Electron Paramagnetic Resonance Data Research Articles

Room‐temperature ferromagnetic organic magnets derived from fluoro‐graphite via facile halide exchange

AbstractRoom‐temperature ferromagnetic organic magnet is developed via a facile halide exchange process using fluoro‐graphite (FG) as a starting material. Structural and chemical analysis reveals that heterogeneous C–F bond cleavage in the defect sites of FG is essentially related to the formation of ferromagnetic hydroxyl‐graphene (HG). Pyrolysis of FG in a polar solvent with iodine ions induces a formation of metastable C–I bond and subsequent replacement of the I sites with the hydroxyl groups. Specifically, defective sites can be formed in FG due to nucleophile attack where the OH groups can be easily generated. As a result, the FG can be transformed into the HG with a network of sp2‐conjugated carbon motifs in a sp3‐based graphene matrix. Hence, the paramagnetic centers in the π‐electron system of FG can be transformed to the long‐range ordered ferromagnetic centers in the sp2‐conjugated system of HG. Electron paramagnetic resonance data demonstrate the weak ferromagnetic property of HG stable at room temperature.

Mapping the Pathway to Organocopper(II) Complexes Relevant to Atom Transfer Radical Polymerization.

The rare organocopper(II) complex [Cu(Me6tren)(CH2CN)]+ (Me6tren = tris(2-(dimethylamino)ethyl)amine) has emerged as an important model of potential byproducts in copper-catalyzed atom transfer radical polymerization. This complex has been generated by controlled potential electrolysis of [Cu(Me6tren)(NCMe)]2+ in the presence of BrCH2CN. Time-resolved UV-vis and continuous wave and pulse electron paramagnetic resonance (EPR) spectra identified [Cu(Me6tren)Br]+ as an intermediate. Hyperfine sublevel correlation and electron nuclear double resonance spectroscopy of samples at different timepoints reveal signals that are assigned to a C-bound cyanomethylate ligand, with distinct 14N and 1H hyperfine coupling constants in comparison with the corresponding N-bound acetonitrile and bromido complexes. The experimental EPR data are supported by density functional theory calculations to understand how the geometries of the species involved produce distinct spectroscopic signatures, and a clear picture of how this unusual organocopper(II) complex is formed has emerged.

Defect formation in photochromic Ca2SnO4: Al3+

Photoluminescence, photochromism and defect formation were analysed in Ca2SnO4 doped with 0–20% Al3+. Bright cyan luminescence attributed to Sn2+ was detected in all samples. Introduction of Al3+ in Ca2SnO4 led to an enhancement of Sn2+ emission by up to 70% in comparison with an undoped sample. After exposure to UV irradiation, two absorbance bands centered at 520 nm and 710 nm and three paramagnetic defects were observed in Al3+-doped Ca2SnO4 samples. The paramagnetic centers were identified as a hole type and two different Sn3+ centers. Electron paramagnetic resonance (EPR) data suggested that one of the Sn3+ centers is located in a close proximity of Al3+. Analysis of the stability of the absorbance bands and paramagnetic centers revealed that the hole type center is related to the 520 nm absorbance band, and one of the Sn3+ centers is associated with the 710 nm band. The effect of Al3+ on defect formation in Ca2SnO4 was discussed.

Addressing Serine Lability in a Paramagnetic Dimethyl Sulfoxide Reductase Catalytic Intermediate.

Two new desoxo molybdenum(V) complexes have been synthesized and characterized as models for the paramagnetic high-g split intermediate observed in the catalytic cycle of dimethyl sulfoxide reductase (DMSOR). Extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) data are used to provide new insight into the geometric and electronic structures of high-g split and other EPR-active type II/III DMSOR family enzyme forms. The results support a 6-coordinate [(PDT)2Mo(OH)(OSer)]- structure (PDT = pyranopterin dithiolene) for a high-g split with four S donors from two PDT ligands, a coordinated hydroxyl ligand, and a serinate O donor. This geometry orients the redox orbital toward the substrate access channel for the two-electron reduction of substrates.

Elucidating Electron Storage and Distribution within the Pentaheme Scaffold of Cytochrome c Nitrite Reductase (NrfA).

