8i Sites Research Articles

Theoretical Study of the Magnetic Properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H Compounds

We present theoretical investigations examining the electronic and magnetic properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H compounds, including magneto-crystalline anisotropy, magnetic moments, exchange-coupling parameters, and Curie temperatures. The spin-polarized fully relativistic Korringa–Kohn–Rostoker (SPR-KKR) band structure method has been employed, using the coherent potential approximation (CPA) to deal with substitutional disorder. Hubbard-U correction was applied to the local spin density approximation (LSDA+U) in order to account for the significant correlation effects arising from the 4f electronic states of Sm. According to our calculations, the total magnetic moments increases with H addition, in agreement with experimental data. Adding one H atom in the near-neighbor environment of the Fe 8j site reduces the magnetic moments of Fe 8j and enhances the magnetic moment of Fe 8f. For every investigated alloy, the site-resolved spin magnetic moments of Fe on the 8i, 8j, and 8f sites exhibit the same magnitude sequence, with msFe (8i) > msFe (8j) > msFe (8f). While the addition of H has a positive impact on magneto-crystalline anisotropy energy (MAE), the increase in Mo concentration is detrimental to MAE. The computed exchange-coupling parameters reveal the highest values between the closest Fe 8i spins, followed by Fe 8i and Fe 8j spins, for all investigated alloys. The Curie temperature of the alloys under investigation is increased by decreasing the Mo concentration or by H addition, which is qualitatively consistent with experimental findings.

Exploring Crystal Structure, Hyperfine Parameters, and Magnetocaloric Effect in Iron-Rich Intermetallic Alloy with ThMn12-Type Structure: A Comprehensive Investigation Using Experimental and DFT Calculation

In this study, we give a thorough evaluation of the structural, magnetic, and magnetocaloric properties in iron-rich PrFe11Ti intermetallic alloy with ThMn12-type structure using a combination of experimental and theoretical analysis. X-ray diffraction coupled with Rietveld refinement was used to characterize the structure, which revealed a unique tetragonal crystal structure with I4/mmm space group. The 8i site was identified as the preferred site for the Ti atom. This finding was confirmed by various techniques, including XRD, DFT, and Mössbauer spectrometry. Magnetic properties were studied through intrinsic magnetic measurements and magnetocaloric effect analysis. Mössbauer spectroscopy was employed to probe the local magnetic environment and for further characterization of the material’s magnetic properties. The experimental results were complemented by theoretical calculations based on density functional theory (DFT). A promising magnetocaloric effect is observed, with a significant maximum magnetic entropy (−ΔSMmax = 2.5 J·kg−1·K−1) and a relative cooling power about 70 J·kg−1 under low magnetic field change μ0ΔH = 1.5 T. Overall, our results provide a deeper understanding of the structural and magnetic properties of the material under study and demonstrate the effectiveness of the combined experimental and theoretical approach in the investigation of complex materials. The insights gained from this study could have implications for the development of advanced magnetic materials with enhanced properties for potential magnetic applications.

Study on neutron diffraction and correlated magnetic properties of (R1−xZrx)Fe12−yMoy with low Mo concentration

In order to determine the Zr occupancy site, neutron diffraction study on (R1−xZrx)Fe12−yMoy (R = rare earth) were performed at 300 K and 573 K respectively. It is found that Zr occupies the 2a site, like the R atoms behavior. Zr is the first neighbor of Y in the periodic table, and has a similar chemic property as Y, but with a smaller atomic radius. Due to the suitable chemic property and atomic dimension, the addition of Zr at 2a site facilitates forming of the 1–12 phase, allowing more Fe atoms to enter the 8i site and decreasing Mo content from 1.5 to 0.8. As a result, Sm0.9Zr0.1(Fe0.75Co0.25)11.2Mo0.8 shows a large saturation magnetization σs of 137.6 emu/g, a high Curie temperature TC of 775 K and a strong anisotropy field Ha of 87.5 kOe, and its estimated value of maximum magnetic energy product (BH)max is up to 50.4 MGOe, favorable for permanent magnet applications. At present, almost rare-earth sintered magnets are made of Nd2Fe14B, causing the price of the raw material Nd to soar. Therefore, the development of low-cost Sm-Fe based hard magnetic materials is of particular significance.

