2017).The state of charge (SOC) of the electrolyte, expressed as a percentage from 0% to 100%, describes the extent of oxidation that has occurred in each electrolyte. In the catholyte, for example, a 0% SOC solution consists of entirely VO2+, while a 100% SOC solution is entirely VO2 +. The characterization of mass transfer in the operating VRFB has yet to fully investigate the role of SOC on electrolyte properties. Here, we study the relationship between SOC and physical constants of the catholyte using electron paramagnetic resonance spectroscopy.[4,5][6] Our work seeks to elucidate some of the questions that remain in characterizing Nafion membrane permeability in the VRFB catholyte, in particular: how vanadium ion permeability varies with SOC; how the presence of VO2+ and VO2 + together in the catholyte influences each ion’s permeability; and how H2SO4 concentration affects all of these factors. 1. Skyllas-Kazacos, M.; Rychcik, M.; Robins, R. G.; Fane, A. G. New All-Vanadium Redox Flow Cell. J. Electrochem. Soc. 1986, 133, 1057–1058, doi:10.1149/1.2108706.2. Lawton, J. S.; Jones, A.; Zawodzinski, T. Concentration dependence of VO2+ crossover of nafion for vanadium redox flow batteries. J. Electrochem. Soc. 2013, 160, 697–702, doi:10.1149/2.004306jes.3. Lawton, J. S.; Jones, A. M.; Tang, Z.; Lindsey, M.; Zawodzinski, T. Ion effects on vanadium transport in Nafion membranes for vanadium redox flow batteries. J. Electrochem. Soc. 2017, 164, A2987–A2991.4. Lawton, J. S.; Smotkin, E. S.; Budil, D. E. Electron spin resonance investigation of microscopic viscosity, ordering, and polarity in nafion membranes containing methanol-water mixtures. J. Phys. Chem. B 2008, 112, 8549–8557, doi:10.1021/jp800222c.5. Lawton, J. S.; Budil, D. E. Investigation of water and methanol sorption in monovalent- and multivalent-ion-exchanged nafion membranes using electron spin resonance. J. Phys. Chem. B 2009, 113, 10679–10685, doi:10.1021/jp902750j.6. Lawton, J. S.; Budil, D. E. Spin Probe ESR Study of Cation Effects on Methanol and DMMP Solvation in Sulfonated Poly (styrene− isobutylene− styrene) Triblock Copolymers at High Ion-Exchange Capacities. Macromolecules 2009, 43, 652–661.

UV emitting Gd incorporated LiBaB9O15 phosphors: An ESR and photoluminescence investigation

An effort was made to get an idea regarding the correlation between structure and radiative properties of Gd incorporated LiBaB9O15 matrix. The samples were synthesized via a combustion route and characterized using X-ray diffraction (XRD). Photoluminescence (PL) experiments were carried out in order to understand the radiative properties and local site symmetry of ‘Gd’ ions in the sample. The site occupancy of the rare earth ions were also evaluated through electron spin resonance (ESR) measurements. The PL data suggested the emanation of UV light from the system with emission maxima at 307 and 311 nm. Both these peaks belong to the UV-B region which has many useful applications in phototherapy. The nature of the peaks suggested the stabilization of the ion predominantly in asymmetric geometry making the electric dipole transitions more prominent. It was observed that the maximum radiative or luminescence intensity could be obtained for the 0.048 mol ‘Gd’ doped sample. Beyond this, concentration quenching reduced the PL intensity. Based on the energy transfer model it was evaluated that ‘Foster’ type mechanism was responsible for the observed concentration quenching. From the ESR data it could be inferred that the rare earth ion stabilises at two different sites with differing geometries. At lower doping levels, it was seen that the Gd3+ ions predominantly existed at relatively more distorted Ba2+ sites and at higher concentrations, the ions are at the distorted sites as well as at Ba2+ sites with less distorted environment.

Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses

Gadolinium (Gd3+)-doped magnesium-lead-borophosphate glasses composed of MgHPO4–PbO–B2O3–Gd2O3 (PPbBMgGd) were fabricated by usual melt-quenching approach. The physical properties such as refractive index (n), density (ρ) and molar volume (Vm) were attained. Thermal analysis of the glass has been performed through differential scanning calorimetry (DSC). Raman, Fourier transform infrared (FTIR), Electron Paramagnetic Resonance (EPR) and steady-state luminescence of PPbBMgGd glasses were investigated. The Raman and FTIR spectra explore the vibrational groups of both phosphate and borate networks in the PPbBMgGd glasses. The EPR spectra display three characteristic signals at room temperature with definite g values of 6.2, 2.8 and 2.0. These characteristic signals were ascribed perhaps the Gd3+ ions that are located respectively at feeble, moderate and sturdy cubic symmetry sites. Moreover, the EPR spectra were also obtained at various low temperatures (123–297 K) for 5.0 mol% of Gd3+-doped PPbBMgGd5.0 glass. Difference in population among the Zeeman levels (N) and paramagnetic susceptibility (χ) were evaluated. Photoluminescence spectrum reveals a broad intense band of Gd3+ ions upon excitation at 278 nm, could be suitable for visible broadband applications.

