4f Sites Research Articles

The partitioning of La and Y in Nd–Fe–B magnets: A first-principles study

Partial replacement of Nd by Y and La helps to decrease temperature coefficients of remanent magnetization for Nd2Fe14 B (2:14:1) based magnets. The efficiency of La/Y doping depends on the distribution of La/Y in the multi-phase microstructure of a Nd–Fe–B magnet. To understand and control the La/Y redistribution, the partitioning of La/Y in 2:14:1 and Nd-rich grain boundary phase have been studied by a first-principles density functional calculation. The total energy calculations indicate that La and Y prefer to enter the 4g and 4f sites in the 2:14:1 phase, respectively. The substitution energies of La and Y in 2:14:1 are positive (0.41eV/atom) and negative (−0.38eV/atom), respectively. The results indicate that Y prefers to enter the 2:14:1 phase while La tends to be expelled from 2:14:1 phase to Nd-rich grain boundary phase. This is the thermodynamic origin for the different partition behavior of La and Y in Nd–Fe–B magnets, which affects the magnetic properties.

From Incommensurate Correlations to Mesoscopic Spin Resonance inYbRh2Si2

Spin fluctuations are reported near the magnetic field-driven quantum critical point in YbRh(2)Si(2). On cooling, ferromagnetic fluctuations evolve into incommensurate correlations located at q(0) = ±(δ,δ), with δ = 0.14 ± 0.04 r.l.u. At low temperatures, an in-plane magnetic field induces a sharp intradoublet resonant excitation at an energy E(0) = gμ(B)μ(0)H with g = 3.8 ± 0.2. The intensity is localized at the zone center, indicating precession of spin density extending ξ = 6 ± 2 Å beyond the 4f site.

IMPROVED MAGNETIC PROPERTIES OF Sm COMBINING Co OR/AND Zn SUBSTITUTED BARIUM M-TYPE HEXAFERRITES

Sm (Co or/and Zn) substituted nanocrystalline barium hexaferrites were synthesized by the sol–gel auto-combustion process, then X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were used to characterize and discuss the phase composition and the magnetic properties of the as-prepared barium hexaferrites. All results showed that the phase composition and magnetic properties were closely related to the doping elements and x. Owing to hyperfine field, canting spin, magnetic dilution and impurity phases, the saturation magnetization (Ms) of all samples increased firstly, and then decreased. Considering the crystallization, the magnetocrystalline anisotropy and the ions occupancy, the doping elements and x critically affected the coercivity (Hc). Compared the magnetic properties of all the samples, it is concluded that Zn 2+ and Co 2+ influenced each other and Zn 2+ occupied 4f2 sites prior to Co 2+, which led to the increase of Ms and the decrease of Hc.

Synthesis and crystal structures of new oxyapatites BiCa4−xLax(VO4)3−x(GeO4)xO, x=1–3

New germanate–vanadates with the general formula BiCa4−xLax(VO4)3−x(GeO4)xO, x=1−3, have been synthesized by the nitrate–citrate method and characterized. Structural refinement based on X-ray powder diffraction data showed that these compounds are isostructural with BiCa4(VO4)3O (space group P63/m (N 176), Z=2, а=9.8327–9.8755(3), and с=7.1203–7.2133(2)). They may be viewed as continuous series of solid solutions where Ca2+ and V5+ cations in the crystal lattice of vanadate BiCa4(VO4)3O are replaced by La3+ and Ge4+ cations. In the process of substitution, the La3+ cations occupy mainly the 4f lattice sites of the germanate–vanadate oxyapatites, but the filling of the 4f and 6h lattice sites in the germanate BiCaLa3(GeO4)3 is equivalent. IR and Raman spectra were studied. The Raman spectra clearly show a two-mode behavior: the lines of GeO4 and VO4 are separated from each other, and the (V/Ge)O4 tetrahedra exhibit a quasi-independent behavior.

Growth, Structure and Spectroscopic Characterization of Nd3+-Doped KBaGd(WO4)3 Crystal with a Disordered Structure

The undoped and the Nd3+:KBaGd(WO4)3 crystals were grown by the top seeded solution growth (TSSG) method from a flux of K2W2O7. The structure of the pure crystal was determined by the single-crystal X-ray diffraction method. It crystallizes in the monoclinic symmetry with space group C2/c. In the structure, K+ and Ba2+ ions share the same 8f site with occupancy of 0.464 and 0.536, respectively. The investigation of spectral properties of Nd3+:KBaGd(WO4)3 crystal indicates that it exhibits broad absorption and emission bands, which are attributed to locally disordered environments around the Nd3+ centers. The broad absorption band is suitable for diode laser pumping.

