4d Sites Research Articles

Dielectric anisotropy in relaxor ferroelectric Pb0.775La0.15(Zr0.4Ti0.6)O3 ceramics

A huge dielectric anisotropy has been found in Pb0.775La0.15(Zr0.4Ti0.6)O3 (PLZT 15∕40∕60) ceramics by analysis of infrared reflectivity spectra. Anisotropy is present in both the relaxor and the ferroelectric phases. Infrared spectra were evaluated using Bruggeman’s formula within the effective medium approximation. X-ray structural analysis supports a local [110] shift of Pb cations and factor-group analysis taking into account the 4d site symmetry for the effective Pb∕La atom gives more infrared and Raman active phonons than the previous published factor-group analysis, in agreement with our infrared data. Symmetry of all observed polar phonons was assigned.

Erbium dinickel disilicide, ErNi2Si2

Erbium dinickel disilicide, ErNi2Si2, crystallizes in the tetra­gonal ThCr2Si2 structure type, which is an ordered superstucture of the BaAl4 type. The coordination numbers are 22 for Er (2a site), 12 for Ni (4d site), and 9 for Si (4e site).

Magnetic structure of Nd3Co29Si4B10

The crystal and magnetic structures of the quaternary rare-earth intermetallic compound Nd3Co29Si4B10 have been investigated by neutron diffraction (1.5 and 300K). Rietveld refinements demonstrate that Nd3Co29Si4B10 is isostructural with the Nd3Ni29Si4B10 tetragonal structure and that Nd3Co29Si4B10 has a canted magnetic structure below the ferromagnetic ordering temperature TC=200K. The refinement of the 1.5-K diffraction pattern leads to mean values for the magnetic moments of μCo∼0.4μB and μNd∼2μB at the 2b and 4d sites.

4D dynamic construction management and visualization software: 2. Site trial

The performance of any system can only be gauged by practical applications. This paper delineates some insight and experience on the on-site use of a prototype four-dimensional (4D) site management software system for a warehouse superstructure in Hong Kong. It links a three-dimensional model and a construction schedule to furnish visualization of the state of a site at any user-specified date. Site personnel generally view this system as potentially very promising and helpful. The 4D visualization is found to have capability to assist cognitive, reflective and analytical activities of site management. Through this site trial, some limitations of the software are also spotted which consequently lead to enhancement to the system. The prototype model may require a large amount of data input for large projects which lead to large computer files and slow processing speed. Hence, careful implementation of details is required in the planning level for practical 4D simulation modeling. This, however, will not be a long-term problem, given the recent advancement of computer technology.

Generalized image models and their application as statistical models of images

A generalized image model (GIM) is presented. Images are represented as sets of four-dimensional (4D) sites combining position and intensity information, as well as their associated uncertainty and joint variation. This model seamlessly allows for the representation of both images and statistical models (such as those used for classification of normal/abnormal anatomy and for inter-patient registration), as well as other representations such as landmarks or meshes. A GIM-based registration method aimed at the construction and application of statistical models of images is proposed. A procedure based on the iterative closest point (ICP) algorithm is modified to deal with features other than position and to integrate statistical information. Furthermore, we modify the ICP framework by using a Kalman filter to efficiently compute the transformation. The initialization and update of the statistical model are also described. Preliminary results show the feasibility of the approach and its potentialities.

4D dynamic management for construction planning and resource utilization

The requirements, nowadays, for more effective planning and management assistance from computer have become even more demanding. In this paper, our 4D model, 4D Site Management Model+ (4DSMM+), is described, a model developed in an attempt to address the requirement for linking scheduling data to a 3D computer graphics building model, which will allow planners to perform graphic simulations of the construction process. Compared to other 4D models and our earlier research, the current model extends 4D technology into the areas of resource management and site space utilization, in addition to the planning of building construction. A new information system platform, 4D Management for Construction Planning and Resource Utilization (4D-MCPRU), has been developed in this research, in order to implement the model 4DSMM+, which integrates dynamic resource management at the project level and decision-making support with the other features. This paper discusses features of the newly extended model and describes the underlying techniques needed for applying the model in construction management practice.