Phase transformation and ionic conductivity mechanism of a low-temperature sintering semiconductor Na2CaV4O12

Alkaline earth metal vanadates have drawn attention because of their potential applications in electrochemical devices. Here, Na2CaV4O12 was prepared at extremely low temperatures (350–550 °C) and showed a semiconductor behavior with a bandgap of 2.92 eV. A phase transition from P4/nbm to P4̅b2 occurred at 510 °C was identified by an in-situ XRD upon heating, where the 16 n site for oxygen atoms in the P4/nbm phase evolves into two distinguishable 8i sites in the P4̅b2 phase. Ionic conduction in Na2CaV4O12 at elevated temperatures was reported for the first time in the present work. A strong correlation between ionic conductivity and phase structure of Na2CaV4O12 is observed. The charged carriers are mainly sodium ions for the low-temperature P4/nbm phase, while mixed conduction contributed by sodium ions and oxide ions happened in the transformed phase. Bond valence-based energy landscape calculations disclosed a two-dimensional interstitial diffusion mechanism for Na+ ions in the Na2Ca-layers, as well as a two-dimensional diffusion mechanism for oxide ions in the V4O12-layers. The novel semiconductor ceramic would have potential applications in all-solid sodium ions batteries or solid oxide fuel cells as electrolytes.

Effect of chromium substitution on structural, magnetic and magnetocaloric properties of GdFe12−xCrx intermetallic compounds, Mössbauer spectrometry and ab initio calculations

The single-phase intermetallic compounds GdFe12−xCrx (x ​= ​2, 2.5 and 3), elaborated by arc-melting method and annealed at 1073 ​K, crystallize in the ThMn12-type structure with the I4/mmm space group. The structural, magnetic and magnetocaloric properties of these intermetallic compounds were studied extensively by means of X-ray powder diffraction (XRD), Mössbauer spectrometry and magnetic measurements. The Rietveld analysis of X-ray diffraction patterns reveals that the lattice parameters of GdFe12−xCrx (x ​= ​2, 2.5 and 3) compounds increase linearly with increasing Cr content up to x ​= ​3, and proves that Cr atom prefers 8i sites in the ThMn12 structure and that Fe occupies 8i, 8f and 8j. The Mössbauer spectra study based on the Wigner-Seitz cell volumes correlation with the isomer shift parameter of each specific site 8i, 8f and 8j, confirms the preferred inequivalent crystallographic site of Cr atoms. Ab initio calculations of GdFe12−xCrx were performed using the density functional theory (DFT) based on the full potential linearized augmented plane wave (FLAPW). The calculated data are in good agreement with the results of the magnetic measurements as well as with the Mössbauer spectrometry. The variation of magnetization (M) vs. temperature (T) reveals that all compounds exhibit a second-order ferromagnetic to paramagnetic phase transition in the vicinity of the Curie temperature (TC). The substitution of Fe by Cr yields enhancement of the magnetic ordering transition temperature. The maximum magnetic entropy change (ΔSM) was estimated from isothermal magnetization curves and it increases from 1.82 ​J/kg.K for x ​= ​2, to 2.95 ​J/kg.K for x ​= ​3, under a field change of 1.5 ​T. The relative cooling power (RCP) is in the range of 12.5 ​J/kg for x ​= ​2 and 30.4 ​J/kg for x ​= ​3. Our results suggest that such substituted compounds could be considered as a potential candidate for magnetic refrigeration technology.

Crystal structures and electron-density distributions of Y(Fe, M)12 (M = Si, Ti, V, Cr, Mo, and W) determined by Rietveld and MEM analyses

The crystal structures of Y(Fe, M)12 (where M = Si, Ti, V, Cr, Mo, and W) were investigated by Rietveld analyses, and electron-density distributions were estimated by the maximum entropy method. There were no substituting elements to distort the lattice similarly. While the sites arranged selectively according to the element did not move necessarily, the distance between two atoms in the 8i site (an iron dumbbell) could change freely. Electron-density distributions revealed isolated iron dumbbells and a bonding network between Y, 8f, and 8j sites. Moreover, the connection in an iron dumbbell was much stronger than any other connection. As the substituting amount of the tungsten element increases, the electron density at interstitial positions increases. However, the high electron density disappears by over-substitution. It was suggested that the network of 22-hedrons, which configured the Y, 8f, and 8j sites, correlates with the thermal stability of the ThMn12 structure.

