Theoretical investigation of interaction of hydrogen and intermetallic compound YCo5

Stability, magnetic properties, electric field gradients and hyperfine fields of YCo5Hx compounds were investigated by using DFT based calculations. Two computational approaches were employed employed in the study-ultrasoft pseudopotentials with plane waves and all-electron FP(L)APW + lo method. It was found that H atoms prefer off-centered or centered octahedral sites. Enthalpies of formation for α→β transition were calculated. Satisfactory agreement was found between theoretical results and previous experimental value of the enthalpy of formation. It was also found that inclusion of spin-polarization reduces stability of the hydrides. Comparison of theoretical and experimental spin magnetic moments of different YCo5Hx compounds resulted in a reasonable agreement betwen present theoretical results and previous experimental and theoretical data. Magnetocrystalline anisotropy energy (MAE) was calculated for intermetallic compound YCo5. Reasonable agreement was found between MAE obtained in the present study and the corresponding theoretical and experimental values obtained in earlier studies

Mössbauer spectroscopic analysis of (Nd,Pr,Dy)2(Fe,Co,Ga)14B/α-Fe permanent magnetic nanocomposites

In this paper, it is reported the structural and magnetic properties of Nd13.7Pr0.7Dy0.2Fe73.1Co6.3Ga0.4B5.6 and Nd7.7Pr0.7Dy0.2Fe79.1Co6.3Ga0.4B5.6 magnetic nanocomposites, synthesized by melt-spinning and annealing methods. The Nd-Fe-B ribbons are melt-spun at v=30 m/s in high vacuum and annealed at 715oC for 4 min. in argon. Furthermore, X-ray diffraction and transmission 57Fe Mössbauer spectra at RT are used to investigate the effects of substituent elements: Dy, Pr, Co, Ga on the hard magnetic properties and microstructure of both nanocomposites. Analysis of Mössbauer spectra for Nd13.7Pr0.7Dy0.2Fe73.1Co6.3Ga0.4B5.6 is done in terms of ten Zeeman sextets, one paramagnetic doublet related to Nd1.1Fe4B4 phase and two hyperfine magnetic fields distributions extracted from spectrum. Similar result of analysis of the second nanocomposite is obtained with eleven sextets, one doublet and one distribution. One sextet corresponds to α-Fe phase, while we have identified six iron sextets corresponding to the six distinct iron sites in the Nd2Fe14B structure: 16k1, 16k2, 8j1, 8j2, 4c and 4e. The three remaining sextets belong to Fe3B structure with three inequivalent Fe sites: FeI(8g), FeII(8g) and FeIII(8g). The eleventh sextet of Nd7.7Pr0.7Dy0.2Fe79.1Co6.3Ga0.4B5.6 belongs to FeB. All relevant parameters for both nanocomposites: magnetic hyperfine field, isomer shift and quadrupole splitting are determined for each of these sites. To highlight the thermally induced structural transformations, the quenched samples have been analysed by differential scanning calorimetry and thermo-magnetic measurements. The magnetic properties, measured at RT on the quenched and annealed ribbons, revealed the relationship between the alloy chemical composition and processing.RoPM AM 2017 : 5th International Conference on Powder Metallurgy & Advanced Materials, 17-20th September 2017, Cluj Napoca, Romani

The influence of fluorine doping on the structural and the electrical properties of LiFePO4 powder

Low intrinsic electronic conductivity is the main weakness of LiFePO4 for the use as cathode material in lithium ion batteries. Here is presented an experimental proof of the theoretical prediction that fluorine doping of LiFePO4 can enhance its electrical conductivity. LiFePO4 and fluorine-doped LiFePO4 olivine type, carbon-free powders are synthesized and examined. Crystal structure refinements in the space group Pnma reveal that doping with fluorine ions preserves olivine structure with the reduction of both the lattice parameters and the antisite defect, and an increase of a crystallite size. A small amount of incorporated fluorine enhances electrical conductivity from 4.6 × 10-7 Scm-1 to 2.3 × 10-6 Scm-1 and has positive impact on the electrochemical performances. Several spectroscopy techniques (Mössbauer, FTIR, and Raman) disclose differences between two powders and additionally support the findings of both the Rietveld refinement and the conductivity measurements

The influence of fluorine doping on the structural and the electrical properties of LiFePO4 powder

Low intrinsic electronic conductivity is the main weakness of LiFePO4 for the use as cathode material in lithium ion batteries. Here is presented an experimental proof of the theoretical prediction that fluorine doping of LiFePO4 can enhance its electrical conductivity. LiFePO4 and fluorine-doped LiFePO4 olivine type, carbon-free powders are synthesized and examined. Crystal structure refinements in the space group Pnma reveal that doping with fluorine ions preserves olivine structure with the reduction of both the lattice parameters and the antisite defect, and an increase of a crystallite size. A small amount of incorporated fluorine enhances electrical conductivity from 4.6 × 10-7 Scm-1 to 2.3 × 10-6 Scm-1 and has positive impact on the electrochemical performances. Several spectroscopy techniques (Mössbauer, FTIR, and Raman) disclose differences between two powders and additionally support the findings of both the Rietveld refinement and the conductivity measurements

A study of defect structures in Fe-alloyed ZnO: Morphology, magnetism, and hyperfine interactions