Revised magnetic phase diagram for FexMn5−xSi3 intermetallics

The magnetic and crystal structures of the series FexMn5−xSi3 (x=4,3.5,3,2,1) have been reinvestigated by neutron powder diffraction between 315 and 8 K. Fe atoms are found to preferentially occupy the 4d site of the hexagonal D88 structure (P63/mcm). The compound Fe4MnSi3 orders ferromagnetically below 300 K. Refinement of the neutron data for this compound indicates that the moments are aligned at ∼40° off the c axis. A high moment at the 6g site is due to a large Mn moment which is forced into a ferromagnetic alignment with the Fe moments. Associated with this moment is a moderate magnetic entropy change, with a maximum value of 4 J kg−1 K−1 for a magnetic field change of 5 T. The magnetic structure of Fe3.5Mn1.5Si3 is similar to that of Fe4MnSi3, but the Curie point is lower, below 240 K. A feature for this composition is the appearance of a spiral structure below 60 K. The compounds with x=1, 2, and 3 are all antiferromagnets with a spiral structure observed for Fe3Mn2Si3 at all temperatures up to the Néel point of 150 K. Two separate antiferromagnetic structures exist for Fe2Mn3Si3, one between 120 and 45 K and another structure for temperatures below 45 K. Similar features in the magnetic ordering are exhibited by FeMn4Si3. At temperatures between 70 and 95 K, the magnetic structure is similar to that of the low temperature phase of Fe2Mn3Si3. Accompanying the magnetic transition at 70 K is a structural distortion to space group Cmcm, a feature also observed for Mn5Si3.

Magnetic properties of (Mn 1-x Ru x ) 3 Ga alloys

We found that the pseudo binary alloys Mn1-xRux 3Ga, with 0.33≤x≤0.67, have an ordered b.c.c. structure. The lattice constant a is almost constant with respect to x: a=6.000 A for x=0.33 and a=5.992 A for x=0.67. For the alloy with x=0.33, i.e. Mn2RuGa, the magnetization is almost saturated in a field of 20 kOe. The saturation magnetization at 4.2 K is 23 emu/g, and the Curie temperature, TC, is 460 K. The TC of (Mn1-xRux)3Ga decreases almost linearly with increasing x, and it vanishes around x=0.67 (MnRu2Ga). We also determined atomic and magnetic structures from neutron diffraction experiments. The alloy Mn2RuGa (x=0.33) has an ordered structure of CuHg2Ti type; the magnetic Mn atoms mainly occupy the 4a (0,0,0) and 4d \(\frac\Box3\Box\Box4\Box\),\(\frac\Box3\Box\Box4\Box\),\(\frac\Box3\Box\Box4\Box\) sites. We also observedthat the magnetic moments of Mn atoms on the 4a and 4d sites are antiparallel to each other; values of the magnetic moment are μa=4.6 and μd=3.3 μB per Mn atom.

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mossbauer spectra of epitaxial α″-Fe16N2 and α′-Fe8N films have been studied and their differences are discussed in detail. The Mossbauer spectrum of α″-Fe16N2 can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mossbauer spectrum of α′-Fe8N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for α″-Fe16N2 than for α′-Fe8N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for α″-Fe16N2. The iron moments tend to locate in the film plane for α″-Fe16N2 and to arrange perpendicularly to the film plane for α′-Fe8N.

ChemInform Abstract: Chromium Sublattice Magnetic Ordering in a Compound of the ThCr2Si2 Type Structure: HoCr2Si2.

Abstract A neutron diffraction investigation of the intermetallic compound HoCr 2 Si 2 has been carried out over the temperature range 1.7–470 K. The data clearly demonstrate that the Cr sublattice in this compound is ordered for the whole range of temperatures investigated. Chromium atoms at 4d sites are aligned anti-parallel along the c axis. The refined value of the ordered Cr moment is 1.48(4) μ B at 293 K and 1.60(4) μ B at 25 K. The magnetic space group is I4′/m′m′m , with G Z − Mn magnetic modes at 4d sites. The Ho and Cr sublattices order independently, with Ho moments at 2a sites ordering below 1.7 K.

Chromium sublattice magnetic ordering in a compound of the ThCr 2Si 2 type structure: HoCr 2Si 2

A neutron diffraction investigation of the intermetallic compound HoCr 2Si 2 has been carried out over the temperature range 1.7–470 K. The data clearly demonstrate that the Cr sublattice in this compound is ordered for the whole range of temperatures investigated. Chromium atoms at 4d sites are aligned anti-parallel along the c axis. The refined value of the ordered Cr moment is 1.48(4) μ B at 293 K and 1.60(4) μ B at 25 K. The magnetic space group is I4′/m′m′m, with G Z − Mn magnetic modes at 4d sites. The Ho and Cr sublattices order independently, with Ho moments at 2a sites ordering below 1.7 K.

Amplitude-modulated spin structure in Er5Sn3 above 10 K

Neutron diffraction has been performed on the intermetallic Er5Sn3 crystallizing in the Mn5Si3-type structure with space group P63/mcm. The unit cell constants are a=8.733 Å and c=6.369 Å. Er5Sn3 orders antiferromagnetically below 32 K with an incommensurate sine wave-modulated magnetic structure. The magnetic propagation vector and the spin orientations are along the c-axis. The magnitude of the Er-moments are 9.44 and 4.33 μB for the 4d and 6g sites, respectively, at 14 K. New magnetic reflections appear below 8 K giving rise to an unknown magnetic structure.

