Low Site Symmetry Research Articles

From solid solution to cluster formation of Fe and Cr in α-Zr

To understand the mechanisms by which the re-solution of Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, the solubility and clustering of Fe and Cr in model binary Zr alloys was investigated using a combination of experimental and modelling techniques — atom probe tomography (APT), x-ray diffraction (XRD), thermoelectric power (TEP) and density functional theory (DFT). Cr occupies both interstitial and substitutional sites in the α-Zr lattice; Fe favours interstitial sites, and a low-symmetry site that was not previously modelled is found to be the most favourable for Fe. Lattice expansion as a function of Fe and Cr content in the α-Zr matrix deviates from Vegard's law and is strongly anisotropic for Fe additions, expanding the c-axis while contracting the a-axis. Matrix content of solutes cannot be reliably estimated from lattice parameter measurements, instead a combination of TEP and APT was employed. Defect clusters form at higher solution concentrations, which induce a smaller lattice strain compared to the dilute defects. In the presence of a Zr vacancy, all two-atom clusters are more soluble than individual point defects and as many as four Fe or three Cr atoms could be accommodated in a single Zr vacancy. The Zr vacancy is critical for the increased apparent solubility of defect clusters; the implications for irradiation induced microstructure changes in Zr alloys are discussed.

Photoluminescence profiles and fast/slow annealing effects of Eu(III)/Tb(III)‐codoped silica phosphor materials

A silica (SiO2) nanoparticle matrix was codoped with luminescent Eu(III) and Tb(III) ions using a modified Stöber method. The effects of fast and slow thermal annealing on photoluminescence profile imaging were examined. Slow annealing treatment suppressed more quenching sites than fast thermal annealing to further increase the photoluminescence signals. The photoluminescence signals observed between 450 and 720 nm were assigned to the (5)D(0) → (7)F(J) (J = 0,1,2,3,4) of Eu(III) and the (5)D(4) → (7)F(J) (J = 6,5,4,3) transitions of Tb(III). Photoluminescence was largely sensitized by indirect excitation and was much stronger than that generated by direct excitation. The Eu(III) and Tb(III) ions were doped at lower symmetry sites in the silica matrix.

Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13−xSix compounds inferred from Mössbauer spectroscopy and magnetometry

Manganese containing La–Fe–Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La–Fe–Mn–Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μFe is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe11.3Mn0.3Si1.4 and, for comparison, LaFe11.6Si1.4 by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn13 structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (FeII) of low local symmetry, and the highly symmetrical (8b) sites (FeI). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(Bhf) of hyperfine magnetic fields Bhf. The average hyperfine field 〈Bhf〉, originating predominantly from FeII sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe11.6Si1.4, and the approximate relation 〈Bhf〉 = AμFe is confirmed for FeII sites, with A = 14.2 T/μB. 〈Bhf〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe11.3Mn0.3Si1.4, providing evidence for a reduction by 9.7% of the average Fe atomic moment μFe from ~2.16 μB to a value of ~1.95 μB by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of Ms = 163.1(1) Am2 kg−1 of LaFe11.6Si1.4 by 10.5% due to Mn substitution.

Enhanced stability of Eu in GaN nanoparticles: Effects of Si co-doping

Ab initio calculations on Eu doped (GaN)n (n = 12, 13, and 32) nanoparticles show that Eu doping in nanoparticles is favorable compared with bulk GaN as a large fraction of atoms lie on the surface where strain can be released compared with bulk where often Eu doping is associated with a N vacancy. Co-doping of Si further facilitates Eu doping as strain from an oversized Eu atom and an undersized Si atom is compensated. These results along with low symmetry sites in nanoparticles make them attractive for developing strongly luminescent nanomaterials. The atomic and electronic structures are discussed using generalized gradient approximation (GGA) for the exchange-correlation energy as well as GGA + U formalism. In all cases of Eu (Eu + Si) doping, the magnetic moments are localized on the Eu site with a large value of 6μB (7μB). Our results suggest that co-doping can be a very useful way to achieve rare-earth doping in different hosts for optoelectronic materials.