Adjustable coupling and in situ variable frequency electron paramagnetic resonance probe with loop-gap resonators for spectroscopy up to X-band.

In standard electron paramagnetic resonance (EPR) spectroscopy, the frequency of an experiment is set and the spectrum is acquired using the magnetic field as the independent variable. There are cases in which it is desirable instead to fix the field and tune the frequency such as when studying avoided level crossings. We have designed and tested an adjustable frequency and variable coupling EPR probe with loop-gap resonators (LGRs) that works at a temperature as low as 1.8 K. The frequency is tuned by adjusting the height of a dielectric piece of sapphire inserted into the gap of an LGR; coupling of the microwave antenna is varied with the height of the antenna above the LGR. Both coupling antenna and dielectric are located within the cryogenic sample chamber, but their motion is controlled with external micrometers located outside the cryostat. The frequency of the LGR (∼4 GHz) can be adjusted by more than 1 GHz (>25%). To cover a wide range of frequencies, different LGRs can be designed to cover frequencies up to X-band. We demonstrate the operation of our probe by mapping out avoided crossings for the Ni4 molecular nanomagnet to determine the tunnel splittings with high precision.

Analysis of Transferred MoS2 Layers Grown by MOCVD: Evidence of Mo Vacancy Related Defect Formation

A low-temperature multi-frequency electron spin resonance (ESR) study has been carried out on 1, 3.5, and 6 layer thick MoS2 films, grown by metal organic vapor deposition (MOCVD) and subsequently transferred on SiO2/Si. This reveals the observation of a previously unreported, nearly isotropic signal at g ≈ 1.9998 with corresponding spin center (spin S = ½) densities ranging from ∼6 × 108 cm−2 to ∼5 × 1011 cm−2. The ESR investigation is closely combined with an in-depth analysis by an assortment of other experimental techniques, including atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), to ultimately result in the assignment of the ESR signal to a defect of intrinsic nature, most likely a Mo vacancy (VMo) related defect located at MoS2 grain edges or boundaries. The oxidation of the 2D material at grain edges and boundaries combined with the applied water-based transfer procedure is demonstrated to play a crucial role in the generation of the newly observed defect, thus advising caution with the currently applied process method. The presented analysis, which combines a variety of experimental techniques, contributes to the fine-tuning of the CVD growth and transfer process of high-quality few-layer MoS2 intended for next-generation nanoelectronic devices.

Synthesis and electronic structure of a mononuclear copper(II) complex supported by tris(2-hydroxyliminopropyl)amine

The blue-green, mononuclear copper complex [CuL(NO3)]NO3, (1), was synthesized by reacting copper(II) nitrate with L = tris(2-hydroxyliminopropyl)amine in methanol. The structure of 1 was determined by single-crystal X-ray diffraction, which revealed a copper(II) ion that exhibits a pseudo-octahedral coordination geometry. While one of the nitrate anions is coordinated in an η2 fashion, the other is non-coordinated and is involved in intra- and inter-molecular hydrogen bonds. The nitrate-promoted intermolecular hydrogen bonding is supported by the protonated ligand and leads to the formation of supramolecular undulating chains. The electronic structure of 1 was investigated using Electron Paramagnetic Resonance (EPR) measurements performed at cryogenic temperatures on both neat powders and frozen solutions. In the solid state we observe a quasi-axial S = 1/2 signal described by gz = 2.256, gy = 2.086, and gx = 2.048. These values were rationalized using coupled-perturbed (CP) Density Functional Theory (DFT) and CASSCF/NEVPT2 calculations. The computational investigation of 1 suggests an (x2 − y2)-type ground state and a g -tensor dominated by the (xy) → (x2 − y2) transition. EPR investigation of frozen solution samples revealed the presence of two similar spectral components indicating that upon dissolution the nitrate anion is displaced from the coordination sphere and 1 speciates into six- and five-coordinated complex-cations. The cyclic voltammograms of 1 revealed a quasi-reversible Cu(I)/Cu(II) redox couple, E1/2 = +0.26 V vs. SHE, and that 1 functions as a precursor for electrodeposited Cu(0)/Cu2O films.

