Body-centered Tetragonal Lattice Research Articles

Precipitates in normalized and tempered 10%Cr ferritic/martensitic steels

Precipitates in the two 10%Cr ferritic/martensitic steels in the normalized-and-tempered condition were investigated through transmission electron microscopy with an energy dispersive X-ray system. A large number of precipitates with large difference in size and block-like, or plate/rod-like, or sphere-like morphology have been observed in the two steels. Dominant precipitate phases in the two steels were Nb-rich carbide NbC and Cr-rich carbide M23C6. Z-phases have been found in the two steels. Two types of modified Z-phases, Cr(V,Nb)(C,N), with similar chemical composition and totally different crystal structures, body-centered tetragonal and NaCl-type cubic lattices, as well as original Z-phase, CrNb(C,N), were identified in the two steels. The modified and original Z-phases were observed to coexist with a clear orientation relationship within the same precipitate particles of the steel. The formation of Z-phase in the steels has also been discussed.

Observation of superconductivity at 30∼46K in AxFe2Se2(A = Li, Na, Ba, Sr, Ca, Yb and Eu)

New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced Tc = 30∼46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a∼3.755–3.831 Å while c∼15.99–20.54 Å. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.

Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V–H system studied by 1H NMR

The diffusion of hydrogen in the monohydride phase (β phase) of the V1−xCrx–H (x≤0.1) system was studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, EH, and the Korringa constant, T1eT, were estimated from the temperature and frequency dependence of 1H spin–lattice relaxation times, T1. The value of EH increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T1eT, which was accompanied by reducing in the stability of the monohydride phase of the V–H system.

Self-assembling of Ge quantum dots in an alumina matrix

In this work we report on a self-assembled growth of a Ge quantum dot lattice in a single 600-nm-thick $\text{Ge}+{\text{Al}}_{2}{\text{O}}_{3}$ layer during magnetron sputtering deposition of a $\text{Ge}+{\text{Al}}_{2}{\text{O}}_{3}$ mixture at an elevated substrate temperature. The self-assembly results in the formation of a well-ordered three-dimensional body-centered tetragonal quantum dot lattice within the whole deposited volume. The quantum dots formed are very small in size (less than 4.0 nm), have a narrow size distribution and a large packing density. The parameters of the quantum dot lattice can be tuned by changing the deposition parameters. The self-ordering of the quantum dots is explained by diffusion-mediated nucleation and surface-morphology effects and simulated by a kinetic Monte Carlo model.

Interplay among helical order, surface effects, and range of interacting layers in ultrathin films

The properties of helical thin films have been thoroughly investigated by classical Monte Carlo simulations. The employed model assumes classical planar spins in a body-centered tetragonal lattice, where the helical arrangement along the film growth direction has been modeled by nearest-neighbor and next-nearest-neighbor competing interactions, the minimal requirement to get helical order. We obtain that, while the in-plane transition temperatures remain essentially unchanged with respect to the bulk ones, the helical or fan arrangement is stabilized at lower and lower temperatures when the film thickness, $n$, decreases; in the ordered phase, increasing the temperature, a softening of the helix pitch wave vector is also observed. Moreover, we show also that the simulation data around both transition temperatures lead us to exclude the presence of a first-order transition for all analyzed sizes. Finally, by comparing the results of the present work to those obtained for other models previously adopted in literature, we can get a deeper insight about the entwined role played by the number (range) of interlayer interactions and surface effects in noncollinear thin films.

Finite-Temperature Transition in the Spin-Dimer Antiferromagnet BaCuSi2O6

We consider a classical XY-like Hamiltonian on a body-centered tetragonal lattice, focusing on the role of interlayer frustration. A three-dimensional (3D) ordered phase is realized via thermal fluctuations, breaking the mirror-image reflection symmetry in addition to the XY symmetry. A heuristic field-theoretical model of the transition has a decoupled fixed point in the 3D XY universality, and our Monte Carlo simulation suggests that there is such a temperature region where long-wavelength fluctuations can be described by this fixed point. However, it is shown using scaling arguments that the decoupled fixed point is unstable against a fluctuation-induced biquadratic interaction, indicating that a crossover to nontrivial critical phenomena with different exponents appears as one approaches the critical point beyond the transient temperature region. This new scenario clearly contradicts the previous notion of the 3D XY universality.