Vibrational Spectroscopic and X-Ray Diffraction Studies of Cerium Zirconium Oxides with Ce/Zr Composition Ratio=1 Prepared by Reduction and Successive Oxidation of t′-(Ce0.5Zr0.5)O2 Phase

Cerium zirconium oxides with the composition CeZrO4 were prepared by oxidizing in O2 at 873 K precursors with the composition CeZrO3.5+δ (δ<0.3). The precursors were prepared by reducing the t′-(Ce0.5Zr0.5)O2 phase at 873≤Tred.≤1573 K. The CeZrO4 compounds together with the precursors were characterized by powder X-ray diffraction and IR and Raman spectroscopies. In the case of 1323≤Tred.≤1573 K, the precursors were identified as having the pyrochlore-type Ce2Zr2O7+2δ phase with δ<0.06. When the reduction temperature was decreased, the ordering level between Ce and Zr ions was decreased in the pyrochlore-type phase. The CeZrO4 compounds obtained by oxidizing the pyrochlore-type precursors were identified as the κ-CeZrO4 phase in which Ce and Zr ions are in an ordered arrangement similar to that of the pyrochlore-type structure. The space group of the κ phase was not the same as that of the pyrochlore-type structure. In the case of Tred.≤1123 K, cubic CaF2-related-type (Ce0.5Zr0.5)O(3.5+δ)/2 phases with 0.1<δ<0.3 were obtained. When the precursor obtained at Tred.=1123 K was oxidized, a tetragonal phase appeared, but its Raman spectrum was different from that seen in the t′ phase. This phase was identified to be a novel t′meta-(Ce0.5Zr0.5)O2 phase, because it transformed into the t′ phase by annealing above 1173 K under an O2 atmosphere. When the precursor obtained at Tred.=873 K was oxidized, the t′-(Ce0.5Zr0.5)O2 phase identical to the starting sample appeared. The structural difference between t′meta and t′ phases was estimated to be in their oxygen parameter, z, for the 4d site in the space group of P42/nmc. The oxygen parameter for the t′meta phase is closer to 0.25 than that for t′ phase. It was shown that the temperature and holding time for precursor preparation and the oxygen content δ are important factors for the structure of the reoxidized CeZrO4 compound.

Rietveld refinement studies of Nb 4N 5- and Nb 5N 6-related phases in the (Mn)–Nb–O–N system

The structures of two Nb and two Mn–Nb (oxy)nitrides have been refined from Cu Kα 1 powder diffractometer data by the Rietveld method and have been characterised by electron diffraction. The phases Nb 3.49N 4.56O 0.44 and Mn 0.54Nb 3.07N 4.40O 0.60 have structures related to that of Nb 4N 5, an NaCl type with ordered metal vacancies. Refinements in space group I4/ m, with R F=1.3% and 1.1%, respectively, have shown that they are disordered and that metal atoms partially occupy both the nominally filled 8h position and the nominally empty 2a position in the Nb 4N 5 type. The observed bond lengths indicate that the 2a position is occupied only by Mn atoms in Mn 0.54Nb 3.07N 4.40O 0.60. The Nb 5N 6 type structure of Nb 5(N,O) 6 and the related structure of MnNb 2(N,O) 3 has been refined in space group P6 3/ mcm to R F=1.5% and 2.9%, respectively. In the latter, the trigonal prism sites 6g are found to be occupied only by Nb, the octahedral 4d sites by approximately equal amounts Mn and Nb and the octahedral 2a site, empty in the Nb 5N 6 type structure, only by Mn. The Nb (oxy)nitride structures exhibit short Nb–Nb distances which indicates localised bonding between Nb atoms. Magnetic susceptibility and electrical conductivity measurements of the tetragonal phases at 15–300 K show, respectively, small magnetic moments which are coupled in an antiferromagnetic manner at low temperatures and poor, nearly temperature-independent, conductivities.

Phase identification of the new 126 K Ba–Ca–Cu–O superconductor

New superconducting oxides that have a composition of Ba 2Ca 2Cu 3(Ca x Cu y )O 8+ δ with 0.4≥ x≥0.9 and 0.7≥ y≥0, and show an optimum T c∼126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common tetragonal unit cell among these oxides, which has lattice constants a=3.85 Å and c=28.2 Å with an I/4mmm symmetry. The cell consists of a rock-salt (BaO) 2 block sandwiched between two perovskite Ca 2Cu 3O 6 blocks with interstitial Ca x +2 and Cu y +2 located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial Ca x Cu y . The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H 2O and CO 2 in open air. The decayed products (Ca,Cu,H,CO 2) 2Ba 2Ca 2Cu 3O 8+ δ have an expanded lattice constant c∼33.8 Å and a lower T c∼90 K. A structure model for the decayed phases was also proposed.

