Nonequivalent Sites Research Articles

Ab initio study of hydrogen diffusion in Be and Be12Ti for fusion applications

Beryllium and Be12Ti are considered as neutron multiplier materials in various tritium breeding blanket designs for future fusion reactors. Tritium and helium are formed due to neutron-induced nuclear transmutation and accumulated in beryllium-based materials. As a β-radioactive isotope, tritium represents additional safety concern with respect to sudden release during operation or hindering consequent recycling. Therefore, detailed study of its accumulation and release is indispensable. In particular, it is important to reveal which process controls tritium release: binding of tritium with traps or diffusion out of material bulk. Ab initio calculations of solution energies at all non-equivalent hydrogen interstitial sites confirm easier dissolution of tritium in Be12Ti in comparison with pure beryllium. The 17 considered diffusion barriers between adjacent hydrogen interstitial sites are calculated using a dimer method instead of a computationally more expensive nudged elastic band approach. Three possible diffusion paths in Be12Ti were revealed: two of them along the c-axis and another one along the a- and b-axes. Considering calculated hydrogen diffusion barriers in Be12Ti, one can conclude that at low hydrogen concentrations and low temperatures, diffusion occurs preferentially along the c-axis. The rate limiting diffusion barriers in Be12Ti are found to be in the range of 0.4–0.5 eV, which are comparable to 0.42 eV in pure beryllium suggesting that tritium diffusion in both materials proceeds at comparable rates. Therefore, binding of tritium atoms with vacancies and other lattice imperfections are expected to play a crucial role in tritium retention.

Influence of local lattice structure on magnetic properties in Y2Fe17 compounds

A theoretical study is performed on the relation between magnetic properties and lattice structures, that is, lattice constants and local atom displacement, for rhombhedral (rh-) ${\mathrm{Y}}_{2}{\mathrm{Fe}}_{17}$ compounds. We use real-space full-orbital tight-binding formalism to calculate electronic states. Magnetic anisotropy (MA) is calculated with high numerical accuracy by adopting a second-order perturbation for spin-orbit interaction. It is shown that the local magnetic moments of Fe atoms on 9d and 18h sites increase with increasing lattice volume. The result is attributed to the high atomic area density of these sites on the hexagonal planes. Those of Fe atoms on the other sites are found to be nearly independent of the volume. We calculate local MA energy of Fe atoms on each nonequivalent site and find that the magnitude of the local MA is strongly affected by the local atom displacement. As a result, the MA of ${\mathrm{Y}}_{2}{\mathrm{Fe}}_{17}$ is shown to be sensitive to volume.

(Ga,Ge)-Analogue of Tourmaline: Crystal Structure and Composition

Crystals of a (Ga,Ge)-analogue of tourmaline have been synthesized by hydrothermal method in the temperature range of 600–650°C under a pressure of 100 MPa. The composition is analyzed, and the structure of single crystals is determined by X-ray diffraction (XRD) analysis. The rhombohedral-cell parameters are found to be a = 16.0102(4) A, c = 7.18423(19) A, sp. gr. R3m, and Z = 3. The distribution of cations over nonequivalent sites in the structure under study is determined to be as follows: Na0.57(Al1.20Ga1.20 $${\text{Fe}}_{{0.60}}^{{2 + }}$$ )(Al4.728 $${\text{Fe}}_{{1.20}}^{{2 + }}$$ Ti0.072)((Si4.80Ge0.78Al0.42)O18)(BO3)3(OH)3(O0.42ОН0.58). The results obtained confirm possibility for obtaining Ge-substituted crystals with tourmaline structure, which (by analogy with the Ge-analogue of quartz) exhibit high piezo- and pyroelectric properties.