Correction to Electron Paramagnetic Resonance Investigation of the Structure of Graphene Oxide: pH-Dependence of the Spectroscopic Response

Correction to Electron Paramagnetic Resonance Investigation of the Structure of Graphene Oxide: pH-Dependence of the Spectroscopic Response

Radical Formation by Fine Particulate Matter Associated with Highly Oxygenated Molecules.

Highly oxygenated molecules (HOMs) play an important role in the formation and evolution of secondary organic aerosols (SOA). However, the abundance of HOMs in different environments and their relation to the oxidative potential of fine particulate matter (PM) are largely unknown. Here, we investigated the relative HOM abundance and radical yield of laboratory-generated SOA and fine PM in ambient air ranging from remote forest areas to highly polluted megacities. By electron paramagnetic resonance and mass spectrometric investigations, we found that the relative abundance of HOMs, especially the dimeric and low-volatility types, in ambient fine PM was positively correlated with the formation of radicals in aqueous PM extracts. SOA from photooxidation of isoprene, ozonolysis of α- and β-pinene, and fine PM from tropical (central Amazon) and boreal (Hyytiälä, Finland) forests exhibited a higher HOM abundance and radical yield than SOA from photooxidation of naphthalene and fine PM from urban sites (Beijing, Guangzhou, Mainz, Shanghai, and Xi'an), confirming that HOMs are important constituents of biogenic SOA to generate radicals. Our study provides new insights into the chemical relationship of HOM abundance, composition, and sources with the yield of radicals by laboratory and ambient aerosols, enabling better quantification of the component-specific contribution of source- or site-specific fine PM to its climate and health effects.

Nonresonant Transmission Line Probe for Sensitive Interferometric Electron Spin Resonance Detection.

Electron spin resonance (ESR) spectroscopy measures paramagnetic free radicals, or electron spins, in a variety of biological, chemical, and physical systems. Detection of diverse paramagnetic species is important in applications ranging from quantum computation to biomedical research. Countless efforts have been made to improve the sensitivity of ESR detection. However, the improvement comes at the cost of experimental accessibility. Thus, most ESR spectrometers are limited to specific sample geometries and compositions. Here, we present a nonresonant transmission line ESR probe (microstrip geometry) that effectively couples high frequency microwave magnetic field into a wide range of sample geometries and compositions. The nonresonant transmission line probe maintains detection sensitivity while increasing availability to a wider range of applications. The high frequency magnetic field homogeneity is greatly increased by positioning the sample between the microstrip signal line and the ground plane. Sample interfacing occurs via a universal sample holder which is compatible with both solid and liquid samples. The unavoidable loss in sensitivity due to the nonresonant nature of the transmission line probe (low Q) is recuperated by using a highly sensitive microwave interferometer-based detection circuit. The combination of our sensitive interferometer and nonresonant transmission line provides similar sensitivity to a commercially available ESR spectrometer equipped with a high-Q resonator. The nonresonant probe allows for transmission, reflection, or dual-mode detection (transmission and reflection), where the dual-mode results in a √2 signal enhancement.

Eu3+ ion distribution in oxyfluoride glass nanocomposites

Assessment of activator distribution in lanthanide doped nanocomposites is an important and challenging task. Oxyfluoride glass ceramics have been chosen as a model system to characterize incorporation efficiency of Eu3+ ions in CaF2 nanocrystals using a combination of X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques. Lattice constant changes induced by size mismatch of Eu3+ and Ca2+ ions have been used to evaluate Eu3+ content in CaF2. The distortion of CaF2 lattice due to incorporation of europium ions can be detected from EPR investigations via Gd3+ paramagnetic probe spectra. It is shown that ratio of different symmetry CaF2:Gd3+ centres and linewidth of EPR transitions can be used to monitor europium content in the crystalline phase of glass ceramics.

Metal Bonding with 3d and 6d Orbitals: An EPR and ENDOR Spectroscopic Investigation of Ti3+-Al and Th3+-Al Heterobimetallic Complexes.

Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cptt2ThH3AlCTMS3 [TMS = Si(CH3)3; Cptt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp2TiH3AlCTMS3 (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, 1H and 27Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (aiso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th3+ complex is ∼5-fold larger than that in the titanium(3+) (Ti3+) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.

A matrix isolation ESR investigation of Mg+-N2.