Structural and magnetic properties of YCo4B compound at high pressures

The crystal structure and magnetic properties of YCo4B intermetallic compound were studied by means of neutron diffraction at pressures p ≤ 6.5 GPa. No structural transitions are evidenced. The cobalt moments at 2c and 6i sites decrease, upon lattice compression, with a rate dlnMCo/dp ≅ 0.11 GPa−1, showing a rather high degree of itinerancy. The evolutions with pressure of band structures have been also analysed and the computed cobalt moments are discussed in correlation with those experimentally determined.

The quaternary system Sm-Fe-Mo-Al and the effect of Al substitution on magnetic and structural properties of its ThMn12 phase

The Fe-rich corner of the quaternary system Sm-Fe-Mo-Al is studied at isothermal conditions. The phase equilibria of ferromagnetic SmAlxFe10−xMo2 with adjacent phases, especially Sm2AlxFe15.7−xMo1.3, are observed for 1000 °C. Existence ranges are studied by SEM/EDX on samples with various compositions. Magnetic and structural properties were obtained by VSM(PPMS) as well as Permagraph measurements while structural relations were tracked by XRD. Al can be substituted for Fe up to x = 6.5 accompanied by a decrease in high field (8 MA/m ≈ 10 T) polarization from 0.66 T (x = 0.0) to 0.44 T (x = 2.0). Respective Curie temperatures of SmAlxFe10−xMo2 and Sm2AlxFe15.7−xMo1.3 initially increase with Al content reaching a maximum of ≈125 °C at x = 0.5 and ≈135 °C at x = 1.0, respectively. Further Al substitution decreases TC. The coercive fields of optimally annealed nanocrystalline powders decrease slightly from 153 kA/m for x = 0.0–139 kA/m for x = 0.8 and drops to 38 kA/m for x = 2.0. XRD data indicates that lattice parameters of SmAlxFe10−xMo2 continuously increase with Al content. The simultaneous refinement of X-ray and neutron diffraction patterns confirms that Mo prefers 8i sites in the ThMn12 structure and that Al occupies equally 8i and 8j in a ratio of 1:2.

The stability of newly developed (R,Zr)(Fe,Co)12−xTix alloys for permanent magnets

(R,Zr)(Fe,Co)12−xTix (R = Nd and Sm) alloys of ten different compositions were prepared by strip-casting. The elemental distribution of the sample particles after annealing was quite homogeneous. Existing phases in the prepared alloys were determined by X-ray diffraction and the occupation sites of the substituted elements (Ti, Co, and Zr) were determined using spherical aberration corrected scanning transmission electron microscopy. The ThMn12 structure is stable in Ti1.0 compounds for both R = Nd and Sm, but exhibited obvious precipitation of α-(Fe,Co) phases in low Ti content compounds with Ti0.5. Ti substitution into Fe(8i) sites is important for the stabilization of the ThMn12 structure. Furthermore, the stability of low Ti content compounds could be recovered by the substitution of Zr into R(2a) sites. Co was mainly in Fe(8j) and Fe(8f) sites, which contributed to the stability of the 1–12 structure by increasing the configuration entropy. Analysis based on three Fe hexagons and two local sub-structural units suggested that the stabilization of ThMn12 structure by Ti and Zr substitutions mainly originates by resolving local mismatches in interatomic distances in the structure.