Magnetic anisotropies of La–Co substituted M-type Sr hexaferrites studied by 57Fe Mössbauer spectroscopy with external magnetic fields

Magnetoplumbite-(M)-type hexaferrites are the most widely used base materials of permanent magnets. La–Co substitution in these M-type Sr hexaferrites improves these magnetic anisotropies. By 57Fe Mössbauer spectroscopy using the single-crystalline samples, we have investigated the Fe electronic states in the La–Co substituted M-type Sr hexaferrites, Sr1−xLaxFe12−yCoyO19, as a function of an external magnetic field to discuss the enhancement of these magnetic anisotropies. All observed 57Fe Mössbauer spectra were well analyzed using the five subspectra corresponding to the five crystallographic nonequivalent Fe sites. At one site of these five Fe sites, the 2a Fe site, the extracted hyperfine interaction parameters clearly show the Co substitution and external magnetic field dependences, revealing the increments of the 3d electrons in the Fe ions, which are caused by the substitution of Co2+ ions for Fe3+ ions at the neighbor 4f1 Fe site. This hybridization between the Fe ions and the Co ions induces the non-negligible unquenched orbital momentum in the Fe ions at the 2a Fe site. Our findings represent an enhancement of the magnetic anisotropies in Sr1−xLaxFe12−yCoyO19.

Effect of Ce substitution on the structural and magnetic properties of Nd2Fe14B

The effect of Ce substitution on the structural and magnetic properties of (Nd1-δCeδ)2Fe14B (0 < δ < 1) series was systematically studied using neutron diffraction, 57Fe Mössbauer spectroscopy and X-ray magnetic circular dichroism (XMCD). An anomaly in the lattice parameters was observed in a sample with composition in the range of 0.2 < δ < 0.4, where a phase separation happens and the 2:14:1 isostructural dual-main-phases (IDMPs) appear. Outside this composition range, Ce shows a preferential occupation at the 4g site than the 4f site and a preferable Ce3+ valence state. The average magnetic moments of Nd/Ce and Fe atoms decrease with the increasing of δ. The magnitude of the Fe moments at non-equivalent sites is related to several factors, such as the Fe coordination number, the Wigner-Seitz cell volume, and the Boron atom effect. These results reveal the microscopic mechanisms for the structural and magnetic properties of Ce-substitution Nd2Fe14B, providing perspective on developing the next-generation low-cost and high-performance permanent magnetic materials.

Magnetic compensation at two different composition ratios in rare-earth-free Mn4−xCoxN ferrimagnetic films

Ferrimagnets in the vicinity of the magnetic compensation point are of particular interest for spin-torque-based spintronics applications, because they require lower switching currents and thus smaller switching energies. Room-temperature compensation is rather uncommon, but can be obtained by tuning the chemical concentrations of compounds such as ${\mathrm{Co}}_{0.79}{\mathrm{Tb}}_{0.21}$ or ${\mathrm{Co}}_{0.56}{\mathrm{Gd}}_{0.44}$, in order to find the single concentration at which one sublattice compensates the other. Here we study the ferrimagnetic ${\mathrm{Mn}}_{4\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}\mathrm{N}$ films, showing that they presumably possess at room temperature not only one but two different Co concentrations at which the magnetic compensation occurs. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we show that this startling behavior can be explained by the existence of two nonequivalent sites for the Co, when it is inserted in the antiperovskite structure of epitaxial ${\mathrm{Mn}}_{4\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}\mathrm{N}$ films.

Raman spectroscopic studies of O–H stretching vibration in Mn-rich apatites: A structural approach

Abstract The O–H stretching vibration mode in crystals of (Mn,Cl)-rich and F-poor minerals of the apatite-supergroup has been studied by micro-Raman spectroscopy. The main purpose was to check if such an analysis can provide a quick and simple method to assess the distribution of Ca and Mn together with traces of Fe + Mg (= Mn*) on nonequivalent cationic sites in the apatite structure, especially in small and strongly heterogeneous crystals directly in thin sections. The O–H stretching vibration mode can then be treated as a useful structural probe giving information on the M2 occupants bonded to XOH. Pieczkaite, with the empirical formula (Mn4.49Fe0.47Ca0.05Mg0.01)Σ5.01P2.99O12[Cl0.83(OH)0.17], displays the O–H stretching mode centered at ~3380 cm–1, which shows that the complete replacement of Ca by Mn* at the M2 site is connected with a shift of the O–H stretching band ~192 cm–1 toward lower wavenumbers in relation to the O–H Raman band position reported for hydroxylapatite. The value is high enough to be an indicator of the M2Mn*···OH content in any sample of Mn-enriched apatite. Studies of the fine structure of the band disclosed its dependence on (1) the local combinations of Ca and Mn* forming triplets of M2 cations bonded to the X anion, (2) the presence of OH+Cl at the two half-occupied X sites that form chemical bonds with the M2 cations varying in strength and length, and (3) the spatial geometry of the X–M2 bonds and polarizability of the monovalent X anion by varying cations in the M2M2M2 triplets. The deconvolution of the band into maximum eight component bands with constant Raman shifts opens the possibility of evaluating the averaged M2M2M2 triplet bonded to oxygen of the XOH group. If the OH/(OH+Cl) fraction is known, the amounts of Ca and Mn* bonded to XOH can also be estimated. Application of the method to the holotype parafiniukite showed a slightly different distribution of Ca in M2M2M2 triplets than had been assumed from single-crystal X-ray diffraction. However, it corroborates suggestions that in the apatite structure there may be a preference for M2Ca to be bonded to XOH and M2Mn* to XCl. Our results show that the proposed method can be used as an independent tool in structural studies of Mn-rich minerals of the apatite-supergroup, providing results complementary to single-crystal X-ray diffraction. This method can easily be adjusted to modern apatite-type nanomaterials synthesized for biomedical and various industrial applications.