The adducts formed between 25Mg+ with 14N2 and 25Mg+ with 15N2 have been trapped in a solid neon matrix and studied with electron spin resonance (ESR) spectroscopy. These radical species were formed through the interaction of laser ablated magnesium and nitrogen gas. The Mg+-N2 radical species was found to have a ground electronic state of 2Σ+ in a linear configuration with discrete coupling to the proximate nitrogen resolved in the spectra. Fitting the ESR spectra allowed magnetic parameters to be determined as follows: g⊥ = 2.0012(5), g∥ = 2.0015(8), A⊥(1-14N) = 32(3) MHz, A∥(1-14N) = 34(5) MHz, A⊥(1-15N) = 45(4) MHz, A∥(1-15N) = 47(6) MHz, A⊥(25Mg) = -581(5) MHz, and A∥(25Mg) = -582(5) MHz, and estimates derived for A⊥(2-14N) = 1(2) MHz, A∥(2-14N) = 2(5) MHz, A⊥(2-15N) = 2(2) MHz, and A∥(2-15N) = 4(6) MHz. Ab initio calculations using the coupled-cluster single double triple methodology showed that the linear form was 59.7 kcal mol-1 more stable than the T-shaped form. The potential energy curve around the equilibrium geometry was explored using the complete active space self-consistent field approach, and Hartree-Fock singles and double configuration interaction and multireference singles and double configuration interaction calculations of the hyperfine coupling constants were undertaken, and reasonable agreement with the experiment was observed.

Enhanced Photoreduction of U(VI) on C3N4 by Cr(VI) and Bisphenol A: ESR, XPS, and EXAFS Investigation.

The effect of Cr(VI) and bisphenol A (BPA) on U(VI) photoreduction by C3N4 photocatalyst was demonstrated by the batch experiments, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) techniques. The batch experiments manifested that Cr(VI) and BPA enhanced the photocatalytic activity of C3N4 for U(VI) photoreduction, whereas U(VI) photoreduction was significantly diminished with increased pH from 4.0 to 8.0. According to radical scavengers and ESR analysis, U(VI) was photoreduced to U(IV) by photogenerated electrons of conduction band edge, whereas Cr(VI) was reduced to Cr(III) by H2O2. BPA and its products such as organic acid and alcohols can capture photoinduced holes, which resulted in the enhancement of U(VI) photoreduction to U(IV). XPS and XANES analyses demonstrated that U(VI) was gradually photoreduced to U(IV) by C3N4 within irradiation 60 min, whereas U(IV) was reoxidized to U(VI) with increasing irradiation time. EXAFS analysis determined that the dominant interaction mechanisms of U(VI) on C3N4 after irradiation for 240 min were reductive precipitation and inner-sphere surface complexation. This work highlights the synergistic removal of radionuclides, heavy metals, and persistent organic pollutants by C3N4, which is crucial for the design and application of a high-performance photocatalyst in actual environmental cleanup.

Localized π-conjugated structure and EPR investigation of g-C3N4 photocatalyst

The π-conjugated structure of graphitic carbon nitride (g-C3N4) is particularly vital to many photocatalytic reactions. Herein, the hybrid structure of tri-s-triazine unit in g-C3N4 framework is chemically analyzed and expounded according to the hybrid orbital theory. The localized π-conjugated structure of g-C3N4 is also monitored by the electron paramagnetic resonance (EPR) or electron spin resonance (ESR) technique. The experimental results indicate that this π-conjugated structure is attributed to the orbital overlapping of the hybrid carbon and nitride atoms in their 2pz orbits. Unlike graphene with the nonlocalized π-conjugated structure, this orbital overlapping in the whole two-dimensional plane of g-C3N4 is separated by the electrons pairs in 2pz orbits of the bridging nitride atoms, leading to the localized π-conjugated structure. Therefore, the g-C3N4 exhibits the typical features of a semiconductor with band gap and visible-light response. Meanwhile, the EPR or ESR technique can be acted as the ideal tool to indirectly evaluate the yield of photoelectrons by detecting the superoxide radicals (O2−) in g-C3N4-based photocatalytic reactions.

Injectable Oxygen Sensitive Chitosan Complex with High Oxygen Sensitivity and Stability to Oxidoreductants.