Cytochrome c nitrite reductases (CcNIR or NrfA) play important roles in the global nitrogen cycle by conserving the usable nitrogen in the soil. Here, the electron storage and distribution properties within the pentaheme scaffold of Geobacter lovleyi NrfA were investigated via electron paramagnetic resonance (EPR) spectroscopy coupled with chemical titration experiments. Initially, a chemical reduction method was established to sequentially add electrons to the fully oxidized protein, 1 equiv at a time. The step-by-step reduction of the hemes was then followed using ultraviolet-visible absorption and EPR spectroscopy. EPR spectral simulations were used to elucidate the sequence of heme reduction within the pentaheme scaffold of NrfA and identify the signals of all five hemes in the EPR spectra. Electrochemical experiments ascertain the reduction potentials for each heme, observed in a narrow range from +10 mV (heme 5) to -226 mV (heme 3) (vs the standard hydrogen electrode). On the basis of quantitative analysis and simulation of the EPR data, we demonstrate that hemes 4 and 5 are reduced first (before the active site heme 1) and serve the purpose of an electron storage unit within the protein. To probe the role of the central heme 3, an H108M NrfA variant was generated where the reduction potential of heme 3 is shifted positively (from -226 to +48 mV). The H108M mutation significantly impacts the distribution of electrons within the pentaheme scaffold and the reduction potentials of the hemes, reducing the catalytic activity of the enzyme to 1% compared to that of the wild type. We propose that this is due to heme 3's important role as an electron gateway in the wild-type enzyme.

EPR of Photoexcited Triplet-State Acceptor Porphyrins.

The photoexcited triplet states of porphyrin architectures are of significant interest in a wide range of fields including molecular wires, nonlinear optics, and molecular spintronics. Electron paramagnetic resonance (EPR) is a key spectroscopic tool in the characterization of these transient paramagnetic states singularly well suited to quantify spin delocalization. Previous work proposed a means of extracting the absolute signs of the zero-field splitting (ZFS) parameters, D and E, and triplet sublevel populations by transient continuous wave, hyperfine measurements, and magnetophotoselection. Here, we present challenges of this methodology for a series of meso-perfluoroalkyl-substituted zinc porphyrin monomers with orthorhombic symmetries, where interpretation of experimental data must proceed with caution and the validity of the assumptions used in the analysis must be scrutinized. The EPR data are discussed alongside quantum chemical calculations, employing both DFT and CASSCF methodologies. Despite some success of the latter in quantifying the magnitude of the ZFS interaction, the results clearly provide motivation to develop improved methods for ZFS calculations of highly delocalized organic triplet states.

Electron spin resonance (ESR) in petrophysical characterization of pure carbonatic rocks and containing oil

Petrophysical evaluation from laboratory and well logging data, such as gamma ray, Nuclear Magnetic Resonance (NMR) and resistivity has been used to identify and evaluate oil zones in reservoirs. Gamma ray log is able to predict lithologies along the well, and X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) techniques also provide important lithological laboratory information. The NMR log directly estimates the porosity of the rock. Resistivity log identifies the water region, but cannot provide information about oil saturation. However, the application of unconventional techniques, such as Electron Spin Resonance (ESR), can be of great relevance in the well logging, adding information about lithologies, oil quality and saturation. The ESR technique, which can directly estimate the oil saturation inside the rock, can also provide important information about the origin and formation of rocks. This technique could be an advance for the oil industry, helping to understand the geothermal processes suffered by the rocks and identifying important regions of the reservoir. Thus, in this work, we studied carbonate rock (limestone and dolomite) samples with and without oil, by ESR technique. Firstly, the effectiveness of ESR in the differentiation of carbonates (calcitic and dolomitic) without oil was tested, through the analysis of manganese ion impurity, which promotes the replacement of calcium and magnesium ions in calcites and dolomites. The samples had a typical manganese ESR signature. The hyperfine parameter, A, was homogeneous for both limestone and dolomites. The zero-field split parameter, D, showed variability, as reported in the literature. The studies were carried out at X-band frequency. The analysis by XRF and XRD provided information on the chemical and mineralogical composition of the carbonates, which were added to the ESR data for application in a correlation matrix, by which it was identified that the components CaO and MgO were directly related to the intensity of the ESR signal and the concentration of CaO was inversely related to the concentration of Al2O3 and SiO2. In one limestone rock sample was incorporated oil, with 25° API and moderate biodegradation level, to identify by ESR, the oil presence inside this rock. The technique was able to identify the oil and its saturation in the rock through the detection of free radical signal, g = 1.9951. The innovative application of the ESR technique to identify oil in varying saturation, in carbonate rock samples from the Illinois Basin (USA), was successful. These results demonstrated that this technique in addition to being accurate, non-destructive and relatively low cost, could be used in mud logging operations providing results for lithological characterization, oil identification and its saturation inside the reservoirs.