Ferromagnetism in ZrFe12−xAlx and HfFe12−xAlx (x = 6.0, 6.5, 7.0)

Results of crystal structure, microstructure, and magnetic properties studies of arcmelted and annealed ZrFe6Al6, ZrFe5.5Al6.5, ZrFe5Al7, HfFe6Al6, HfFe5.5Al6.5, and HfFe5Al7 are reported. These compounds adopt the ThMn12 structure-type (space group 139, I4/mmm), like their more common rare-earth analogues. Analysis of powder x-ray diffraction data show that the 8f site is occupied by Fe, the 8i site by Al, and the 8j site by a mixture of Fe and Al. All of the compounds undergo a ferromagnetic transition at temperatures ranging from 170 to 270 K. Three effects on the magnetic properties are noted as the Fe content is increased: decrease in the Curie temperature, decrease in the magnetic moment per Fe at high fields, and the development of non-monotonic temperature dependence of the low-field magnetization below the Curie temperature. First principles calculations indicate a ferromagnetic ground state, but find antiferromagnetic interactions among Fe moments on 8j sites which is likely important in understanding the observed behavior.

Crystal structure and magnetic properties of novel R3Co2.2Si1.8 compounds (R=Y, Gd–Tm)

The crystal structure of new ternary Dy3Co2.2Si1.8 compound has been established using single crystal X-ray diffraction, and those of R3Co2.2Si1.8 phases (R=Y, Gd–Tm) phases have been verified through the powder diffraction techniques. The Dy3Co2.2Si1.8 structure is a new structure type, which is obtained from the Er3Ge4 with an ordered Er3Ge4-type structure (space group Cmcm) one by coloring the two Ge sites, 4a and 4c, with Si and the 8f site with Co atoms.Tb3Co2.2Si1.8, Dy3Co2.2Si1.8, Ho3Co2.2Si1.8 and Er3Co2.2Si1.8 compounds display ferromagnetic-type ordering.

Neutron scattering study of ionic diffusion in Cu–Se superionic compounds

Paper reports the results of the neutron scattering study of crystal structure and diffusion of Cu2−δSe compounds in superionic α-phase. We found that the crystallographic model with Cu atoms in the 8c (0.25, 0.25, 0.25) and 32f (x, x, x) (x=0.33–0.39) sites in fluorite lattice provides the best description of the average structure. The quasi-elastic neutron scattering data reveal the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu atoms between diffusion hops. Combination of neutron diffraction, quasi-elastic scattering experimental data with the Bond-Valence Method simulations strongly suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.

Synthesis, Crystal Structure, Chemical Bonding, and Physical Properties of the Ternary Na/Mg Stannide Na2MgSn

A ternary stannide of sodium and magnesium, Na(2)MgSn, was synthesized from the elements, and the crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the Li(2)CuAs structure type (hexagonal, P6(3)/mmc, Z = 2, a = 5.0486(11) Å, c = 10.095(2) Å), and its structure is built up of two-dimensional honeycomb layers of (2)(∞)[(MgSn)(2-)] stacked along the c-axis, with Na atoms as "space fillers". First-principles computations at various levels of density functional theory (DFT) verify that the most stable configuration is the one in which Na and Mg atoms occupy the 4f and 2b sites, respectively, and thus DFT provides a necessary complement to X-ray structural elucidation. Our computations also predict that Na(2)MgSn must be a semiconductor with a small band gap. In accord with these predictions, the electrical resistivity measured for a polycrystalline sample of Na(2)MgSn is 9.6-10.4 mΩ cm in the range of 90-635 K, and the Seebeck coefficient decreases from +390 μV K(-1) (at 300 K) to +150 μV K(-1) (at 430 K).

The partitioning of Dy and Tb in NdFeB magnets: A first-principles study

Doping with Dy/Tb is a current solution to improve the coercivity and the operating temperature of NdFeB magnets, by increasing the magnetocrystalline anisotropy of the (Nd, Dy/Tb)2Fe14B (2:14:1) phase. The efficiency of Dy/Tb doping depends on the distribution of Dy/Tb in the multi-phase microstructure of the NdFeB magnet. To understand and control the Dy/Tb redistribution, the partitioning of Dy/Tb between 2:14:1 and Nd-rich phases has been studied by a first-principles density functional calculation. The total energy calculations indicate that Dy and Tb prefer to enter the 4f sites in the 2:14:1 phase. The substitution energies of Dy and Tb in 2:14:1 are negative and are −0.35 eV/atom and −0.33 eV/atom, respectively, as Nd is replaced with 25% of Dy/Tb, i.e., stabilizing the 2:14:1 structure. However, the substitution energies of Dy and Tb in NdO have large positive values. They are 0.83 eV/atom and 0.73 eV/atom for Dy and Tb, respectively, when 25% of Nd is replaced by Dy/Tb. The results indicate that Dy/Tb prefer to enter the 2:14:1 phase rather than the Nd-rich phase (NdO). This is the thermodynamic origin for the selective occupation of Dy and Tb in the 2:14:1 structure which enhances the magnetic anisotropy field.

