6h Sites Research Articles

Neutron diffraction and Mössbauer effect study of several Nd2Fe17−xAlx solid solutions

Aluminum-substituted Nd2Fe17−xAlx solid solutions with x=2.30, 4.44, 6.13, 8.00, and 9.40 have been studied by neutron diffraction and Mössbauer spectroscopy. A Rietveld analysis of the neutron scattering indicates that the aluminum atoms have a high initial affinity for the 18h site, show a high affinity for the 6c site at high aluminum concentrations, and are absent from the 9d site at all aluminum concentrations. The Mössbauer spectra show a maximum in both the maximum and weighted average hyperfine field for x≊2. This indicates that the lattice expansion which occurs with aluminum substitution is sufficient, at least for small values of x, where magnetic dilution is small, to enhance the intrasublattice coupling between the iron moments, and to increase the magnetic hyperfine fields.

Site occupation in57Fe doped NdCo12B6: M�ssbauer analysis

The analysis of room temperature and4.2 K Mossbauer spectra of NdCo12B6 doped with57Fe is presented. A clear preference of the57Fe atoms for the 18h sites, of increased number of Co near neighbour atoms compared with the 18g site, is obtained in good agreement with recent studies of related 1:12:6 compounds.

Magnetic structure of the Laves phase compound TiFe2

The magnetic structure of TiFe2 which crystallizes with the C15 Laves phase structure has been determined from neutron diffraction measurements made on both single-crystal and polycrystalline samples. In the antiferromagnetic structure the Fe2 atoms in the (6h) sites with the same z coordinate are coupled ferromagnetically with the moments in successive layers alternately parallel and antiparallel to the c-axis. The Fe1 atoms which are in the (2a) sites are at an anti-centre of symmetry and carry no ordered moment. Magnetization measurements support this structure, giving a positive paramagnetic Curie temperature and showing the metamagnetic transition characteristic of ferromagnetically coupled sheets. Polarized neutron measurements indicate that in the antiferromagnetic phase the two types of Fe atom contribute almost equally to the susceptibility and suggest that the magnetic moment on the Fe1 atoms is only induced by the molecular field due to the aligned moment on the surrounding Fe2 atoms.

The influence of interstitial N, C and H atoms on the hyperfine fields at the yttrium and cobalt sites in Y 2Co 17

The spin echo nuclear magnetic resonance spectra of Y 2Co 17A x (A≡N, C, H) are presented. On the basis of the satellite pattern of the 89Y spectra, we have deduced that N and C atoms occupy the 9e sites (rhombohedral compounds) or 6h sites (hexagonal compounds) around the yttrium site. A decrease in the hyperfine fields at the yttrium and cobalt sites caused by neighbouring interstitial atoms is observed, with a magnitude dependent on the site and the type of interstitial atom. The effect is discussed in terms of the influence of N, C and H atoms on the local electronic structure at the yttrium and cobalt sites and compared with the effect observed in the RE 2Fe 17-based compounds (RE, rare earth).

Electric field gradient at the 59Co nucleus of ferromagnetic YCo 3

The structure of YCo 3 is the PuNi 3 type with three inequivalent cobalt sites in the lattice. The spin-echo decay of the 59Co NMR at 43 MHz (18h site) was found to have an oscillatory component, indicating an electric field gradient at the nucleus equivalent to a quadrupole splitting frequency of 210 kHz. No clear oscillations could be detected at the NMR frequencies of the 3b and 6c sites but a signal, attributed to nuclei in domain walls, was observed near 48 MHz.

A Mössbauer effect study of Nd 2Fe 17 and Nd 2Fe 17N 2.6

The Mössbauer spectra of Nd 2Fe 17 and Nd 2Fe 17N 2.6 have been measured as a function of temperature between 85 and 295 K. The spectral fits and hyperfine parameters for Nd 2Fe 17 are consistent with a Wigner-Seitz analysis of the iron near-neighbor environment. The presence of nitrogen has a major effect on the hyperfine parameters of the iron on the 18f and 18h sites, the only iron sites which have nitrogen near neighbors in Nd 2Fe 17N 2.6.

High field Mössbauer investigations of the magnetic phase transition in Sc 0.25Ti 0.75Fe 2 at 4.2 K

Mössbauer measurements in applied fields up to 13.5 T on Sc 0.25Ti 0.75Fe 2 are reported. The hysteretic behaviour observed in the field dependence of the bulk magnetization is also visible in the Mössbauer spectra, but hysteresis in the hyperfine fields is only found for Fe on 6h sites. In low external fields the hyperfine fields on these sites form a cone (opening angle ≈140°) with the resulting field parallel to those of 2a sites. At high fields the cone angle drops to zero and all hyperfine fields are antiparallel to the applied field. The field induced phase transition can be described as a transition from canted ferromagnetism to ferromagnetism of the Fe atoms occupying the 6h positions.