Rapid microwave-assisted hydrothermal synthesis of SrWO4:Eu3+ nanowires and their luminescence properties

SrWO4:Eu3+ nanowires were synthesized at 160 °C within 10 min via a microwave-assisted hydrothermal method. In examining the influences of synthesis temperature and reaction time on the morphology of nanowires, it was found that any temperatures and reaction time except 160 °C and 10 min gave rise to poorer morphologies under otherwise equal conditions. The synthesized nanowires were characterized by means of X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), high-resolution transmission electron microscopy(HRTEM), energy dispersive X-ray(EDX) and Raman spectrometry, respectively. The results suggest that the samples are homogenous and dispersive single phase nanowires. The photoluminescence properties of the nanowires were determined with a spectrofluorometer. Two obviously sharp peaks at 395 and 464 nm and a broad peak centered at 290 nm were found in their excitation spectrum. Under excitation at 395 and 464 nm, the 5D0→7F2 transition is the dominant process which means Eu3+ ion is located at a low symmetry site, while the 5D0→7F1 transition dominates under the excitation at 290 nm, showing a highly symmetric field around the Eu3+ ion, which indicates the presence of the two local Eu3+ environments.

High precision wavefunctions for hyperfine states of low symmetry materials suitable for quantum information processing

We report on a method of employing spin Hamiltonian data to enhance the accuracy of crystal-field fits for rare-earth impurity sites of low symmetry in crystalline hosts. As an initial test of this method, we apply it to orthorhombic sites in Li+ and Na+ ion charge compensated Sm3+ centers in codoped CaF2 and SrF2 crystals. This yields a good agreement between theory and experiment, in addition to accurately modelling crystal strains due to the differing ionic radii of Li+ and Na+.

Structural effects on the photophysical properties of mono-β-diketonate and bis-β-diketonate Eu(III) complexes.

Two ligands, mono-β-diketone (p-methoxylbenzoyl)trifluoroacetone (MBTF) and bis-β-diketone 1,2-bis(4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl))phenoxyl ethane (BTPE) with similar chemical structures, have been designed and prepared for the purpose of building the relationship between the structures and luminescence properties of Eu(iii) complexes. Structures of the Eu(iii) complexes [Eu(MBTF)3(DMSO)(H2O)] and [Eu2(BTPE)3(DMSO)4] have been defined by single crystal X-ray crystallography. The mono-β-diketone complex [Eu(MBTF)3(DMSO)(H2O)] is a mononuclear structure, the central Eu(iii) ion is coordinated by eight oxygen atoms from three ligands and two solvents, in a distorted trigonal-dodecahedral (8-TDH) geometry. Whereas, the bis-β-diketone complex [Eu2(BTPE)3(DMSO)4] adopts a triple-stranded dinuclear structure in which the two Eu(iii) ions are helically wrapped by three bis-bidentate ligands, and each Eu(iii) ion is eight-coordinated by six oxygen atoms from the ligands and two oxygen atoms from the coordinated DMSO molecules, in a distorted square-antiprismatic (8-SAP) geometry. The photophysical properties related to the electronic transition are characterized by the absorbance spectra, the emission spectra, the emission quantum yields, the emission lifetimes, and the radiative (kr) and nonradiative rate constants (knr). The mono-β-diketone complex [Eu(MBTF)3(DMSO)(H2O)] offers a relatively high emission quantum yield (38%, in solid) compared to that observed in the bis-β-diketone complex [Eu2(BTPE)3(DMSO)4] (25%, in solid). This enhancement of emission quantum yield in the mono-β-diketone complex can be attributed to its lower site symmetry around the Eu(iii) ion, lower non-radiative rate constant and higher energy transfer efficiency from the ligand to the metal ion.

Crystal structure and photoluminescent properties of BaZn1−xEuxV2O7 nanoparticles

The structure of novel BaZn1−xEuxV2O7 nanoparticles (synthesised employing solution combustion process) was refined and determined from X-ray diffraction (XRD) profiles by the Rietveld refinement method. It was found to crystallize in orthorhombic lattice with the Pnma space group. Further structural informations and particle morphology were investigated using FT-IR spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence spectral analysis showed that these nanophosphor can be effectively excited by near-UV light (305 nm, 395 nm), emitting intense red luminescence (620 nm) corresponding to the hypersensitive 5D0→7F2 transition of Eu3+ ion, located at low-symmetry site with no inversion center in BaZnV2O7 crystal lattice. The dependence of luminescence intensity and chromaticity index on Zn2+ substitution by Eu3+ ions in BaZnV2O7 was also investigated which unveil 5 mol% as the optimum concentration. These nanophosphors are potential candidate for white LEDs using near ultra-violet (NUV) LED chip excitation.