On the magnetic structure of PrMn2O5: a neutron diffraction study

The long-range magnetic ordering of PrMn2O5 has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at TN ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k1 = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k2 = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k1 and k2. The magnetic structure of the k1-domain is given by the basis vectors (Cx,0,0) and (Cx′,0,0) for Mn(4h) and Mn(4f), respectively. In the k2-domain, the magnetic structure is defined by the basis vectors (0,0,Gz) and (Fx′,Gy′,0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k1, the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μB, respectively; for the magnetic phase associated with k2, the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μB, respectively.

Crystal structure of novel compounds in the systems Zr–Cu–Al, Mo–Pd–Al and partial phase equilibria in the Mo–Pd–Al system

The crystal structures of three Al-rich compounds have been solved from X-ray single crystal diffractometry: τ(1)-MoPd(2-x)Al(8+x) (x = 0.067); τ(7)-Zr(Cu(1-x)Al(x))(12) (x = 0.514) and τ(9)-ZrCu(1-x)Al(4) (x = 0.144). τ(1)-MoPd(2-x)Al(8+x) adopts a unique structure type (space group Pbcm; lattice parameters a = 0.78153(2), b = 1.02643(3) and c = 0.86098(2) nm), which can be conceived as a superstructure of the Mo(Cu(x)Al(1-x))(6)Al(4) type. Whereas Mo-atoms occupy the 4d site, Pd(2) occupies the 4c site, Al and Pd(1) atoms randomly share the 4d position and the rest of the positions are fully occupied by Al. A Bärnighausen tree documents the crystallographic group-subgroup relation between the structure types of Mo(Cu(x)Al(1-x))(6)Al(4) and τ(1). τ(7)-Zr(Cu(1-x)Al(x))(12) (x = 0.514) has been confirmed to crystallize with the ThMn(12) type (space group I4/mmm; lattice parameters a = 0.85243(2) and c = 0.50862(3) nm). In total, 4 crystallographic sites were defined, out of which, Zr occupies site 2a, the 8f site is fully occupied by Cu, the 8i site is entirely occupied by Al, but the 8j site turned out to comprise a random mixture of Cu and Al atoms. The compound τ(9)-ZrCu(1-x)Al(4) (x = 0.144) crystallizes in a unique structure type (space group P4/nmm; lattice parameters a = 0.40275(3) and c = 1.17688(4) nm) which exhibits full atom order but a vacancy (14.4%) on the 2c site, shared with Cu atoms. τ(9)-ZrCu(1-x)Al(4) is a superstructure of Cu with an arrangement of three unit cells of Cu in the direction of the c-axis. A Bärnighausen tree documents this relationship. The ZrCu(1-x)Al(4) type (n = 3) is part of a series of structures which follow this building principle: Cu (n = 1), TiAl(3) (n = 2), τ(5)-TiNi(2-x)Al(5) (n = 4), HfGa(2) (n = 6) and Cu(3)Pd (n = 7). A partial isothermal section for the Al-rich part of the Mo-Pd-Al system at 860 °C has been established with two ternary compounds τ(1)-MoPd(2-x)Al(8+x) and τ(2) (unknown structure). The Vickers hardness (H(v)) for τ(1) was found to be 842 ± 40 MPa.

Order–disorder transition in the (1−x)Li2TiO3–xMgO system (0 ≤ x ≤ 0.5)

The order–disorder phase transition of magnesium lithium titanate solid–solution (1−x)Li2TiO3–xMgO (0 ≤ x ≤ 0.5) ceramics prepared by conventional solid-state processing has been examined. The phase and structural analysis was carried out using electron diffraction, neutron diffraction and high-resolution transmission electron microscopy. Both electron and neutron diffraction results revealed the onset of an order-to-disorder transition at 0.3 < x < 0.4. Superlattice reflections found in certain regions of x = 0.2 samples and most areas of x = 0.3 samples were caused by a twin structure stabilized by Mg incorporation. Rietveld refinements of neutron diffraction data suggested a random distribution of Mg on the Li 4e sites and equal distribution of Mg on the two Ti 4e sites for x ≤ 0.3. As the Mg content continues to increase, the crystal symmetry transforms from monoclinic to cubic rocksalt. Consequently, the cation ordering on the 8f and 4d sites of the C2/c structure became corrupted and turned into short-range ordering on the 4a sites of a cubic structure with symmetry, resulting in diffuse scattering in electron diffraction patterns.