Nitrogen atom location in rhombohedral and hexagonal RE2Fe17Nx compounds

High resolution neutron powder diffraction studies were performed on Nd2Fe17Nx and Y2Fe17Nx. A strong preferential site occupation of the N atoms on the 9e position of the rhombohedral compound Nd2Fe17Nx was observed. In the hexagonal compound Y2Fe17Nx the N atoms occupy primarily the 6h sites (82%) and only a small fraction goes into the 12i site (12%). The refined nitrogen occupation numbers correspond to the formula compositions Nd2Fe17N2.6 and Y2Fe17N2.9.

X-ray diffraction and magnetic susceptibility studies of Lu(Fe xAl 1- x) 2 compounds (0.55 ⩽ χ ⩽ 0.65)

Powdered LuFe 1.1Al 0.9 and LuFe 1.2Al 0.8 samples analysed by X-ray diffraction have the hexagonal MgZn 2 structure, whereas the structure of LuFe 1.3Al 0.7 sample is of the cubic MgCu 2 type. A tendency for the Fe atoms to occupy the 6h sites in hexagonal structure samples was observed. The temperature dependence of magnetic susceptibility measurements χ( T) exhibit a cusp-like peak (at T M) for all examined samples. Moreover, below T M the χ( T) runs depend on the cooling process (without and with a magnetic field applied prior to the measurement). This behaviour of the magnetic susceptibility suggests that all the investigated compounds are cluster glasses.

A neutron diffraction study of the atomic ordering in Er(Fe 1 − xCo x) 3

Neutron diffraction studies of the intermetallic compounds ErFe 2.28 − Co 0.72, ErFe 1.68Co 1.32, and ErFe 1.16Co 1.84, above the magnetic ordering temperatures, indicate that iron atoms preferentially occupy the 6c site, while cobalt atoms show a strong preference for the 3b site and a slight preference for the 18h site. The room-temperature pattern for ErFe 1.16 − Co 1.84, below the Curie temperature, indicates a collinear arrangement of the magnetic moments along the c axis. A more complex, non-collinear, array of magnetic moments is indicated for ErFe 2.28Co 0.72.

Calculation of deuterium site occupancy in LaNi 5D 6.5, LaNi 4AlD 4.8 and LaNi 4MnD 5.9

By adopting our model for the calculation of the enthalpy of hydride formation, and by assuming that the number of bonds formed between the hydrogen and metal atoms in an alloy is proportional to the number of “broken” bonds for the metal atom, the interactive energy and site occupancy of deuterium in LaNi 5D 6.5, LaNi 4AlD 4.8 and LaNi 4MnD 5.9 are calculated and compared with experimental results. The agreement between the calculated and experimentally measured site occupancies is quite good with the exception that the occupation of the 3f and 4h sites in LaNi 5D 6.5 and LaNi 4MnD 5.9 cannot be predicted. The enthalpies of deuterium sorption are also calculated for the three alloys. The addition of aluminium or manganese increases the enthalpy of sorption.

Cadmium-ion distribution and motion in beta -alumina: An x-ray-diffraction and Raman-scattering investigation.

The structure and dynamics of a mixed cadmium-sodium \ensuremath{\beta}-alumina crystal, with composition (${\mathrm{Cd}}_{0.79}$${\mathrm{Na}}_{0.42}$O${)}_{1.22}$\ensuremath{\cdot}${11\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$, was investigated by x-ray diffraction and Raman spectroscopy. The conventional crystal-structure determination was carried out at room temperature, attaining R=0.043 in the least-squares refinement. Cadmium ions are close to mid-oxygen sites, but displaced by 0.168(5) A\r{} from the conduction plane. Sodium ions are located in 6h sites near the Beevers-Ross positions, and the bridging oxygen atoms are disorderly distributed slightly off the threefold axes. Al interstitial-vacancy pairs correlated with oxygen interstitials were detected. Polarized Raman spectra, obtained in the low-frequency region at some temperatures from 300 K down to liquid-helium temperature region, are analyzed. The results of the analysis of features connected with cadmium-ion vibrational dynamics in room-temperature spectra are consistent with the model of ionic distribution obtained by refining x-ray-diffraction data. The temperature evolution of some Raman bands is discussed in terms of cadmium-ion rearrangement.

Crystal structure of a barium hexagallate: Barium β-gallate

Single crystals of barium hexagallate have been grown in a Na 2OGa 2O 3 flux. The compound is hexagonal a = 5.859(2) A ̊ , c = 23.192(9) A ̊ . The crystal structure was determined by single crystal X-ray reflection data and refined to a final R-value of 0.018. The structure was revealed to be of β-alumina type, with a formula Ba 0.65Ga 10.80O 16.84, and not of magnetoplumbite type as it was conventionally considered. The Ba 2+ ion is located only in a 6h site, near the Beevers-Ross one. The charge compensation mechanism is that of gallium vacancies in the tetrahedra of the middle of the spinel block instead of that of Frenkel defects.