Structure, magnetic and thermodynamic properties of heterometallic ludwigites: Cu2GaBO5 and Cu2AlBO5

We present an extensive study of the structural, magnetic and thermodynamic properties of high-quality monocrystals of two heterometallic oxyborates from the ludwigite family: Cu2GaBO5 and Cu2AlBO5 in the temperature range above 2 K. The distinctive feature of the investigated structures is the selective distribution of Cu and Ga/Al cations. The unit cell of Cu2GaBO5 and Cu2AlBO5 contains four nonequivalent crystallographic sites of metal ions. Two sites in the structure of Cu2GaBO5 are fully occupied by Cu atoms which form the quasi one-dimensional chains along the a-axis. For Cu2AlBO5 all sites are partially occupied by Al and Cu atoms. The joint analysis of low-temperature data on magnetic susceptibility and magnetic contribution to the specific heat showed that Cu2AlBO5 and Cu2GaBO5 exhibit an antiferromagnetic transition at TN=2.4 and 4.1 K, respectively. The magnetic response below TN can be represented as the sum of two contributions: the ordered component and the paramagnetic contribution from defects of the same relative weight. It is shown that the external magnetic field above 2.5T and 2.8T for Cu2GaBO5 and Cu2AlBO5, respectively, leads to a broadening of the magnetic phase transition indicating suppression of the long-range antiferromagnetic order.

Polymorph and Polytype Identification from Individual Mica Particles Using Selected Area Electron Diffraction

AbstractDioctahedral micas are composed of two tetrahedral sheets and one octahedral sheet to form TOT or 2:1 layers. These minerals are widespread and occur with structures differing by (1) the layer stacking mode (polytypes), (2) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (3) the charge-compensating interlayer cation and isomorphic substitutions. The purpose of the present study was to assess the potential of parallel-illumination electron diffraction (ED) to determine the polytype/polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns were calculated along several zone axes close to the c*- and c-axes using the kinematical approximation for trans- and cis-vacant varieties of the four common mica polytypes (1M, 2M1, 2M2, and 3T). When properly oriented, all ED patterns have similar geometry, but differ by their intensity distribution over hk reflections of the zero-order Laue zone. Differences are enhanced for ED patterns calculated along the [001] zone axis. Identification criteria were proposed for polytype/polymorph identification, based on the qualitative distribution of bright and weak reflections. A database of ED patterns calculated along other zone axes was provided in case the optimum [001] orientation could not be found. Various polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered.

Proving the Dual Electronic Structure of Charged Metal-Molecule Interfaces: Surface-Enhanced Raman Scattering of Cyanide Adsorbed on a Nanostructured Silver Electrode