Tuning biological activity in dinuclear Cu (II) complexes derived from pyrazine ligands: Structure, magnetism, catecholase, antimicrobial, antibiofilm, and antibreast cancer activity

In a thrust to establish a structure–activity relationship of dinuclear Cu (II) complexes for derived biological activities, two complexes bearing composition [Cu2(pza)2(2‐methoxybenzoate)4] (1) and [Cu2(pza)2(3‐methoxybenzoate)4] (2) are designed. The complexes are characterized thoroughly via single crystal X‐ray diffraction (XRD), spectroscopic, and magnetic analysis. The exact structure of complexes was elucidated as dinuclear bridged by carboxylates with Cu (II) in each complex present in a distorted square‐pyramidal geometry. The ortho and meta substituted benzoates are used in order to investigate its effect on catecholase and other biological activities. The magnetic data on variable temperature dispense the presence of moderate antiferromagnetic interactions in both the complexes with J = −88.7(2) cm−1, R = 2.13 × 10−4 for 1 and J = −86.3(2) cm−1, R = 3.11 × 10−4 for 2 (g = 2.0). As corroborated from UV–visible titrations, cyclic voltammetry (CV), and Electron paramagnetic resonance (EPR) data, the 2 shows enhanced catecholase activity due to higher redox‐active Cu (II) center in 2, which is directly related to the inductive effect of the methoxy substituent. Both the complexes were screened against some bacteria and fungi, and 2 has shown potent activity Escherichia coli. Moreover, the complexes have also shown better antibiofilm properties as compared to their parent salts. The excellent antibreast cancer activity has been observed against MCF‐7 cell line and, 2has greater IC50 (lower activity) than 1. Thus, a structure–activity relationship can further direct us to design other substituted complexes with enhanced biological activity in the future course.

Combustion behaviors of various coals and chars: From covalent bonds’ and radicals’ perspective

The combustion behaviors of chars under reaction control are important for utilization, correlating work from bonds’ and radicals’ perspective are needed for further quantitative works. In this work, the effects of minerals, and mass and heat transfer were eliminated using deashed samples and temperature-programed combustion with low heating rate. The combustion of 15 coals and 45 chars prepared under different temperatures were studied. Using differential thermal analysis, nuclear magnetic resonance, electron spin resonance data coupled with model compounds, the combustion behaviors of coals and chars can be divided into three regions, with temperature lower than 325 °C, higher than 325 °C and lower than 430 °C, higher than 430 °C, and the eight covalent bonds with or without effect of radicals can be sorted into three steps corresponding to the three regions. The initial step rely highly on aliphatic carbon bonds and the last step are mainly determined by the aromatic bonds in poly-aromatic rings. Therefore, pyrolysis processes with less volatile-char interaction or sufficient bonds’ dissociation with less emission of volatiles are preferred to prepare chars as alternative feeds, and the three-region division can help to build generic kinetic networks on combustion of complex hydrocarbons.

Structures of Impurity Defects in Lithium Niobate and Tantalate Derived from Electron Paramagnetic and Electron Nuclear Double Resonance Data

Point intrinsic and extrinsic defects, especially paramagnetic ions of transition metals and rare-earth elements, have essential influence on properties of lithium niobate, LN and tantalate, LT, and often determine their suitability for numerous applications. Discussions about structures of the defects in LN/LT have lasted for decades. Many experimental methods facilitate progress in determining the structures of impurity centers. This paper gives current bird’s eye view on contributions of Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) studies to the determination of impurity defect structures in LN and LT crystals for a broad audience of researchers and students. Symmetry and charge compensation considerations restrict a number of possible structures. Comparison of measured angular dependences of ENDOR frequencies with calculated ones for Li and Nb substitution using dipole–dipole approximation allows unambiguously to determine the exact location of paramagnetic impurities. Models with two lithium vacancies explain angular dependencies of EPR spectra for Me3+ ions substituting for Li+ like Cr, Er, Fe, Gd, Nd, and Yb. Self-compensation of excessive charges through equalization of concentrations of Me3+(Li+) and Me3+(Nb5+) and appearance of interstitial Li+ in the structural vacancy near Me3+(Nb5+) take place in stoichiometric LN/LT due to lack of intrinsic defects.