Enhanced electrochemical properties of Mg2+ doped Li2Na2Ti6O14 anode material for lithium-ion batteries

A series of Mg2+ doped Li2Na2Ti6O14 in the form of Li2-xMgxNa2Ti6O14 (x=0.0, 0.05, 0.1, 0.15, 2.0) are successfully synthesized via a solid-state method. Phase structure and surface morphology analyses show that all samples maintain high purity and low dose of Mg2+ doping does not change the crystal structure and surface morphology of Li2Na2Ti6O14. According to the Rietveld refinement result, it is known that Mg2+ takes the tetrahedral 8i sites shared with Li+ in the structure. Galvanostatic charge/discharge tests demonstrate that Li1.95Mg0.05Na2Ti6O14 exhibits higher reversible capacity (264.6mAhg−1) and better cycling performance (capacity retention of 82.7% after 50 cycles) than other materials. Even cycled at high current density of 500mAg−1, Li1.95Mg0.05Na2Ti6O14 still provides the specific capacity of 167.4mAg−1 after 100 cycles, indicating that low dose of Mg2+ doping can effectively improve the lithium storage capability of Li2Na2Ti6O14. The enhanced electrochemical properties are contributed to the improved electronic conductivity and ionic diffusion coefficient of Li2Na2Ti6O14 via Mg2+ doping. Besides, the lithiation/delithiation behavior of Li1.95Mg0.05Na2Ti6O14 is also studied by homemade in-situ X-ray diffraction device, which reveals that Li1.95Mg0.05Na2Ti6O14 exhibits high structural and electrochemical reversibility as lithium storage material.

Structure, nonstoichiometry, and geometrical frustration ofα-tetragonal boron

Recent discoveries of supposedly pure alpha-tetragonal boron require to revisit its structure. The system is also interesting with respect to a new type of geometrical frustration in elemental crystals, which was found in beta-rhombohedral boron. Based on density functional theory calculations, the present study has resolved the structural and thermodynamic characteristics of pure alpha-tetragonal boron. Different from beta-rhombohedral boron, the conditions for stable covalent bonding (a band gap and completely filled valence bands) are almost fulfilled at a composition B_52 with two 4c interstitial sites occupied. This indicates that the ground state of pure alpha-tetragonal boron is stoichiometric. However, the covalent condition is not perfectly fulfilled because non-bonding in-gap states exist that cannot be eliminated. The half occupation of the 4c sites yields a macroscopic amount of residual entropy, which is as large as that of beta-rhombohedral boron. Therefore, alpha-tetragonal boron can be classified as an elemental crystal with geometrical frustration. Deviations from stoichiometry can occur only at finite temperatures. Thermodynamic considerations show that deviations delta from the stoichiometric composition B_(52+delta) are small and positive. For reported high-pressure syntheses conditions delta is predicted to be about 0.1 to 0.2. An important difference between pure and C- or N-containing alpha-tetragonal boron is found in the occupation of interstitial sites: the pure form prefers to occupy the 4c sites, whereas in C- or N-containing forms a mixture of 2a, 8h, and 8i sites are occupied. The present article provides relations of site occupation, delta values, and lattice parameters, which enable us to identify pure alpha-tetragonal and distinguish the pure form from other ones.

Anomalies in magnetoelastic properties of DyFe11.2Nb0.8 compound

The structural and magnetic properties of DyFe11.2Nb0.8 compound have been investigated by high intensity, high resolution synchrotron x-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements as well as Mössbauer spectroscopy (5–300 K). The easy magnetization direction at room temperature is along the c-axis. With decreasing temperature, the magnetocrystalline anisotropy changes from easy-axis to easy-cone at Tsr2 (∼202 K), then to easy plane at Tsr1 (∼94 K). The thermal expansion coefficient obtained from the synchrotron x-ray study exhibits clear anomalies around both of the spin reorientation transition temperatures. The 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j, and 8f sites from the Mössbauer spectra.