Infrared spectroscopic study of SO42− ions included in M′2M′′(SeO4)2⋅6H2O (Me′ = K, NH4+; M′′ = Mg, Co, Ni, Cu, Zn) and NH4+ ions included in K2M(XO4)2⋅6H2O (X = S, Se; M′′ = Mg, Co, Ni, Cu, Zn)

Infrared spectra of Tutton compounds, M′2M′′(XO4)2⋅6H2O (M′=K, NH4+; M′′=Mg, Co, Ni, Cu, Zn; X=S, Se), as well as those of SO42− guest ions included in selenate host lattices and of NH4+ guest ions included in potassium host lattices are presented and discussed in the regions of ν3 and ν1 of SO42− guest ions, ν4 of NH4+ guest ions and water librations.The SO42− guest ions matrix-isolated in selenate matrices (approximately 2mol%) exhibit three bands corresponding to ν3 and one band corresponding to ν1 in good agreement with the low site symmetry C1 of the host selenate ions. When the larger SeO42− ions are replaced by the smaller SO42− ions the mean values of the asymmetric stretching modes ν¯3 of the included SO42− ions are slightly shifted to lower frequencies as compared to those of the same ions in the neat sulfate compounds due to the smaller repulsion potential of the selenate matrices (larger unit-cell volumes of the selenates). It has been established that the extent of energetic distortion of the sulfate ions matrix-isolated in the ammonium selenates as deduced from the values of Δν3 and Δν3/νc is stronger than that of the same ions matrix-isolated in the potassium selenates due to the formation of hydrogen bonds between the SO42− guest ions with both the water molecules in the host compounds and the NH4+ host ions (for example, Δν3 of the sulfate guest ions have values of 30 and 51cm−1 in the nickel potassium and ammonium compounds, and 33 and 49cm−1 in the zinc potassium and ammonium compounds, respectively).The infrared spectra of ammonium doped potassium sulfate matrices show three bands corresponding to Δν4 of the included ammonium ions in agreement with the low site symmetry C1 of the host potassium ions. However, the inclusion of ammonium ions in selenate matrices (with exception of the magnesium compound) leads to the appearance of four bands in the region of ν4. At that stage of our knowledge we assume that some kind of disorder of the ammonium ions included in selenate lattices occurs due to the different proton acceptor capability of the SO42− and SeO42− ions. The latter ions are known to exhibit stronger proton acceptor abilities. This fact will facilitate the formation of polyfurcate hydrogen bonds of the ammonium ions in the selenate matrices, thus leading to increasing in the coordination number of these ions, i.e. to a disorder of the ammonium guest ions.The strength of the hydrogen bonds formed in the title Tutton compounds as well as that of the hydrogen bonds in potassium compounds containing isomorphously included ammonium ions as deduced from the wavenumbers of the water librations are also discussed. The bands corresponding to water librations in the spectra of the mixed crystals K1.8(NH4)0.2M(XO4)2⋅6H2O (M=Mg, Co, Ni, Cu, Zn; X=S, Se) broaden and shift to lower frequencies as compared to those of the potassium host compounds, thus indicating that weaker hydrogen bonds are formed in the mixed crystals. These spectroscopic findings are owing to the decrease in the proton acceptor capacity of the SO42− and SeO42− ions due to the formation of hydrogen bonds between the host anions and the guest ammonium cations additionally to water molecules (anti-cooperative or proton acceptor competitive effect). Furthermore, the band shifts in the spectra of the selenate matrices are generally larger than those observed in the spectra of the respective sulfates due to the stronger proton acceptor ability of the selenate ions.