In order to study the effect of Fe cation substitution on the local structure, defect formation, and hyperfine interactions in ZnO, Mössbauer spectroscopy measurements of the microwave processed Zn1−xFexO (x=0.05, 0.10, 0.15, and 0.20) nanoparticles, together with ab initio calculations, were performed. Complementary information on the distribution of particle size and morphology, as well as magnetic properties, were obtained by X-ray diffraction, transmission electron microscopy, and squid-magnetometry. The selected model for analyzing the Mössbauer spectra of our samples is a distribution of quadrupole splittings. The fitting model with two Lorentz doublets was rejected due to its failure to include larger doublets. The Fe3+ ions do not yield magnetic ordering in the samples at room temperature. The results from first-principles calculations confirm that the major component of the Mössbauer spectra corresponds to the Fe-alloyed ZnO with Zn vacancy in the next nearest neighbor environment. The magnetic measurements are consistent with the description of the distribution of iron ions over the randomly formed clusters in the ZnO host lattice. While at room temperature all the samples are paramagnetic, magnetic interactions cause a transition into a cluster spin-glass state at low temperatures

Modification of ZnO surfaces with oxygen vacancies: density functional study

ZnO is a complex material, whose properties are very sensitive to preparation conditions due to the presence of various intrinsic defects. Using the modified density functional calculations, we study the relative stability of the nonpolar (1010 and 1120) and the polar (zinc-terminated (0001)-Zn and oxygen-terminated (0001)-O) ZnO surfaces, as well as the influence of oxygen vacancies on their stability and electronic structure. In our study we consider models of surfaces with different positions and concentrations of oxygen vacancies. Particular attention is given to the charge state of the oxygen vacancies and charges of the individual atoms. We find that the 1010 surface is the most stable, while the polar (0001)-Zn surface is the least stable. Our calculations indicate that the configuration with oxygen vacancies at the top of the surface is the most favorable for all studied surfaces. The obtained results for the structural relaxations and energetics are compared with previous theoretical and experimental data

The influence of fluorine doping on the structural and the electrical properties of LiFePO4 powder

Low intrinsic electronic conductivity is the main weakness of LiFePO4 for the use as cathode material in lithium ion batteries. Here is presented an experimental proof of the theoretical prediction that fluorine doping of LiFePO4 can enhance its electrical conductivity. LiFePO4 and fluorine-doped LiFePO4 olivine type, carbon-free powders are synthesized and examined. Crystal structure refinements in the space group Pnma reveal that doping with fluorine ions preserves olivine structure with the reduction of both the lattice parameters and the antisite defect, and an increase of a crystallite size. A small amount of incorporated fluorine enhances electrical conductivity from 4.6 × 10-7 Scm-1 to 2.3 × 10-6 Scm-1 and has positive impact on the electrochemical performances. Several spectroscopy techniques (Mössbauer, FTIR, and Raman) disclose differences between two powders and additionally support the findings of both the Rietveld refinement and the conductivity measurements

A Three - Detector High Resolution TDPAC Spectrometer

An advanced fast-slow coincidence scintillation spectrometer for the Timx Differential Perturbed y-y Angular Correlation (TDPAC) experiments is presented. It consists of three detectors with BaF2 scintillators and was developed in the Group for Hyperfine Interactions in the Institute for Nuclear Sciences in Vinca. The spectrometer is characterized with high detection efficiency, excellent time resolution, high counting rate performance and is operating in the temperature range 78-1300 K

Structural deformation in Hf2Ni observed by TDPAC method

The electric quadrupole interaction in Hf2Ni was measured at 181Ta probe using the time-differential perturbed angular correlation method (TDPAC) in the temperature range 78 to 1314 K. Analysis of the obtained spectra revealed the presence of a single nuclear quadrupole interaction (NQI) and its linear descending dependence with increasing temperature. In the region 1134 to 1314 K, the presence of structural deformation is evident from the coincidence spectra.YUCOMAT 2007 : 9th Annual Conference YUCOMAT 2007 : Programme and the book of abstracts; September 10-14, 2007; Herceg Novi, Montenegr

Mössbauer spectroscopy study of nanosized spinel CoFe2O4 ferrite obtained during coprecipitation followed by mechanochemical treatment

The powdery cobalt ferrite (CoFe2O4) is prepared by coprecipitation followed by mechanochemical synthesis in a planetary ball mill. Obtained nanomaterial has been studied using a variety of characterization techniques: X-ray diffraction (XRD), Raman spectroscopy, far infrared (FIR) reflectivity and attenuated total reflectance (ATR) in combination with Fourier transform infrared (FTIR) spectroscopy in mid IR spectra. The investigated CoFe2O4 nanomaterial showed a typical XRD pattern of cubic spinel. In the Raman and IR spectra are observed all of first-order Raman and IR active modes. Weak sub bands activated by structure disorder are seen also. Since nano-CoFe2O4 is macroscopically cubic, its main Raman and IR modes are assigned as in normal cubic spinel. Raman spectrum is fitted with 8 Lorentzian peaks. It is observed that the value of x 0.58 obtained from Raman spectrum, is in good agreement with the value obtained by XRD-structural analysis (0.51). To analyze the IR spectra, we used Decoupled Plasmon - Phonon (DPP) model of the complex dielectric function. Measurement of magnetization in the range of magnetic fields H>>Hc enable the calculation of the anisotropy coefficient K1 = 4.02·105 J cm-3 , which is very high in cobalt ferrite. The 57Fe-Mössbauer spectrum of the CoFe2O4 sample was measured at room temperature in ± 12 mm s-1 Doppler velocity range. The 57Fe-Mössbauer spectrum of the CoFe2O4 sample was fitted with the extended Voigt-based fitting method