VASP Calculation on Lithium Reaction to Tin Phosphides

Studies on electrochemical reaction of SnPx materials were carried out by VASP ab initio calculation, where. SnPx having four different crystalline compositions, SnP0.75(Sn4P3), SnP0.94(SnP17/18), SnP and SnP3. The reaction potential of the lithiation of SnP0.94(SnP17/18) material was compared between intercalation and conversion reaction. The total-energy calculation and full structural optimization of SnPx, SnPx related materials and Li materials were performed using the Vienna ab initio Simulation Package (VASP)[1]. The possibility of intercalation of lithium to SnP0.94 was analyzed to obtain reaction potential of 1.853 V (Li/Li+) accommodating lithium to 6i site. 6i site is a vacant site in Sn18P17 (SnP0.94). Through further intercalation of lithium, reaction potential of Li0.5SnP0.94, Li1.5SnP1.56 and Li6.06SnP0.94 can also be calculated as 0.9026, 0.705 and -1.5 V(Li/Li+), respectively.Considering conversion reaction, SnP0.94 can react to produce 2 kinds of products such as LiP and Li3P as shown in Eq. 1. SnP0.94 + (0.94z+y) = 0.94LizP + LiySn ---------(1) At LiySn formation, y of zero means that only conversion reaction takes place without the formation of LiySn. Reaction potential of y=0 were 1.35 V and 0.948 V for the formation of LiP and Li3P. Reaction potential of the hole reaction [SnP0.94 + xLi = products], including intercalation and conversion, was plotted against reacted lithium extent of x in Fig. 1. It is observed that the reaction potential of conversion is higher than that of the lithium intercalation. In case of conversion reaction to form LiP with reacted lithium extent of 0.94 was 1.35 V which was higher than 0.937 V of lithium intercalation to form Li0.56SnP0.94 . Therefore, conversion reaction is labile compared to intercalation. Moreover, the reaction potential of conversion to form Li3P with reacted lithium extent of 2.833 was 0.948 V which is higher than that of the conversion reaction potential of 0.704 V to form LiP with reacted lithium extent of 3.28. Therefore conversion reaction to form Li3P is easier than to form LiP. We will discuss VASP calculation on lithium reaction to Tin Phosphides in details[1] Kresse,G.; Hafner, J. Phys. Rev. B , 47 (1993) 558; 49(1994) 14251.

On the YFe11Mo intermetallic characterization

Rare-earth intermetallic compounds adopting the tetragonal ThMn12–type structure and containing high Fe concentrations have attracted considerable attention in the field of permanent magnets. Among them, the Y-Fe-Mo series has been extensively investigated, especially by X-ray diffraction (XRD), but the microstructural characterization was very limited.In the present work, Y:11Fe:Mo has been prepared by melting Y, Fe and Mo in an arc furnace followed by splat-quenching and/or annealing treatments. The structure, chemistry and magnetic domain configurations of the resulting polycrystalline aggregates have been investigated by XRD, scanning and transmission electron microscopies (SEM and TEM, respectively), energy dispersive X-ray spectroscopy (EDS) and Lorentz microscopy.A special emphasis was given to Lorentz microscopy, where contrast is based on the Lorentz deflection imposed on electrons by the passage through a magnetic specimen. The magnetic domains have been imaged by the intermediate lens in Fresnel mode, with the objective lens switched off. In these conditions the intermediate lens is defocused so that out-of-focus images of the specimen are formed: the magnetic domain walls are imaged as alternate bright (convergent) and dark (divergent) lines. For the overfocused image, bright lines occur at the position of domain walls for which the magnetisation on either side is deflecting the electrons towards the wall, whereas dark lines are observed at the walls for which the magnetisation on either side is deflecting the electrons away from the wall. The opposite contrast is observed at the underfocused image.The results have shown that the tetragonal YFe11-xMoy phase is predominant, with a cellular dendritic morphology (Figure 1), but a considerable presence of -Fe(Mo) could be inferred, forming a coarse intercellular eutectic mixture. Significant Fe segregation occurred during annealing. However, this composition variation corresponded to an extremely limited evolution of the lattice parameters, the Rietveld analysis pointing to Fe vacancies at the 8i sites on the annealed material. XRD and EDS results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. The current study also showed that grain boundaries are usually associated with domain walls and that YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials (Figure 2), while residual -Fe(Mo) grains exhibit vortex configurations (see arrows in Figures 2 (a-c)).The work was supported by the Portuguese Science Foundation through the CTM/48617/2002 and PEst-OE/CTM-UI0084/2011 grants.