Superoxide radical scavenging by sodium 4,5-dihydroxybenzene-1,3-disulfonate dissolved in water: Experimental and quantum chemical studies

Tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate) is often used as a probe for detection of superoxide radicals. In this work, we have studied the interaction of Tiron with superoxide radicals dissolved in water solutions. The density functional theory (DFT) method was used for scrutinizing electron and energy characteristics of Tiron radicals in the systems modeling freshwater and salt water. In water solution at neutral pH, deprotonated form of superoxide radical, O2•-, dominates over the protonated form HO2•. Fully reduced (TH2) and fully oxidized (T) species of Tiron react with superoxide radicals, leading to the formation of the Tiron semiquinone radicals. DFT calculations show that the reactions of O2•- with TH2 and T in water solution will advance if these reactions are catalyzed by hydronium ions: TH2 + O2•- + H3O+ → TH• + H2O2 + H2O and T + O2•- + H3O+ → TH• + O2 + H2O. Electron paramagnetic resonance (EPR) and DFT data show that Tiron radicals dissolved in water solution with neutral pH exist predominantly in deprotonated state T•-. The constants of the hyperfine coupling between an unpaired electron and the hydrogen atoms of deprotonated radicals T•-, computed for freshwater and seawater solutions, are in a fairly good agreement with experimental constants derived from the EPR spectra of Tiron radicals.

Ce3+ triggers fenton-like processes in neutral solutions for effective catechol degradation

Classical Fenton and Fenton-like processes destruct organic pollutants in water non-selectively to complete mineralization. However, the usage of classical Fenton or Fenton-like processes is often limited due to the narrow operational pH window, sludge accumulation, inefficient H2O2 and efficiency decline. To overcome these constraints, in this study, we used a homogeneous Fe3+-Ce3+-H2O2 Fenton-like process to degrade catechol at different experimental conditions. At pH 7, almost 97% of 10 mM catechol can be destructed within 60 min while the degradation by Classical Fenton or Fe3+-H2O2 Fenton-like process only 36.2% and 23.7%. The resultant solution after the degradation contains only traces of cerium ions. The sludge created by the process was extensively characterized by FTIR and XPS spectroscopy to elucidate the fate of cerium ions. Electron spin resonance (ESR) data confirmed •OH as the major free radical in Fe3+-Ce3+-H2O2 process. Our Fenton-like process widens the optimal pH values to neutral condition.

Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni-2S Structures and Implications for Biological 2M-2S Sites.

Sulfur-bridged bimetallic 2M-2S type structures are essential cofactors that participate in biological long-range electron transport and metabolism. Metal-sulfur bond covalency is a decisive property for inner sphere (through-bond) type electron transfer that dominates in buried or hydrophobic protein environments. This work reports on a combined experimental and computational study of the effect of ligand charge on the electronic structure of a 2Ni-2S model site that adopts the biologically relevant S = 1/2 redox state. Starting out from an isostructural dinickel(1.5+)-dithiophenolate platform with sulfur-bridged tetrahedral Ni sites, η2:η2-μ-coordination of the S = 1/2 [2Ni-2S]+ core to either a neutral π-system or strongly σ-donating cyclohexadienido renders its electronic structure substantially different. Density functional theory analysis corroborates pulse and continuous wave electron paramagnetic resonance data that associate co-ligand charge with the significant change in the mechanism and size of electron-31P nuclear spin hyperfine coupling to a phosphine reporter ligand at each nickel center. An increasing level of charge donation attenuates direct and through-bridge electronic coupling of the metal sites, resulting in a stronger electronic coupling of the 2Ni-2S core to its terminal phosphine donors. Drawing a connection to biological 2M-2S sites, our 2Ni-2S system indicates that a fine balance of intracore and core-protein electronic coupling is key to biological function for which the degree of charge donation by peripheral donors appears to be a significant parameter.