Nuclear Magnetic Resonance Study of YCo3

The nuclear magnetic resonance of 59 Co nuclei in magnetic domains of YCo 3- x Ni x (0≦ x ≦1) has been measured under external magnetic fields up to 55 kOe at 4.2 K. The hyperfine fields at the 3b, 6c and 18h sites in YCo 3 are found to be 35 kOe, 61 kOe and -42 kOe, respectively. The nuclear magnetic resonance of YCo 3- x Ni x with x =0.65, 0.8 and 1.0 under external magnetic fields shows a nearly paramagnetic state of Co atoms. Consistently, the magnetic moment of these compounds rapidly drops down around these concentrations.

Neutron diffraction in Ti 1.2Mn 1.8 deuteride: Structural and magnetic aspects

The hydrogen storage compound Ti 1.2Mn 1.8 is an off-stoichiometric hexagonal Laves phase (C14) where the excess titanium atoms are inserted in the manganese sublattice and are located exclusively on the 2a sites. This leads to the formation of Ti 3Mn tetrahedra which do not exist in stoichiometric TiMn 2. Our neutron diffraction study revealed that the three deuterium atoms per formula unit do not enter TiMn 3 or Mn 4 sites but are distributed among the Ti 2Mn 2 and the Ti 3Mn sites, where the latter have an increased occupancy because of their higher hydrogen affinity. An exclusive volume exists around each deuterium atom; owing to the high deuterium concentration this results in liquid-like short-range order with the first and second coordination spheres at about 2.1 Å and 2.8 Å respectively. The manganese atoms on the 6h sites in the magnetically ordered state of Ti 1.2Mn 1.8D 3 form ferromagnetic layers, whereas the other manganese atoms and the titanium atoms remain non-magnetic.

Conduction plane and structure of Li-stabilized Na+ β″-alumina: A powder neutron diffraction study

The structure of Li-stabilized Na+ β″-alumina has been refined by Rietveld analysis of high resolution time-of-flight powder diffraction data. The sample was a sintered tube of the type used for battery separators. The Li was found to reside in the Al(2) site in the spinel block. Fourier mapping and Rietveld analysis revealed that Na is located in both 6c and 18h sites (hexagonal setting of space group R3m) separated 1.03(3) Å in the conduction slab.

On an intermediate electronic configuration of cobalt (+III)

In a D 4h site with a strong elongation the 3A 2g term of cobalt (+III) can be stabilized with an electronic configuration intermediate between low spin ( 1A 1g) and high spin ( 5E g): d 2 xzd 2 yzd 1 xyd 1 z2d 0 x2 −y2. The La 2Li 0.5Co 0.5O 4 phase, whose layer structure favours strongly such a distortion of the CoO 6 octahedra, illustrates very well this possibility for cobalt (+III). 1A 1g → 3A 2g and 1A 1g → 5E g transitions appear as the temperature increases.

Fluoroberyllate mit apatitstruktur und ihre beziehungen zu sulfaten und silicaten

The fluoroberyllates Na 6Pb 4(BeF 4) 6F 2 and Na 6Ca 4(BeF 4) 6F 2 as well as the silicate Ca 6Th 4(SiO 4) 6O 2 have been synthesized. According to x-ray diffraction studies these phases crystallize in the apatite structure type. Intensity calculations reveal a distribution of the large cations for the Pb and Th compounds, where the Pb 2+ or Th 4+ ions are accumulated on the 6h sites of the apatite structure. The fluoroberyllates show interesting relations to sulfate apatites and certain silicate oxyapatites.

Magnetic properties of compounds RCoAl ( R = Pr, Nd, Gd, Tb, Dy, Ho, Er, TmandLu)

The magnetic properties of compounds RCoAl have been studied in fields up to 50 kOe in the range 4–300 K. The random environment of rare earth atoms in the MgZn 2-type structure (Co and Al occupy 2a and 6h sites in a statistical manner) dictates a state of magnetic scatter order. As a result, intrinsic magnetic hardness develops in these materials. This phenomenon is absent in crystallographically well-ordered ternary ferromagnetic compounds such as RNiAl (Fe 2P type). The strength of intrinsic magnetic hardness depends on the strength of exchange.

Model for the Composition-Dependent Ferromagnetic to Antiferromagnetic Transition in Fe2Ti

Mössbauer effect and magnetic susceptibility measurements on single-phase Fe2+xTi1−x confirm the finding of Nakamichi that the bulk magnetic properties are very sensitive to deviations from stoichiometry. Well below the Néel temperature antiferromagnetic titanium-rich material exhibits two narrow-line 57Fe Mössbauer spectra coresponding to the 2a and 6h sites. Ferromagnetic iron-rich material has additional components and a greatly broadened spectrum. This indicates that Fe2Ti is basically antiferromagnetic and suggests that the ferromagnetism arises from iron atoms in titanium sites which serve as nuclei for ferromagnetic clusters. These account quantitatively not only for the details of the Mössbauer spectra of iron-rich material, but also for the saturation magnetization and other features of the susceptibility data.