(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys

Amorphous silicon and its alloys are widely used in semiconductor industry. Due to short range order, doping can be achieved by substituting phosphorus or boron for silicon. Moreover, the optical bandgap can be increased by alloying with C, N or O or decreased by alloying with Ge. Due to its amorphous nature the network can accommodate a variety of elements up to relatively high concentrations at the expense of an increased density of defects in the bandgap. Rare earth elements (RE) have been introduced in the form of trivalent ions in amorphous silicon or its wider bandgap alloys. They present characteristic intra-4f luminescence if their chemical neighborhood is non-centrosymetric and the electric dipole forbidden transitions become partially allowed. In the amorphous alloys excitation through the network can be efficiently achieved when a RE transition corresponds to roughly half the bandgap. The RE act as nucleation centers to induce formation of Si nanocrystals under high temperature annealing. Samples were prepared by reactive RF sputtering from a Si target partially covered with metallic RE platelets using appropriate reactive atmospheres. Annealing was used either to optimize the photoluminescence or to induce the formation of Si nanocrystals.We will present characterization by photoluminescence and EXAFS of Er, Nd, Eu and Tb in a-Si:H, a-SiOx:H and a-SiNx:H. The RE ions tend to occupy low coordination low symmetry sites in the network in the amorphous samples. This favors relatively strong wide photoluminescence lines. The RE present two luminescence lifetimes, the fast component determined by the host and the slow associated to the local symmetry of the RE ions.Models for the excitation of luminescence and recombination processes and their relation with the local nanostructure around the RE ions will be presented.

The Solid Solutions (Ce1-xLax)RuSn

X-Ray-pure samples of the solid solutions (Ce1-xLax)RuSn were obtained up to x=0.5. Powder diffraction data show the CeRuSn-type superstructure up to x ≈ 0:3 and the CeCoAl-type subcell for higher lanthanum contents. The structure of a single crystal with x=0.5 was refined on the basis of single-crystal X-ray diffractometer data: CeCoAl type, C2=m, a=1160.8(2), b=477.6(1), c=511.6(1) pm, β = 102:97(2)°, wR=0.0510, 444 F2 values, 20 variables. Magnetic investigations were performed for all samples up to a lanthanum content of x=0.4. No cooperative phenomena could be observed, and all samples show Curie-Weiss behavior above a certain temperature. The cerium valence is about 3.32(2) for all samples of the solid solution. Hence, La3+ has to replace Ce3+ as well as Ce4+ in a particular quantity. The electrical resistivity measurements confirm the suppression of the magnetic ordering and the structural transition upon replacement of cerium by lanthanum. 119Sn Mössbauer spectra of samples with x=0.2 and 0.5 are indicative of single tin sites with isomer shifts of δ =1.86(1)mm s-1 for x=0.2 and δ =1.88(1)mm s-1 for x=0.5. Both signals are subject to significant quadrupole splitting, a consequence of the low site symmetry. Results of XANES measurements are perfectly in line with the cerium valences determined by susceptibility measurements and yield a constant value of 3.16(1) for all investigated compounds

Crystal-field study for Yb3+ doped KY(WO4)2 crystal – Revisited based on superposition model

The optical spectra and electron paramagnetic resonance (EPR) parameters (gi and hyperfine structure constants Ai, i=x, y, z) for Yb3+:KY(WO4)2 crystal have been simultaneously explained by a crystal-field study based on superposition model (SPM). The deficiencies in a recent paper by Trabelsi et al. (2010) have been clarified and solved in the present work. The more reliable fitted phenomenological crystal-field parameters (CFPs) have been confirmed and monoclinic standardization of the obtained CFPs for Yb3+:KY(WO4)2 crystal is performed to ensure direct comparison with other rare earth ion doped potassium rare-earth double tungstates. By SPM calculations in this work, two individual EPR experiments with greatly different y-directional principal g factors for Yb3+:KY(WO4)2 crystal have been analyzed and possible reason for this large difference is also given. It indicates that magnetic dipolar and exchange interactions among different rare earth ions will play an important role in determining their magnetic properties in crystal under low temperature and more investigations should be carried out. The present procedures for SPM analysis can be applied to other crystal field environment with low site symmetry.

Luminescence and structural properties of Eu3+ doped BaY2ZnO5 for LED solid-state lighting application

Eu3+ doped BaY2(1−x)ZnO5 phosphor was successfully synthesized by a single step solution combustion process. The crystal structure and particle morphology were investigated by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. The XRD results suggest that BaY2ZnO5 crystallizes in a single phased orthorhombic structure with space group Pbnm at 1,100 °C. The phosphor can be effectively excited by near-UV light, emitting intense red luminescence (628 nm) corresponding to the hypersensitive 5D0 → 7F2 transition of Eu3+ ions, located at low-symmetry site with no inversion center in BaY2ZnO5 crystal lattice. Fluorescence decay analysis was carried out to understand the energy transfer mechanism and quenching behavior of luminescence of Eu3+ ions in the BaY2ZnO5 phosphor. The BaY2ZnO5: Eu3+ emission (λex = 395 nm) could be tuned from blue to white and red light by varying the Eu3+ ions concentration, making this phosphor as a promising candidate for LEDs application.