Atomistic simulation of site preference, Curie temperature and lattice vibration of ZrT12−xMx (T=Fe, Co; M=Al, Ga)

A series of lattice inversion pair potentials are used to evaluate the phase stability and site preference of ZrT12−xMx (T=Co, Fe; M=Al, Ga; x=6.0, 6.5, 7.0) compounds. The calculated preferential occupation site of the M atom is found to be the 8i site, which is in good agreement with experimental results. And the calculated lattice constants coincide quite well with experimental values. Further, the energy DOS of the relaxed structures are calculated and the variation in Curie temperature is explained qualitatively by the spin-fluctuation theory. Meanwhile, the phonon densities of states and Debye temperature are evaluated for the ZrT12−xMx compounds.

Structure Properties of the ${\rm YFe}_{11}{\rm Mo}$ Intermetallic Compound

The structure, chemistry and magnetic domain configurations of YFe11Mo polycrystalline aggregates have been investigated by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and Lorentz microscopy. Although extensive reports on the magnetic properties of this compound can be found in the literature, its exact equilibrium structure remains elusive. The compound was prepared by arc-melting followed by splat-quenching and subsequent annealing. The quenched material consisted of a ThMn12-type matrix and α-Fe(Mo) impurity phase. Annealing lead to iron segregation from the matrix, as evidenced by a clear shift to higher angles of the α -Fe(Mo) peak observed in the diffraction pattern. Nevertheless, the composition variation in the intermetallic compound was accompanied by an extremely limited evolution of the lattice parameters, suggesting the presence of Fe vacancies after annealing. Rietveld analysis carried out on the annealed material points to Fe vacancies at the 8i sites. The present results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials, while residual α -Fe(Mo) grains exhibit vortex configurations. Annealing hardly affected the magnetic domain configurations of the YFe11Mo and α-Fe(Mo) aggregates.

Synthesis, crystal structure and physical properties of Yb11Bi10 −xSnx

The intermetallic phase Yb11Bi10 −xSnx (x = 0 and 4.8) crystallizing in the Ho11Ge10 structure type was synthesized and characterized. The crystal structure was established by single-crystal X-ray diffraction data in the tetragonal space group I4/mmm (no.139), Pearson code tI84, Z = 4, a = 12.2043(4) Å, c = 17.7227(9) Å, and V = 2639.7(2) Å3, Rgt(F) = 0.040, 763 observed reflections for Yb11Bi10, and a = 12.0183(5) Å, c = 17.413(1) Å, and V = 2515.1(2) Å3, Rgt(F) = 0.027, 762 observed reflections for Yb11Bi5.2Sn4.8. The crystal structure of Yb11Bi10 contains three discrete anionic moieties: isolated Bi3− anions, Bi24− dimers and Bi44− squares. In Yb11Bi5.2Sn4.8, the square units are formed solely by Sn, and the structure shows a mixed Bi/Sn occupancy at the 8i and 16m Wyckoff sites. Magnetization measurements show that the title phase contains ytterbium exclusively in the 4f14 configuration up to 400 K. The Yb-LIII X-ray absorption spectrum attests also the presence of Yb with a 4f14 (Yb2+) configuration for Yb11Bi10, while the average valence of ytterbium was found to be 2.09 for Yb11Bi5.2Sn4.8.

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 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.

ChemInform Abstract: First‐Principles Study of the Crystal and Electronic Structures of α‐Tetragonal Boron.

The crystal and electronic structures of α-tetragonal (α-t) boron were investigated by first-principles calculation. Application of a simple model assuming 50 atoms in the unit cell indicated that α-t boron had a metallic density of state, thus contradicting the experimental fact that it is a p-type semiconductor. The presence of an additional two interstitial boron atoms at the 4c site made α-t boron semiconductive and the most stable. The cohesive energy per atom was as high as those of α- and β-rhombohedral boron, suggesting that α-t boron is produced more easily than was previously thought. The experimentally obtained α-t boron in nanobelt form had about two interstitial atoms at the 8i sites. We consider that the shallow potential at 8i sites generates low-energy phonon modes, which increase the entropy and consequently decrease the free energy at high temperatures. Calculation of the electronic band structure showed that the highest valence band had a larger dispersion from Γ to Z than from Γ to X; this indicated a strong anisotropy in hole conduction.