Various therapeutic approaches have been developed for vascularization of diseased tissues. To quickly and accurately evaluate efficacy of a therapy, reproducible monitoring of tissue oxygen concentration changes at the same tissue location by a minimally invasive or noninvasive spectroscopic approach represents a compelling strategy. Yet this cannot be achieved by any clinically available approaches. Magnetic-resonance-based method, electron paramagnetic resonance (EPR), has the potential to accomplish the goal. However, the existing small molecule EPR probes cannot maintain a consistent concentration at the same tissue location during vascularization period to acquire a stable EPR signal, and they cannot be implanted and/or retrieved by a minimally invasive approach such as injection. Herein, a polymeric, injectable, and degradable EPR probe was developed. The probe was based on the complex of chitosan and tetrathiatriarylmethyl (TAM) radical. The probe had high oxygen sensitivity and reproducibility. It was more stable than free TAM radical when exposed to oxidoreductants that commonly appear in diseased tissues. The probe maintained EPR signal intensity for 4 weeks in vitro and in vivo without changing oxygen sensitivity. It also exhibited excellent biocompatibility. The developed TAM/chitosan complex may be used for long-term detection of tissue oxygen content during therapies.

Electronic active defects and local order in doped ZnO ceramics inferred from EPR and 27Al NMR investigations

Doped ZnO based ceramics were fabricated by using a solid state reaction of ZnO co-doped by TiO2, Al2O3 and MgO and sintered in different atmospheres (Air, N2, N2 + CO). The crystalline structures consist in wurtzite ZnO and a minor spinel phase Zn2TiO4. The electrical conductivity is modulated by the sintering conditions with the highest value (˜105 S m−1) obtained in the reducing atmosphere (N2 + CO). The role of defects and vacancies on the electrical behavior was exhaustively investigated by Raman, electron paramagnetic resonance (EPR) and solid state NMR methods. The paramagnetic centres inferred from EPR studies show a Pauli-like spin susceptibility. Their origin was assigned to shallow donors from interstitial defects (Zni) favored by substitutional Al ions (AlZn). The NMR spectral features with a characteristic 185 ppm line which correlates with the electrical conductivity are presumed to be caused by the Knight shift effect from the conduction electrons and the involved paramagnetic centres.

An Electron Paramagnetic Resonance and Photoluminescence Investigation of UVB Radiation Emitting Gadolinium-Activated CaY2Al4SiO12 Garnet Compound

Trivalent gadolinium (Gd3+) activated CaY2Al4SiO12 garnet compounds with a variable concentration of Gd3+ ions were synthesized by a sol–gel method. The synthesized garnet compounds were characterized for their structural, morphological, luminescent, and magnetic properties. The x-ray diffraction data of Gd3+ activated CaY2Al4SiO12 garnet compound reveal the cubic structure with Ia3d space group. The scanning electron microscope images represent the morphology of the particles. The photoluminescence properties were analyzed from the excitation and emission spectra. The emission spectra of the CaY2Al4SiO12:Gd3+ garnet compound under 273 nm excitation exhibit two emission peaks at (308 nm and 314 nm) nm and are attributed to 6P5/2 → 8S7/2 and 6P7/2 → 8S7/2 transitions, respectively. The emission peak observed at 314 nm is more intense than the peak observed at 308 nm. The high-intensity peak at 314 nm is used to cure various skin disorders, and hence is beneficial in phototherapy. The intensity of the emission peaks increases up to 0.09 mol of Gd3+ ion concentration in the CaY2Al4SiO12 garnet compound, and beyond, then decreases, showing concentration quenching. The electron paramagnetic resonance of Gd3+ ions in the prepared garnet compound exhibits resonance signals with effective g values at g ≈ 6.5, 5.0, 2.6, 2.2, 2.0, and 1.6 of the U-spectrum, due to cubic, octahedral, or tetrahedral distorted sites.

Thermo-mechanical analysis of a probe for electron paramagnetic resonance spectroscopy operating at cryogenic temperatures.

In this article, we present the thermo-mechanical analysis of an electron paramagnetic resonance (EPR) probe operating at cryogenic temperatures using finite element analysis. Thermo-mechanical analysis plays a key role in the mechanical design evaluation process as EPR probes are often subjected to large stresses under such extreme conditions. For simplification, we assume thermal conduction to be the dominant mode of heat transfer over convection and radiation. The simulation model consists of a cryostat-probe assembly with appropriate thermal and structural boundary conditions. The predicted temperature distributions from the steady-state thermal analysis is then used for the stress analysis of the EPR probe. The stress analysis indicated that stresses in the EPR probe are below the ultimate strengths of each component, and thus safe for running EPR experiments. Furthermore, the simulation results were confirmed experimentally, and we found that the predicted heat losses for the EPR probe assembly and the sample holder are in excellent agreement with the experimental measurements.