Nanoparticle characterization of Er-doped SnO2 pellets obtained with different pH of colloidal suspension

SnO2:2 at. %Er xerogel samples were obtained by sol-gel technique from colloidal suspensions with distinct pHs. The evaluation of critical regions inside the nanocrystallite is fundamental for the interpretation of the influence of pH on the emission data. In this way, the nanocrystal depletion layer thickness was obtained with the help of photoluminescence, Raman, X-ray diffraction, and field-emission gun scanning electron microscopy measurements. It was observed that acid suspensions (pH < 7) lead to high surface disorder in which a larger number of cross-linked bonds Sn-O-Sn among nanoparticles are present. For these samples, the nanoparticle depletion layer is larger as compared to samples obtained from other pH. Photoluminescence measurement in the near infrared region indicates that the emission intensity of the transition 4I13/2 → 4I15/2 is also influenced by the pH of the starting colloidal suspension, generating peaks more or less broadened, depending on location of Er3+ ions in the SnO2 lattice (high or low symmetry sites).

Eu3+ probe ion for rare-earth dopant site structure in sol–gel derived LiYF4 oxyfluoride glass–ceramic

Sol–gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass–ceramic containing Eu3+-doped LiYF4 nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass–ceramic Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably as Eu–O center and/or dimer centers in low symmetry (C2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization.

Novel polyhedron Y1−x Dy x VO4 nanocrystals: hydrothermal synthesis and luminescence properties

Novel polyhedron Y1−x Dy x VO4 (0 ≤ x ≤ 0.1) nanocrystals were synthesized via a hydrothermal route in ethylene glycol–water mixtures at 150 °C for 25 h. The characterizations from X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra revealed that polyhedron-like Y1−x Dy x VO4 single crystals with size of 20–100 nm were structurally uniform. The as-prepared polyhedron Y1−x Dy x VO4 performed excellent chromaticity and higher ratio of yellow (4F9/2 → 6H13/2) to blue (4F9/2 → 6H15/2) emissions compared with Y1−x Dy x VO4 irregular nanoparticles, which were mainly due to low site symmetry resulting from the crystallite size and crystallization increased. The formation mechanism had been proposed. Crucial influencing factors such as the pH value, reaction temperature, and time on the morphologies of the final products had also been discussed. The newly discovered uniform morphology of the products maybe has good potential in related future applications.

Infrared spectroscopic study of Rb2M(XO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn; X = S, Se) and of [formula omitted] guest ions included in rubidium Tutton selenates

The solubility in the three-component systems Rb2SeO4–MSeO4–H2O (M=Mg, Ni, Cu) is studied at 25°C by the method of isothermal decrease of supersaturation. It has been established that double compounds, Rb2M(SeO4)2·6H2O (M=Mg, Ni, Cu), crystallize from the ternary solutions within wide concentration ranges.Infrared spectra of rubidium Tutton compounds, Rb2M(XO4)2·6H2O (X=S, Se; M=Mg, Co, Ni, Cu, Zn) (in the region of 1300–400cm−1) as well as those of SO42- ions included in selenate matrices (up to 15mol%) in the region of the stretching modes ν3 and ν1 are presented and discussed. The matrix-isolated SO42- ions exhibit three bands for ν3 and one band for ν1 in good agreement with the low site symmetry C1 of the SeO42- host ions. The extent of energetic distortion of the isomorphously included ions as deduced from the values of Δν3 (site group splitting) and Δν3/νc (where νc is the centro-frequency value of the asymmetric stretches) is discussed. The comparison of the vibrational behavior of the matrix-isolated SO42- ions (about 2mol%) in different Tutton selenate matrices show that the distortions of the sulfate guest ions increase on going from potassium selenates to ammonium ones. The stronger distortion of the SO42- guest ions in ammonium compounds as compared to that of the same ions included in the potassium and rubidium selenates is due to the formation of hydrogen bonds between the guest ions and the NH4+ host cations additionally to those with water molecules of the host compound. The matrix-isolated sulfate guest ions are stronger distorted in the case of the Rb2Cu(SeO4)2·6H2O, (NH4)2Cu(SeO4)2·6H2O, (NH4)2Zn(SeO4)2·6H2O, and (NH4)2Ni(SeO4)2·6H2O as compared to those in the respective magnesium, cobalt and nickel selenate matrices is due probably to the formation of stronger hydrogen bonds in the former compounds (strong synergetic effect of the copper, zinc and nickel ions) as well as to the formation of hydrogen bonds between the guest ions and the ammonium host ions. The water librations are also briefly discussed.

