Lattice Parameter Ratio Research Articles

Structural and Thermodynamic Properties of TiAl intermetallics under High Pressure

The structural and thermodynamic properties of TiAl intermetallics under high pressure have been investigated by ab initio plane-wave pseudopotential density functional theory method. It is found that the ratio of lattice parameter c to a keeps almost constant with a value of 1.02 under the pressure from 0 to 20 GPa, which agrees well with the experimental results. With the pressure increasing from 20 to 45 GPa the values of c/a decrease almost linearly from 1.02 to 0.99. These calculated results indicate under low pressure the variation rate for a-axis is almost the same to that for c-axis, but under higher pressure the variation for a-axis is smaller than along c-axis. Through the quasi-harmonic Debye model, the equation of state (EOS) of TiAl intermetallics, as well as the thermal expansion and heat capacity at various pressures and temperatures are also studied.

Structure and electrical properties of lanthanum doped Bi_2Sr_2Ca_{2-x}La_xCu_3O_{10+ \delta} superconductor

In this work we studied the effect of La-doping on phase purity, crystal structure and electrical resistivity of Bi_2Sr_2Ca_2Cu_3O_{10+\delta} superconductor, synthesized by usual solid state reaction method. The X-ray florescence (XRF) show the stoichiometry of samples and X-ray diffraction (XRD) analysis showed the change of structure from orthorhombic to tetragonal by increasing La doping. It was found that the change of lanthanum concentrations of all our samples produce a change in the oxygen content, the ratio of lattice parameters c/a and mass density \rho_m. Electrical resistivity, using the four-probe technique, was used to find the critical temperature T_c and to put in evidence metallic and semiconducting behavior in normal state.

Complex investigations into V2 ± δO3 compounds within the region of homogeneity

This paper reports on a comprehensive study of the structure, electrical resistivity, and heat capacity of V2 ± δO3 compounds within the region of homogeneity. Some parameters of the metal-insulator phase transition are determined. The dependences of the phase transition temperature on the ratio of lattice parameters “c/a” and the dependences of the jump of electrical resistivity and hysteresis as a function of the change in entropy of the sample at the phase transition are established.

High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides

High pressure–high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO2 and Li[NixLi1/3−2x/3Mn2/3−x/3]O2 where x = 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO2 remain almost identical to the pristine sample. The high pressure treatment of LiNi0.5Mn0.5O2 (x = 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li1/6Ni1/4Mn7/12]O2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li1/6Ni1/4Mn7/12]O2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li1/6Ni1/4Mn7/12]O2 deliver capacities upon cycling near 200 mA h g−1, even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li1/6Ni1/4Mn7/12]O2; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi0.5Mn0.5O2. In that sense, the Li excess materials are superior to LiNi0.5Mn0.5O2, whose electrochemical characteristics are very sensitive to structural modifications.

Structural and Magnetic Properties of Fe2P under Pressure at Low Temperature

We have carried out X-ray diffraction and 57 Fe nuclear forward scattering using synchrotron radiation to study structural and magnetic properties of Fe 2 P under high pressure at low temperature. The lattice parameter ratio changes at 1.8 and 7.0 GPa as a function of pressure at 24 K without structural change, where the transition at 1.8 GPa corresponds to a ferromagnetic-to-antiferromagnetic one. We deduced the magnetic structures of Fe 2 P at 3 K and 2.5 GPa from refined hyperfine interaction parameters extracted from 57 Fe nuclear forward scattering spectra. Without external magnetic field where Fe 2 P is in the antiferromagnetic state, the magnetic structure is a complex one with a propagation vector along . The distinct change of 57 Fe nuclear forward scattering spectra with external magnetic field along [001] indicates that a metamagnetic-like transition occurs below 5 kOe at 3 K and 2.5 GPa. With external magnetic field, the magnetic structures are not simple collinear ones. Furthermore, the...

Enhancement of piezoelectric response in (100)/(001) oriented tetragonal Pb(Zr, Ti)O3 films by controlling tetragonality and volume fraction of the (001) orientation

(100)/(001)-oriented 2 μm-thick Pb(Ti, Zr)O3, films with the Zr/(Zr + Ti) ratio of 0.39 and 0.50 were prepared by metalorganic chemical vapor deposition. The volume fraction of (001)-oriented domain, Vc, was controlled by selecting the kinds of substrates with different thermal expansion coefficient. The effective piezoelectric constant increased up to 310 pm/V with decreasing the Vc and tetragonality (lattice parameter ratio of c axis to a axis) determined by the decrease of the Zr/(Zr + Ti) ratio. Observed large piezoelectric response was considered to be mainly contributed by the extrinsic effect, such as the 90o domain switching. These experimental inputs suggest the new concept for enhancement of the piezoelectric properties.