The influence of oxygen deficiency on structural and electronic properties of layered superconductor (Fe2As2)(Sr4V2O6−x )

By means of the first-principles FLAPW-GGA approach, we studied the influence of oxygen deficiency on the structural and electronic properties of the superconducting phase (Fe2As2)(Sr4V2O6−x). We find that the formation of vacancies in 2c sites of oxygen sublattice [when the initial tetragonal structure of the stoichiometric crystal (Fe2As2)(Sr4V2O6) is kept] is energetically more preferable than in 4f sites, when orthorhombic distortions arise. The influence of oxygen vacancies on the structure of (Fe2As2) blocks is very insignificant; thus, using the known structural indicators, it is impossible to explain the observed drop of transition temperature TC for oxygen-deficient systems by structural factors. On the other hand, the results of the band structure calculations allow us to assert that the vacancy-induced drop of TC may be caused by the electronic factor, i.e., by the established decrease in Ntot(EF) for the oxygen-deficient system.

Dynamic orientational disorder in crystals of fluoroelpasolites, structural refinement of (NH4)3AlF6, (NH4)3TiOF5 and Rb2KTiOF5

Room-temperature crystal structures of triammonium hexafluoroaluminate, (NH(4))(3)AlF(6) (I), and triammonium oxopentafluorotitanate, (NH(4))(3)TiOF(5) (II), were refined, and the crystal structures of dirubidium potassium oxopentaflourotitanate, Rb(2)KTiOF(5), at 297 K (III) and 218 K (IV) were determined using single-crystal X-ray diffraction techniques. In ammonium fluoroelpasolites [(I) and (II)], the ligand (O, F) atoms are located in the mixed 24e + 96j position of the Fm3m unit cell. The 24e position is occupied by the ligand atoms predominantly in (III) and fully in (IV). 'Ordered' N1 and Rb atoms are tetrahedrally displaced from the 8c position into the 32f site, and the H atoms of the disordered ammonium group N2 are statistically distributed in the 96k and 32f positions. The Ti atoms in (II) and (IV) are shifted from the 4a position to 24e thus allowing identification of the O and F atoms in the octahedron on a local scale. The disorder in the crystals is of a dynamic nature. Unique Raman spectra of Rb(2)KTiOF(5) under the laser beam of 1064 nm indicate fast octahedral reorientations resulting in physical equalizations of the Ti-O and Ti-F distances as well as in the appearance of totally synchronous Ti-O and Ti-F stretching vibrations at 750 cm(-1). This phenomenon is assumed to also take place under X-rays.

Diffusion of water in molecular magnet Cu0.75Mn0.75[Fe(CN)6]⋅7H2O

Here we report the dynamical behaviour of water in Prussian blue analogue (PBA) Cu0.75Mn0.75[Fe(CN)6]⋅7H2O molecular magnet in the temperature range 260–360 K as studied using the quasielastic neutron scattering technique. While significant quasielastic broadening is observed in the hydrated sample, no broadening was observed in the dehydrated one. Data analysis showed that the observed quasielastic broadening in Cu0.75Mn0.75[Fe(CN)6]⋅7H2O corresponds to the dynamics of the non-coordinated water molecules at the 32f site and the coordinated water molecules at the 24e site, existing in the cavities created by the absence of Fe(CN)6 units. The non-coordinated water molecules at 8c interstitial sites do not contribute to the broadening, suggesting that they are immobile at least within the time window of the spectrometer used. Behaviour of the elastic incoherent structure factor is consistent with the model where the water molecules undergo translational diffusion localized within the cavity of 5.1 Å. While all the non-coordinated water molecules at the 32f site are dynamic over the entire range of temperatures, the coordinated ones at the 24e site become progressively dynamic with temperature. The water molecules were found to undergo hindered ( ∼ 1.16 × 10−5 cm2 s−1 at 300 K) diffusion compared to bulk water and the diffusivity followed Arrhenius behaviour within the measured temperature range with an activation energy of 1.26 kcal mol−1.