Optical properties of Tb and Eu doped cubic YAlO3 phosphors synthesized by sol–gel method

In this work, terbium and europium doped YAlO3 phosphors were synthesized by sol–gel method. It has been shown that after annealing at 1000°C these phosphors crystallize in the cubic garnet phase, similar to Y3Al5O12. As evidenced by the time-resolved photoluminescence spectroscopy and Maximum Entropy Method, the photoluminescence decay of both europium 5D0 and terbium 5D4 states has two lifetime components: short and long, both of the order of milliseconds. Moreover, for both phosphors the short photoluminescence lifetime was ascribed to the ions occupying low-symmetry sites in the vicinity of the nanocrystal surface, while the long decay component was related to the ions present in the bulk of YAlO3 nanocrystal. For both dopants, the long lifetime component of Tb3+ and Eu3+ emission is longer than the equivalent lifetimes in bulk Y3Al5O12 crystal.

Calculating Hamiltonian parameters for Yb3+ in a low-symmetry lattice site, and fitting the structure and levels of Yb3+:RETaO4 (RE = Gd, Y, and Sc)

An iterative method is used to find the values of the Hamiltonian parameters for Yb3+ in a given low-symmetry crystalline site. Samples of Yb3+:RETaO4 (RE = Gd, Y, and Sc) were prepared and their structures were determined. Based on the obtained structural data, their orbital-spin parameters and crystal field parameters were fitted by the superposition model (SM). Using the crystal field parameters obtained by the SM fitting as the initial parameters, the Hamiltonian parameters were fitted iteratively. The calculated and experimental energy levels for Yb3+:RETaO4 are consistent, and the maximal mean-root-square deviation is only 2.84 cm−1, indicating that the method is effective to determine the Hamiltonian parameters of Yb3+ in low-symmetry crystalline sites.

Responses of Mn2+speciation inDeinococcus radioduransandEscherichia colito γ-radiation by advanced paramagnetic resonance methods

The remarkable ability of bacterium Deinococcus radiodurans to survive extreme doses of γ-rays (12,000 Gy), 20 times greater than Escherichia coli, is undiminished by loss of Mn-dependent superoxide dismutase (SodA). D. radiodurans radiation resistance is attributed to the accumulation of low-molecular-weight (LMW) "antioxidant" Mn(2+)-metabolite complexes that protect essential enzymes from oxidative damage. However, in vivo information about such complexes within D. radiodurans cells is lacking, and the idea that they can supplant reactive-oxygen-species (ROS)-scavenging enzymes remains controversial. In this report, measurements by advanced paramagnetic resonance techniques [electron-spin-echo (ESE)-EPR/electron nuclear double resonance/ESE envelope modulation (ESEEM)] reveal differential details of the in vivo Mn(2+) speciation in D. radiodurans and E. coli cells and their responses to 10 kGy γ-irradiation. The Mn(2+) of D. radiodurans exists predominantly as LMW complexes with nitrogenous metabolites and orthophosphate, with negligible EPR signal from Mn(2+) of SodA. Thus, the extreme radiation resistance of D. radiodurans cells cannot be attributed to SodA. Correspondingly, 10 kGy irradiation causes no change in D. radiodurans Mn(2+) speciation, despite the paucity of holo-SodA. In contrast, the EPR signal of E. coli is dominated by signals from low-symmetry enzyme sites such as that of SodA, with a minority pool of LMW Mn(2+) complexes that show negligible coordination by nitrogenous metabolites. Nonetheless, irradiation of E. coli majorly changes LMW Mn(2+) speciation, with extensive binding of nitrogenous ligands created by irradiation. We infer that E. coli is highly susceptible to radiation-induced ROS because it lacks an adequate supply of LMW Mn antioxidants.