Experimental and theoretical calculations confirm, for the first time, the existence of two different kinds of electronic structures of a surface complex formed by a particular molecule bonded to charged metal electrodes, clusters, or nanoparticles. Surface-enhanced Raman scattering (SERS) of cyanine adsorbed on a silver electrode shows three regions, which are selected by the electrode potential and characterized by the differentiated response of the vibrational wavenumbers of the ν(CN) stretching band to the electrode potential. The combination between the experimental SERS and DFT calculations has allowed for relating the three regions to chemisorbed (C-hybrid) and physisorbed (P-hybrid) surface complexes, where cyanide is bonded through the carbon on top of a single silver atom of the surface and to bidentate species, respectively. The electrode potential selects one or another type of electronic structure of the surface complex, which are of different natures and with a differentiated response to the applied potential. The electric potential tunes smoothly the wavenumbers, bond energies, and injected charges of the P-hybrid at more negative potentials than that of the zero charge of the electrode, but the very strong C-hybrid prevents significant changes of these properties at positive excesses of charge. The existence of the dual electronic structure of metal-molecule interfaces might require reinterpreting experiments that are usually discussed by resorting to, for instance, the reorientation of the adsorbate, the formation of complexes with different stoichiometries, the existence of nonequivalent local sites on the surface, or to instrumental artifacts.

Equilibrium solute segregation to matrix- θ′ precipitate interfaces in Al-Cu alloys from first principles

Particular combinations of solute atoms segregated to the interface of the Al and ${\ensuremath{\theta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{2}\mathrm{Cu}$ in Al-Cu alloys can help stabilize ${\ensuremath{\theta}}^{\ensuremath{'}}$ precipitates at high temperatures. Stabilization of such precipitates is determined by a combination of thermodynamics (including driving forces for coarsening and transformation and solute segregation tendencies) and kinetic effects (including solute diffusion and interfacial mobility in the presence of interfacial solute segregation). For some alloys such as recent Al-Cu-based alloys, multiple solutes segregate in significant quantities to interfaces, and solute-solute interactions at the interface are important, with multiple types of solutes competing for similar interfacial sites. To treat this situation, we develop and apply a statistical mechanics approach to calculate the temperature-dependent equilibrium solute atoms distribution near the coherent and semicoherent interfaces between the Al matrix and the ${\ensuremath{\theta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{2}\mathrm{Cu}$ precipitates. The developed approach is applied to the investigation of Si, Mn, and Zr segregation at the interface, as particular combinations of these elements affect the thermal stability of the ${\ensuremath{\theta}}^{\ensuremath{'}}$ precipitates. We demonstrate that because Si and Mn atoms segregate on the same semicoherent interface, the presence of Si reduces the concentration of Mn solutes at the interface. Si atoms preferably occupy the first layer of the interface and compete with Mn atoms for one type of particular sites in the layer. Mn atoms preferably occupy the second layer of the semicoherent interface, and the Mn-Mn interaction plays an important role in their distribution. Zr atoms mostly segregate on one of the two nonequivalent sites of the second layer of the coherent interface. Due to symmetry properties of the coherent interface, the calculations show that the segregation Zr of atoms to this interface will likely lead to the formation of ${\mathrm{L}1}_{2}$ ordered ${\mathrm{Al}}_{3}\mathrm{Zr}$ layer.

Octacyanidorhenate(V) Ion as an Efficient Linker for Hysteretic Two-Step Iron(II) Spin Crossover Switchable by Temperature, Light, and Pressure.

A two-step hysteretic FeII spin crossover (SCO) effect was achieved in programmed layered Cs{[Fe(3-CNpy)2 ][Re(CN)8 ]}⋅H2 O (1) (3-CNpy=3-cyanopyridine) assembly consisting of cyanido-bridged FeII -ReV square grid sheets bonded by Cs+ ions. The presence of two non-equivalent FeII sites and the conjunction of 2D bimetallic coordination network with non-covalent interlayer interactions involving Cs+ , [ReV (CN)8 ]3- ions, and 3-CNpy ligands, leads to the occurrence of two steps of thermal SCO with strong cooperativity giving a double thermal hysteresis loop. The resulting spin-transition phenomenon could be tuned by an external pressure giving the room-temperature range of SCO, as well as by visible-light irradiation, inducing an efficient recovery of the high-spin FeII state at low temperatures. We prove that octacyanidorhenate(V) ion is an outstanding metalloligand for induction of a cooperative multistep, multiswitchable FeII SCO effect.

Probing structural and photophysical features of Eu3+ activated NaCdPO4 orthophosphate phosphor.