Neutron and electron diffraction studies of ZnGa 2Se 4

ZnGa 2Se 4 is a II–III–IV compound semiconductor with a zincblede structure basically. Convergent-beam electron diffraction (CBED) and neutron diffraction were applied to determine its crystal structure unambiguously. A CBED pattern clearly exhibits 4 Rmm R symmetry, which suggests that ZnGa 2Se 4 belongs to space group I 4 2m. Rietveld analysis of neutron powder diffraction data on the basis of this space group shows that the 2a site is fully occupied by Ga and that the 2b site is completely vacant. On the other hand, the 4d site is statistically occupied by Zn and Ga atoms.

Giant moment of Fe16N2 as evidenced by 57Fe NMR studies.

A nuclear-magnetic-resonance study of ${\mathrm{Fe}}_{16}$${\mathrm{N}}_{2}$, ${\mathrm{Fe}}_{4}$N, and bcc \ensuremath{\alpha}-Fe phases, along with magnetization experiments on samples containing these phases, show that the average Fe moment for ${\mathrm{Fe}}_{16}$${\mathrm{N}}_{2}$ is between 2.7${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and 2.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. This is in contrast to previous studies which report values ranging from 2.3${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ to 3.5${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. The results reported here indicate an average Fe moment for ${\mathrm{Fe}}_{16}$${\mathrm{N}}_{2}$ which is significantly enhanced compared with that for \ensuremath{\alpha}-Fe, and a moment for Fe atoms at the 4d sites which is greater than 3.0${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. \textcopyright{} 1996 The American Physical Society.

Metastable iron nitrides by mechanical alloying

Mechanical alloying of iron nitrides, e-Fe 3 N and γ'-Fe 4 N together with α-Fe has been carried out in an attempt to produce the metastable α-Fe 16 N 2 phase. The distribution of nitrogen in as-milled and annealed materials is determined by Mossbauer spectroscopy. The distinctive feature in the Mossbauer spectrum of α-Fe 16 N 2 , the hyperfine component with B hf = 40 T attributed to Fe in 4d sites which are furthermost from nitrogen is not seen in any of the Mossbauer spectra. The as-milled product is nano-crystalline α-Fe with an amorphous intergranular phase of approximate composition Fe 80 N 20 . The magnetization of samples decreased on milling and returned to the starting values after annealing which recovers the equilibrium starting mixtures. In the case of the Fe 3 N + Fe experiments, annealing produced some Fe 4 N together with Fe 3 N and Fe.

Influence of carbon introduced by arc melting on the structural and magnetic properties of RFe 12− xV x (R = Y and Dy)

Carbon was introduced by arc melting into RFe 12− x V x compounds, where R = Y or Dy and x = 2, 3 or 4, and its effects on the crystallographic and magnetic properties of these intermetallics were investigated by X-ray, magnetostatic and Mössbauer effect measurements. Experimental results show that the carbon atoms prefer the 4d sites to the 2b sites, or occupy the sites reserved for Fe atoms in the ThMn 12-type crystal structure. Carbon solution increases with increasing vanadium concentration, giving rise to an effect opposite to that of vanadium. An increase in vanadium content causes an increase in the cell volume and a linear decrease in Curie temperature and in the Fe magnetic moment, whereas the addition of carbon leads to a reduction in unit volume while maintaining a high T c and enhancing the Fe magnetic moment despite decreasing iron content in the samples.

Magnetic properties of (Mn 1− xTE x) 5 Ge 3: (TE = Cr and Fe)

The site occupancies and the magnetic moments of the transition-metal atoms, TE = Cr and Fe, in the compounds (Mn 1− x TE x ) 5Ge 3 have been studied by magnetization and neutron-scattering measurements. Chromium and iron atoms were substituted for the Mn atoms in the compound up to x = 0.3 and 0.2, respectively. In the case of Cr, the magnetization and the Curie temperature decrease with increasing Cr content. The fractions of Cr atoms distributed on the 4d and 6g sites are 60% and 40%, respectively, and their magnetic moments are almost zero at 200 K. In the case of Fe, the magnetization and the Curie temperature increase with increasing Fe content x up to 0.10. The magnetization then decreases with increased Fe content. All the Fe atoms occupy the 4d site. Their magnetic moments at 10 K are + 2.6 ± 1.1 μ B, substantially larger than that (1.85 μ B) of Mn on the site. The increase in the bulk magnetization on substitution of Fe for Mn is attributed to the large Fe magnetic moment.