Effects of Copper Metallic Nanoparticles on Structural and Optical Properties of Antimony Phosphate Glasses Co-Doped with Samarium Ions.

New antimony phosphate glasses doped with samarium (III) oxide and co-doped with copper metallic nanoparticles (CuNPs) were obtained by the melt quenching technique. The samples were analyzed by X-ray diffraction analysis (XRD), electron paramagnetic resonance (EPR), ultraviolet-visible (UV–Vis) and photoluminescence (PL) spectroscopies. XRD data suggested that all the obtained samples showed an amorphous nature. EPR data suggested the existence of Cu2+ ions octahedrally surrounded by six oxygen atoms. The dipole–dipole interactions between Cu2+ ions were predominant. UV–Vis spectra revealed the presence of Sm3+ and Cu2+ ions in the samples. The values for nephelauxetic and bonding parameters were also calculated. The negative values obtained for bonding parameter indicate an ionic character of the bonds from the glass network. Photoluminescence spectra exhibited emissions from samarium ions and revealed the influence of dopant nature on of rare-earth ions emissions. The obtained results indicate that the studied materials are suitable for solid state lasers.

The unique speciation of iron in calc-alkaline obsidians

Obsidians found in silicic domes and rhyolitic tuffs are an ideal system to understand the formation conditions of calc-alkaline felsic magmas. The Optical Absorption and Electron Paramagnetic Resonance (EPR) spectroscopic data presented in this study demonstrate that Fe-speciation in obsidians is different from that observed in other geological glasses (e.g., basaltic glasses) or in synthetic Fe-bearing silicate glasses. The presence of unusual symmetric Fe2+ octahedral sites explains the characteristic black to brown color of obsidian glasses, much distinct from that of synthetic Fe-bearing glasses. The Fe3+ sites partly occur in superparamagnetic domains, as evidenced by EPR data. The dramatic intensification of the optical absorption in the visible range indicates the presence of Fe-rich domains with Fe2+-Fe3+ and/or Fe2+-Ti4+ Inter-Valence Charge Transfer (IVCT) processes. These data are consistent with the presence of Fe-rich, mixed valence clusters, located outside the glass structure. The largest of these domains contribute to the black color of obsidians because they cause a background absorption extending throughout the visible spectrum as observed in magnetite. This unusual Fe-speciation reflects the last events of the magmatic history during emplacement and cooling, indicating that obsidians cannot be considered as a quenched witness of the initial magma.

Structural Elucidation and Position Identification of Cu(II) ion in Hexaaquazinc(diaquabismalonto)zincate: Single Crystal EPR and Optical Studies

Single crystal electron paramagnetic resonance (EPR), powder X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) and UV–Visible (UV–Vis) spectroscopy studies of Cu(II) doped in hexaaquazinc(diaquabismalonto)zincate (HZDMZ) have been investigated at room temperature. Angular variation of copper hyperfine lines in the three orthogonal planes shows the presence of a single site in substitutional position, with spin Hamiltonian parameters calculated from EPR spectra are gxx = 2.3940; gyy = 2.0919; g-zz = 2.0700; A–xx = 13.85 mT; Ayy = 1.79 mT; Azz = 1.39 mT. Molecular orbital coefficients β2 = 0.765 and e2 = 0.275 have been calculated, which reveal a moderately covalent metal–ligand bond. Optical and EPR data have been collaborated to obtain crystal field parameters, admixture and molecular orbital coefficients. Powder XRD, FT-IR and UV–Vis data confirms the structure of host complex and presence of the dopant.