Structural stability and electronic properties of the new superconductor(Ca4Al2O6 − x)(Fe2As2) from first-principles study

Density-functional theory calculations are performed on(Ca4Al2O6 − x)(Fe2As2) to address the structural stability and electronic properties. The total-energycalculations show that the ground state of the parent compound(Ca4Al2O6)(Fe2As2) has striped anti-ferromagnetic order with anti-ferromagnetic couplingbetween Fe layers. The oxygen vacancy formation energy at the 4f site (∼6.02 eV) is smaller thanthat at the 2c site (∼6.33 eV) for the (Ca4Al2O6 − x)(Fe2As2) (x = 1) phase, which means oxygen vacancies can be easily formedat the 4f site. Further studies show that the ground state of(Ca4Al2O6 − x)(Fe2As2) (x = 0.25) is identical tothe case of (Ca4Al2O6)(Fe2As2), which implies that the spin density wave (SDW) may coexist with superconductivity in the(Ca4Al2O5.75)(Fe2As2) phase. The densitiesof states (DOSs) of (Ca4Al2O6 − x)(Fe2As2) (x = 0, 0.25) show that both spin-up and spin-down DOSs at the Fermi energy level become largerobviously due to oxygen vacancies being introduced, which means introducing oxygen vacancies in(Ca4Al2O6)(Fe2As2) is beneficial for improving its conductivity. These results will be helpful for synthesizingnew 42 622-type structured iron-based superconductors with oxygen vacancies.

Effect of titanium ion substitution in the barium hexaferrite studied by Mössbauer spectroscopy and X-ray diffraction

A series of M-type barium hexaferrite has been synthesized in a glass melt by partially substituting the Fe2O3 with TiO2 for investigation of their structure. The glass melt has the basic composition (mol%): 40 BaO + 33 B2O3 + (27-x) Fe2O3 + x TiO2 with x = 0, 3.6, 5.4 and 7.2 mol% TiO2. The substituted ferrites were studied by means of X-ray diffraction, Mössbauer spectroscopy and vibration sample magnetometer. X-ray diffraction studies revealed that not all samples have a single ferritic phase, a small second phase corresponding to BaTi6O13 was also observed to form. The Mössbauer spectra changed from magnetically ordered (x = 0) to magnetically ordered with strong line broadening. Moreover, the broadening increases with TiO2 content. The Mössbauer parameters suggested that Ti4 + occupies the 2a and 12k crystal sites, and the Ti4 + substitution on the 2b and 4f2 site also occurs at high melt dopings. Therefore, coercivity and saturation magnetization decreased.

Atomistic configurational effects on piezoelectric properties of La3Ta0.5Ga5.5O14 and a new piezoelectric crystal design

Single crystalline langatate (LTG, La3Ta0.5Ga5.5O14) has been widely used as a sensor material in high temperature applications because of its thermally stable piezoelectric properties. In this research, to elucidate the relationship between piezoelectric tensors and local ionic configurations, first-principles calculations based on density functional perturbation theory (DFPT) were performed on various local ionic structures. The results indicate that two independent relaxed-ion piezoelectric coefficients, e11 and e14, increased with increases in La(3e)–O and Ta(1a)–O distances or decreases in Ga(3f,2d)–O distances. Thus, to obtain high piezoelectric constants in this crystal, ions larger than La3+ should be incorporated at 3e sites to open the distance between 3e ions and oxygen ions, and ions smaller than Ga3+ should be introduced at 2d and 3f sites to reduce the distance between Ga and O ions. Finally, from this design rule, a new crystal, BTAS (Ba3TaAl3Si2O14), which belongs to the same P321 group, is proposed. The calculated relaxed-ion piezoelectric coefficient e11 of BTAS was 17.7% higher than the coefficient of a LTG crystal. This significant increase confirms BTAS as a useful new piezo-material, especially in applications where there is also a need to reduce the use of more expensive elements.

Remarkable influence of carbon nanotubes on microwave absorption characteristics of strontium ferrite/CNT nanocomposites

In this research work, magnetic multi-walled carbon nanotube (MWCNT) nanocomposites have been created by the assembly of Zn–Sn substituted strontium ferrite film onto the surface of MWCNTs. X-ray diffraction and transmission electron microscopy were used to demonstrate the successful attachment of ferrite films to MWCNTs. Mössbauer spectroscopy indicates that the Zn–Sn ions preferentially occupy the 2b and 4f2 sites. Vibrating sample magnetometry confirms the relatively strong dependence of saturation magnetization with the volume percentage of MWCNTs. Microwave absorption of the MWCNTs/doped strontium ferrite nanocomposites is evidently enhanced compared to that of pure MWCNTs and ferrite. The maximum reflection loss increased significantly with an increase in volume percentage of MWCNTs in nanocomposites. Reflection loss evaluations indicated that the nanocomposites display a great potential application as wide-band electromagnetic wave absorbers.