Pseudoelasticity of Cu–Zr nanowires via stress-induced martensitic phase transformations

Atomistic simulations were performed to investigate the pseudoelastic effects induced by martensitic phase transformation from body-centered cubic (B2) to body-centered tetragonal (BCT) lattice in Cu–Zr nanowires. The phase transformation occurs through nucleation and propagation of {100} twin boundary, which differs from the {101} twin boundary for B2 Ni–Al nanowires. During unloading, extension strain up to 45% can be fully recovered through inverse phase transformation. The BCT lattice has also been verified to be metastable for Cu–Zr nanowires with an energy analysis along the epitaxial Bain path. Our work implies Cu–Zr nanowires may be excellent functional components for nanoelectromechanical systems.

Nanostructures and defects in thermoelectric AgPb18SbTe20 single crystal

Nanoparticles play key roles in reducing thermal conductivity, and hence increasing figure of merit for many thermoelectric materials. We have studied the structure of AgPb18SbTe20 (LAST-18) using high resolution imaging, nanoelectron diffraction, energy dispersive spectrum, and electron energy loss spectrum, and observed a range of nanoparticles with different sizes (from less than 1nm to more than 10nm) and shape (sphere, ellipse, square, etc.). The lattice parameters of the nanoparticles have a wide range from 0.601to0.655nm, while those of the matrix have a range from 0.633to0.646nm. The nanoparticles are formed due to the ordering of Pb and Ag–Sb. There are four ordered structures with primitive cubic, primitive tetragonal (T1, a≈a0∕√2, c≈a0, here, a0 is the lattice parameter of the rocksalt-type matrix), primitive tetragonal (T2, a≈a0∕√2, c≈2a0), and body-centered tetragonal (T3, a≈a0∕√2, c≈3a0) lattices, respectively. Antiphase domains, twins, and phase separations were often observed in the nanoparticles. The strain field in the surrounding matrix due to the presence of nanoparticles was retrieved from the high resolution images. The characteristic that the strain field is anisotropic and extends to large area is considered to enhance the scattering of the phonons. The results provide quantitative structure information about nanoparticles, that is essential for the understanding of the origin of the high thermoelectric performance in this class of materials.

Low-energy excitations in DTN below Tc: ESR studies

NiCl2-4SC(NH2)2 (known as DTN) is an S = 1 system with an easy-plane anisotropy dominating over the exchange interaction and exhibiting a field-induced antiferromagnetic ordering with critical fields Bc1=2.1 T, Bc2=12.6 T and temperature Tcmax ∼ 1.2 K. A systematic study of the low-energy excitation spectrum of DTN in the ordered phase at temperatures down to 0.45 K is presented. It is showed that two observed gapped modes can be consistently interpreted within a four-sublattice antiferromagnet model with a weak isotropic corner-center interaction of magnetic ions in the body-centered tetragonal lattice.

Nitride precipitation and coarsening in Fe–2.23 at.% V alloys: XRD and (HR)TEM study of coherent and incoherent diffraction effects caused by misfitting nitride precipitates in a ferrite matrix

Specimens of Fe–2.23 at.% V alloy were nitrided in a NH 3/H 2 gas mixture at 580 °C. The nitrided microstructure was investigated by X-ray diffraction (XRD), and by conventional and high-resolution transmission electron microscopy ((HR)TEM). For specimens homogeneously nitrided for relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the α-Fe-2 0 0 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single body-centered tetragonal lattice, whereas the remaining part of the ferrite is described by a body-centered cubic lattice. Analysis of the microstructure of the nitrided specimens using (HR)TEM confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 °C) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes the VN platelets to appear segmented in the diffraction–contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequences of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix.

Structure of Polydisperse Inverse Ferrofluids: Theory and Computer Simulation

By using theoretical analysis and molecular dynamics simulations, we investigate the structure of colloidal crystals formed by nonmagnetic microparticles (or magnetic holes) suspended in ferrofluids (called inverse ferrofluids), by taking into account the effect of polydispersity in size of the nonmagnetic microparticles. Such polydispersity often exists in real situations. We obtain an analytical expression for the interaction energy of monodisperse, bidisperse, and polydisperse inverse ferrofluids. Body-centered tetragonal (bct) lattices are shown to possess the lowest energy when compared with other sorts of lattices and thus serve as the ground state of the systems. Also, the effect of microparticle size distributions (namely, polydispersity in size) plays an important role in the formation of various kinds of structural configurations. Thus, it seems possible to fabricate colloidal crystals by choosing appropriate polydispersity in size.