EPR and magnetization studies of the manganites La0.7-xEuxSr0.3MnO3 (x = 0.4, 0.5, 0.6, 0.7) and La0.3Nd0.4Sr0.3MnO3 at different temperatures: Conductivity due to hopping of small polarons

Four Eu-Sr manganites, La0.7-xEuxSr0.3MnO3; x = 0.4, 0.5, 0.6, 0.7, and one Nd-Sr manganite Nd0.4La0.3Sr0.3MnO3, were studied by EPR (electron paramagnetic resonance) at 9.6 GHz at temperatures in the range 175 – 400 K. These studies were further supplemented by magnetization measurements. The positions and widths of the EPR lines of all the Eu-Sr manganite samples, containing Eu3+ ions, changed with temperature in a similar manner. The EPR data showed that these Eu-Sr manganites are ferromagnetic, whose magnetic transition temperature from the ferromagnetic to the paramagnetic state (TC) decreased gradually as the Eu content (x) increased, specifically from TC ~ 111 K (x = 0.5) to TC ~ 57 K (x = 0.7). Furthermore, in these samples the FMR (ferromagnetic resonance) lines appeared significantly above the respective TC, specifically, at 290 K (x = 0.4), 280 K (x = 0.5), 250 K (x = 0.6), together with the EPR lines. As for the sample with x = 0.7 no FMR lines were observed in the temperature range investigated here. The temperature dependence of the EPR linewidth is found to be linear in the various La0.7-xEuxSr0.3MnO3 samples, caused by the presence of conductivity due to small-polaron hopping The peak-to-peak first-derivative EPR linewidths, ΔBpp, of these Eu-doped samples fitted well above the temperature, at which the minimum of ΔBpp occurs, to the expression: ΔBpp (T) = ΔBpp,min + ATexp(-Ea/kBT), with the values of the activation energies being Ea = 0.20 eV, 0.17 eV, 0.11 eV, 0.09 eV for x = 0.4, 0.5, 0.6, 0.7, respectively. On the other hand, for the Nd-doped sample, the ΔBpp in Nd0.4La0.3Sr0.3MnO3 decreased monotonically with temperature above TC in accordance with the Curie-Weiss law as ΔBpp (T) = ΔBpp,0 + C/(T-TC). The magnetization measurements were exploited to determine TC for the various samples. The results agreed with those determined by the EPR measurements presented here. In addition, the magnetization data was analyzed to obtain the values of the critical exponents in the phase transition from the paramagnetic to the ferromagnetic state. The critical exponents, as determined for the La0.7-xEuxSr0.3MnO3 sample with x = 0.5, are β = 0.24 ± 0.02, γ = 1.10 ± 0.5, δ = 5.58.

Mn Distribution in ZnO:Mn Ceramics: Influence of Sintering Process and Thermal Annealing

Manganese spatial distribution in ZnO:Mn ceramics (0.1–5 at.% Mn) sintered at 1100 °C in air has been investigated using Electron paramagnetic resonance (EPR), micro-Raman and Diffuse reflectance spectroscopy. The Raman and EPR spectra show that concentration of Mn incorporated in ZnO grains decreases from the surface of ceramic sample to its depth. Mn distribution in ZnO grains is also inhomogeneous. The concentration of Mn on Zn site centers (MnZn2+) evaluated from the EPR data is found to be lower than the Mn content in the charge indicating a large amount of Mn remains in the intergranular spaces. Thermostimulated Mn redistribution between the ZnO grain and the intergranural spaces have been observed under post sintering annealing. The influence of ZnO native donors on MnZn2+ center formation is found. It is supposed that this effect is due to interaction of Mn with interstitial zinc. The impact of native donor concentration on current-voltage characteristics of ZnO:Mn ceramics is demonstrated. The influence of Mn distribution on varistor characteristic is discussed.

DeerLab: a comprehensive software package for analyzing dipolar electron paramagnetic resonance spectroscopy data.

Dipolar EPR spectroscopy (DEER and other techniques) enables the structural characterization of macromolecular and biological systems by measurement of distance distributions between unpaired electrons on a nanometer scale. The inference of these distributions from the measured signals is challenging due to the ill-posed nature of the inverse problem. Existing analysis tools are scattered over several applications with specialized graphical user interfaces. This renders comparison, reproducibility, and method development difficult. To remedy this situation, we present DeerLab, an open-source software package for analyzing dipolar EPR data that is modular and implements a wide range of methods. We show that DeerLab can perform one-step analysis based on separable non-linear least squares, fit dipolar multi-pathway models to multi-pulse DEER data, run global analysis with non-parametric distributions, and use a bootstrapping approach to fully quantify the uncertainty in the analysis.

Free and defect-bound (bi)polarons inLiNbO3: Atomic structure and spectroscopic signatures fromab initiocalculations

Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO3). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_Li antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_Li antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.