A series of Eu3+ doped NaCd1-xPO4: x Eu3+ (0≤x≤0.1) phosphors were synthesized and a systematic investigation of structural, morphological, optical and photophysical properties were carried out. Structural characteristics were analyzed both qualitatively as well as quantitatively to establish the phase purity, supported by FTIR and Raman spectroscopy methods. The valance state of the dopant, as well as surface elemental compositions were investigated by XPS technique. The optical characteristics were analyzed by employing UV-Visible DRS, which shows that the host bandgap is getting broadened upon doping as a result of the Burstein-Moss (BM) effect. Eu3+ doped phosphor exhibits emissions rich in red (617nm) under suitable excitations at 393nm while the optimal doping concentration was found to be around x=0.06. The effective non-radiative transfer of energy from host to activator is governed by the dominant dipole-dipole transitions. The hypersensitive transition line 5D0→7F2 is observed owing to the induced electric dipole (ED) transition of Eu3+ centered at 617nm. Local site symmetry has been analyzed to ascertain the environment around Eu3+ and presence of any non-equivalent cation sites. The concentration quenching effect of phosphors was explained on the basis of Dexter's theory and charge compensation mechanism. The Commission Internationale de l'Eclairage (CIE 1931) chromaticity coordinates for the prepared phosphors were estimated and found to lie in the red region of color space. The photoluminescence decay time was measured for the most intense emission line 5D0→7F2 at 617nm under 393nm excitation and the results indicate that Eu3+ activated NaCdPO4 can be a suitable red phosphor for white LED using UV-LED chip owing to it's near UV-excitation characteristics.

Atomic cooperation in enhancing magnetism: (Fe, Cu)-doped CeCo5

Abstract Developing permanent magnet alloys with decreased critical elements (e.g., Nd, Dy, and Co) requires identifying compositions and structures with uniaxial magneto-crystalline anisotropy energy (MAE), large magnetization, and a high ferromagnetic transition temperature (Curie temperature - TC). One approach to minimizing the critical elements in potential permanent magnet alloys is to use overly produced Ce, which is less critical. Furthermore, reducing Co content in RCo5 (R = Rare Earth) alloys is necessary since Co is also a critical element. An obvious choice for decreasing Co content is a substitution with non-critical Fe. However, the Fe is not stable in the lattice due to the reduced number of d-electrons. Concomitant substitution of Cu stabilizes Fe substitution. Employing first-principles electronic structure theory, we identify the weakly localized nature of cobalt in CeCo5, which causes high uniaxial magnetic anisotropy of ∼10 MJ/m3. In contrast, substituted Cu delocalizes the Co’s 3d-states, resulting in lower anisotropy. Calculations show that 10% Cu can stabilize 20% Fe subsituted for Co, which significantly enhances magnetic moment in the Ce (Co, Fe, Cu)5. This prediction is in good agreement with a single-crystal experiment in which the optimal composition was identified to be 15% of Fe and 12% Cu. The competitive non-equivalent Co sites preferred by Cu and Fe, a unique electronic structure including exchange and crystal field splitting, and rigid band shift variation are borne by 3d states of Co, Fe, and Cu around the Fermi level, all play an essential role in tuning the magnetic properties.

Fe2+ partitioning between the M1 and M2 sites in silicate crystals in some stony and stony-iron meteorites studied using X-ray diffraction and Mössbauer spectroscopy

Study of the Fe2+ partitioning among the crystallographically non-equivalent M1 and M2 sites in silicate crystals such as olivine, orthopyroxene and clinopyroxene is of interest to analyze their features and thermal history. Especially, this is important for investigation of extraterrestrial silicate crystals in meteorites. We discuss the results of the study of Fe2+ partitioning among the M1 and M2 sites in silicate crystals in some stony and stony-iron meteorites based on the data obtained by means of two independent techniques: X-ray diffraction and Mössbauer spectroscopy with a high velocity resolution. It is possible to evaluate the ratios of the Fe2+ occupancies of the non-equivalent M1 and M2 sites in olivine, orthopyroxene and clinopyroxene crystals. Estimation of the Fe2+ and Mg2+ cations distribution coefficient and the temperature of equilibrium cation distribution for olivine and orthopyroxene in some stony and stony-iron meteorites is also considered. A good agreement between results obtained by means of two independent techniques is shown.