Density-functional-theory calculations of matter in strong magnetic fields. II. Infinite chains and condensed matter

We present calculations of the electronic structure of one-dimensional infinite chains and three-dimensional condensed matter in strong magnetic fields ranging from $B={10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{G}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}\mathrm{G}$, appropriate for observed magnetic neutron stars. At these field strengths, the magnetic forces on the electrons dominate over the Coulomb forces, and to a good approximation the electrons are confined to the ground Landau level. Our calculations are based on the density functional theory, and use a local magnetic exchange-correlation function appropriate in the strong field regime. The band structures of electrons in different Landau orbitals are computed self-consistently. Numerical results of the ground-state energies and electron work functions are given for one-dimensional chains ${\mathrm{H}}_{\ensuremath{\infty}}$, ${\mathrm{He}}_{\ensuremath{\infty}}$, ${\mathrm{C}}_{\ensuremath{\infty}}$, and ${\mathrm{Fe}}_{\ensuremath{\infty}}$. Fitting formulae for the $B$ dependence of the energies are also provided. For all the field strengths considered in this paper, hydrogen, helium, and carbon chains are found to be bound relative to individual atoms (although for $B$ less than a few $\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{G}$, carbon infinite chains are very weakly bound relative to individual atoms). Iron chains are significantly bound for $B\ensuremath{\gtrsim}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{G}$ and are weakly bound if at all at $B\ensuremath{\lesssim}{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{G}$. We also study the cohesive property of three-dimensional condensed matter of H, He, C, and Fe at zero pressure, constructed from interacting chains in a body-centered tetragonal lattice. Such three-dimensional condensed matter is found to be bound relative to individual atoms, with the cohesive energy increasing rapidly with increasing $B$.

Magnetic model of the tetragonal-orthorhombic transition in the cuprates

It is shown that a quasi two dimensional (layered) Heisenberg antiferromagnet with fully frustrated interplane couplings ({\it e.g.} on a body-centered tetragonal lattice) generically exhibits two thermal phase transitions with lowering temperature -- an upper transition at $T_{TO}$ (``order from disorder without order'') in which the lattice point-group symmetry is spontaneously broken, and a lower N\'{e}el transition at $T_{N}$ at which spin-rotation symmetry is broken. Although this is the same sequence of transitions observed in La$_2$CuO$_4$, in the Heisenberg model (without additional lattice degrees of freedom) $(T_{TO}-T_N) /T_N$ is much smaller than is observed. The model may apply to the bilayer cuprate La$_2$CaCuO$_6$, in which the transitions are nearly coincident.

Investigation of crystal structure and associated electronic structure of Sr 6BP 5O 20

Strontium borophosphate phosphate (Sr 6BP 5O 20, SrBP), activated by divalent europium ions is a bluish-green phosphor emitting in a broad band with the emission peak near 480 nm. In this paper, we report the crystal structure of SrBP determined from an analysis of the X-ray diffraction pattern of a prismatic single crystal (size 60 μm×50 μm×40 μm). This crystal was chosen from undoped phosphor powder samples prepared for this purpose by solid-state reaction. SrBP is observed to crystallize in a body-centered tetragonal lattice with the lattice parameters a = 9.7 8 9 5 ( 7 ) Å and c = 1 9.0 3 2 ( 3 ) Å , the associated space group being I 4 ¯ c 2 (space group 120). Using the structural data from this study, we have also calculated its electronic structure using the augmented spherical wave method and the local density approximation (LDA). We show the ordering of the electronic states by the density of states (DOS) and the partial DOS plots. The LDA gives a direct optical band gap at the Γ point of about 5 eV. The significance of the crystal structure and associated electronic structure is discussed with respect to maintenance of this phosphor in Hg-discharge lamps.