Ordering Phase Transition with Symmetry-Breaking from Disorder over Non-Equivalent Sites: Calorimetric and Crystallographic Study of Crystalline d-Sorbose

Phase transitions in the crystalline state of chiral sorbose were examined using precise heat capacity calorimetry and X-ray crystallography. The calorimetry established heat capacity below room temperature. Besides the known transition (main transition) at 199.5 K, the calorimetry detected plural thermal anomalies assignable to new phase transitions (around 210 K) and a glass transition (at ca. 120 K). The X-ray diffraction at low temperatures established the crystal structure of the lowest temperature phase. The identification of the broken symmetry upon the main transition solves an apparent contradiction that the structural disorder reported previously does not contribute seemingly to the symmetrization.

Theoretical Simulation of the Binding Energies and Stretching Frequencies of CO Molecules on PtSn Bimetallic Nanoparticles

The adsorption of CO molecules by Pt and Pt–Sn nanoparticles is studied using ab initio calculations. The Pt–C and C–O bond lengths, adsorption energies, and stretching frequencies of adsorbed molecules on the surface of nanoparticles at different sites are calculated. The calculated values are in good agreement with the experimental data. The segregation of bimetallic nanoparticles is also observed. The configuration of a nanoparticle in which platinum atoms are in the nucleus and tin atoms are on the surface is found to be energetically most favorable. The effect of Sn atoms near Pt on the density of electronic states of platinum and the stretching frequency of adsorbed CO molecules is also studied. Tin in the first coordination sphere is found to decrease the CO stretching frequency. The results obtained will allow quantitative interpretation of the infrared spectra of bimetallic nanoparticles during the catalytic reaction to analyze the surface morphology. The constructed adsorption energy maps will make it possible to predict the occupancy of nonequivalent adsorption sites at different temperatures and gas pressures.

Dimer Physics in the Frustrated Cairo Pentagonal Antiferromagnet Bi_{2}Fe_{4}O_{9}.

The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in Bi_{2}Fe_{4}O_{9} materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.

Combining XRD and SEM techniques with site selective spectroscopy for structural and spectroscopic studies of Nd3+-doped LuPO4 micro-powders

We present and discuss the structural and spectroscopic properties of micro-crystalline Nd3+-doped LuPO4 zircon-(xenotime)-type orthophosphates (0.5–20 mol% of Nd3+ ions) synthesized by high-temperature solid-state reaction, in which Nd3+ optical rare earth ion plays the role of a structural probe. Surprisingly, in the literature there are not many studies performed for this optical material. The basic structural research with detailed information about the non-equivalent symmetry sites for the powdered samples with different Nd3+ ion concentrations is still missing. It is very important to fulfill this gap in order to properly apply this optical material. A series of Nd3+-doped LuPO4 micro-powders were obtained by using molar ratio of Lu2O3, Nd2O3, and NH4H2PO4 as the starting materials. Also, for some samples the 10% of ammonium dihydrogen phosphate excess was applied. The XRD and SEM analysis reveal some dependence of Nd3+ ion concentration and synthesis route on the crystal structure. The low-temperature high-resolution techniques like absorption spectroscopy at 4.2 K and site-selective tuneable laser spectroscopy at 77 K allowed to take advantage of Nd3+ ion as a probe for local ordering due to the close relation between its spectroscopic properties and the local structure at the ion site. To the best of our knowledge such a study has not been reported for micro-powdered Nd3+-doped LuPO4.

Nuclear forward scattering application to the spiral magnetic structure study in ε−Fe2O3

The $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure has been analyzed using the synchrotron radiation source. Time spectra of nuclear forward scattering for isolated nanoparticles with an average size of 8 nm immobilized in a xerogel matrix have been recorded in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in applied magnetic fields of $0--4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ in the longitudinal direction at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It has been found that the external magnetic field does not qualitatively change the ${H}_{\mathrm{hf}}(\mathrm{T})$ behavior, but makes a strong opposite impact on the hyperfine fields in the nonequivalent iron sites, leading to the divergence of ${H}_{\mathrm{hf}}$ polar angle dependences below 80 K. A complete diagram of the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is proposed. At 300 K, the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ compound is confirmed to be a collinear ferrimagnet. The experimental results show that the magnetic transition at $150--80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ leads to the formation of a noncollinear magnetic structure. Furthermore, in the range of the 80--4 K, the ground state of a magnetic spiral is established. The experimental results are supplemented by the analysis of the exchange interactions and temperature dependence of the magnetization in a magnetic field of 7 T.