Precipitation Processes in a Cu–0.9 mass% Be Single Crystal

The precipitation processes in a Cu-0.9.9mass%Be alloy single crystal containing only G.P. zones parallel to the matrix (001)a plane have been studied by high-resolution transmission electron microscopy. The precipitation sequence found is: G.P. zones -» ->y'i -> Y\+ / → y. The phase has a two-layer structure of Be atoms separated by a matrix layer parallel to (001 )a, and a body-centered tetragonal (bct) lattice. The y'i phase consisting of five to ten Be-layers is body-centered monoclinic. The Yi phase continuously transforms from the G.P. zones via and y'i. The heterogeneous formation of y' occurs on the yi phase. The yi or y' phase is bct with a = b = 0.254nm and c = 0.352nm or a = 0.254nm and b = c = 0.9.268nm. The y, y'i or y phase aligns with the matrix according to the Bain orientation relationship. The plate-shaped y' precipitates have the (112)a habit plane and the orientation relationship, (112)a // (012) Y .; [110]a // [100]y. (OR I), or the (113)a habit plane and the orientation relationship, (113)a //(013)y; [110]a // [100]y. (OR 11). The structure of y' successively changes into that of the phase with a = 0.280 nm for OR I or 0.268 nm for OR II. The precipitates of OR I and OR II are elongated along approximately [111]a and [332]a, which are in good agreement with the invariant-line directions.

AC electrokinetics of the microparticles in electrorheological solids

Under the application of electric fields, the structure of electrorheological (ER) solids can be changed from the body-centered tetragonal lattice (ground state) to other lattices. For a particle in the lattice, we derive its dipole factor by taking into account both the local-field effect arising from all the other particles and the multipolar interaction between two touching particles, through the Ewald–Kornfeld formulation and multiple image method. For simplifying the study, the dipole factors are expressed in the dielectric dispersion spectral representation exactly. It is found that the electrorotation (EOR) spectrum of ER solids can be affected significantly by the structure transformation, and the local-field effect as well as the multipolar interaction. Our results are well understood in the spectral representation theory. Thus, it is possible to monitor the structure of ER solids by detecting the EOR spectrum.

Force acting on the microparticles in electrorheological solids under the application of a nonuniform ac electric field

Under the application of electric fields, the structure of electrorheological (ER) solids can be changed from the body-centered tetragonal lattice to other lattices. We have derived the dipole factor for the lattice by taking into account the local-field effect through the Ewald–Kornfeld formulation, and expressed it in the spectral representation exactly. It is found that when the ER solid is subject to a nonuniform ac electric field, the force acting on the microparticle can be affected by the structure transformation, and local-field effect as well as field frequency. Our results are very well understood in the spectral representation theory.

Crystallography of a new metastable phase in Zr-N alloy.

Selected-area and convergent-beam electron diffraction were used to determine the lattice, point and space groups of a new metastable phase (designated the pi phase) in the Zr-N system. The pi phase has a body-centered tetragonal lattice with a approximately 0.66 nm and a c/a ratio of 0.81. The point-group symmetry of the phase is 4/mmm and the space group is I4/mmm.

Dipole Interaction in Triplet Superconductivity with Horizontal Line Nodes: Orientation of the Superconducting Order Parameter in Sr2RuO4

Dipole interaction in the triplet superconductivity is studied. If the Cooper pairs are formed by the electrons at $\mib{r}$ and $\mib{r}+(\pm\frac{a}{2},\pm\frac{a}{2},\pm\frac{c}{2})$ in the body centered tetragonal lattice, (in this case the line nodes run horizontally on the cylindrical Fermi surface), the dipole energy is low when the ${\mib{d}}$-vector is perpendicular to the direction of the angular momentum of the Cooper pairs ($\mib{l}$-vector). This result is in contrast with the dipole energies in the ABM state of superfluid $^3$He, where $\mib{d}$-vector is forced to be parallel or antiparallel to the $\mib{l}$-vector by the dipole interaction. The recent NQR experiment in Sr$_2$RuO$_4$ by Ishida et al. can be explained by this result.

Martensitic Iron-Carbon-Boron Alloy Electrodeposit with Improved Mechanical Properties

Iron-carbon-boron (Fe-C-B) alloy films have been prepared by cathodic deposition from an aqueous solution containing iron(II) chloride hydrate, malic acid, and dimethylamineborane (DMAB). Effects of DMAB concentration on the structural and mechanical characteristics were investigated with an analysis of evolved gas, X-ray diffraction, X-ray photoelectron spectroscopy, and measurements of Vickers hardness number, fracture toughness, and wear volume. The Fe-C-B alloy films had a martensitic structure with body-centered tetragonal lattice with interstitial carbon and boron atoms, and showed high Vickers hardness number of around 700 by solid solution hardening, irrespective of DMAB concentration. The Fe-0.96 mol % C-0.64 mol % B alloy film showed excellent fracture toughness and decreased wear volume, compared with those for Fe-C alloy films. The improvement in fracture toughness and wear resistance was achieved by decreasing the content of oxygen impurity in the films. © 2002 The Electrochemical Society. All rights reserved.