Structure and magnetic state of the films deposited by laser ablation of composite nickel and palladium targets

Transmission electron microscopy, electron diffraction, and vibrating-sample magnetometry are used to show that a metastable hcp structure can form in both nickel and Ni-Pd alloy films during alternating sputtering of the Ni and Pd components of composite targets. The hcp lattice parameters increase monotonically when the palladium content in a sputtered target increases in the range 0–75%. The ratio of the hcp lattice parameters c/a is close to the ideal ratio for the hcp lattice (1.63) within the limits of experimental error. In the as-deposited state, nickel and Ni-Pd alloy films with an hcp structure have no magnetic moment. Upon annealing, the films transform into a ferromagnetic state with an fcc structure. The concentration dependence of the lattice parameter of the fcc solid solution a 0 is found to exhibit a positive deviation from Vegard’s law, which is characteristic of alloys with a concave liquidus line.

Structural distortion and orbital ordering in the triangular-lattice antiferromagnet NaVO2from first principles

Triangular-lattice antiferromagnets with the general formula $\mathit{AT}$O${}_{2}$ ($A$ $=$alkali metal, $T$ $=$3$d$ transition metal) often adopt a slightly distorted crystal structure at low temperatures, accompanying a lifting of magnetic frustration and the appearance of long-range magnetic ordering and sometimes a particular orbital ordering. Taking NaVO${}_{2}$ as an example, we successfully demonstrate that the tiny structural distortion with a ratio of lattice parameters, ${a}_{m}$/${b}_{m}$ $=$1.755, and the formation of orbital ordering observed in recent neutron-diffraction experiments can be well interpreted by first-principles calculations including 3$d$ electron correlations with parameter ${U}_{\mathrm{eff}}$ $=$3.6 eV. This distinct study on ``pure'' structural distortion is expected to be applied in other triangular-lattice antiferromagnetic systems.

Structural and magnetic properties on F-doped LiVO2 with two-dimensional triangular lattice

ABSTRACTThe layered oxide LiVO2 recently has received more attention due to its interesting structural and magnetic behaviors involving the two-dimensional magnetic frustration in these systems. We synthesized a series of F-doped LiVO2 samples, and reported the F-doping effect on the structure and transition temperature Tt. The samples LiVO2-xFx (x=0, 0.1, 0.2 and 0.3) were characterized by X-ray diffraction, scanning electron microscope (SEM), differential scanning calorimetry (DSC), magnetic susceptibility and specific heat measurement. The structural analysis shows that with increasing x, the ratio of lattice parameter c/a increasing, i.e. in the a-b plane the lattice is compressed while in the c-axis direction the lattice expands. The DSC measurements show that a first-order phase transition happens at around 500 K, and the thermal hysteresis around phase transition temperature Tt increases with increasing x. Substitution of O with F ions results in a change of two dimensional characteristics and the distortion of the VO6 block in structure, which significantly influence the magnetic ordering transition temperature Tt.

Analytical O-line solutions to phase transformation crystallography in fcc/bcc systems

This paper presents the progress in phase transformation crystallography of the fcc/bcc system based on the O-line model. In contrast to previous numerical methods, closed-form expressions have been derived to describe the orientation relationship (OR), habit plane (HP) normal, dislocation direction and spacing as functions of the lattice parameter ratio (λ) and the angle between the Burgers vectors of the dislocations. Possible directions of the dislocation and the HP normal are presented as elliptical curves in stereographic projections and explicit expressions for the curves are provided. In the 3-D space defined by the OR, the O-line criterion only restricts two degrees of freedom in the OR, while various constraints have been applied to the remaining degree of freedom. A comparison of the calculations with those from existing models indicates the general applicability of the method. Furthermore, the property that the HP normal lays on the surface of an elliptic cone can be readily extended to other systems.

Crystallography of Martensite in TiAu Shape Memory Alloy

The twin structure, habit plane orientation, and morphology of B19 martensite in TiAu, which is a candidate shape memory alloy (SMA) for high-temperature and biomedical applications, were investigated by conventional transmission electron microscopy. Almost all internal twins were {111} type I twins as lattice-invariant deformation (LID). The type II twin was scarcely observed in TiAu, unlike in TiPd and TiPt SMAs. The habit plane roughly corresponded to the twinning plane (K1 plane) of the type II twin because of the superb lattice parameter ratio of TiAu. As a result, an energy-minimizing microstructure referred to as “twins within twins” appears as the major microstructure. The selection rules for the twinning of LID are also discussed considering the results of extensive studies on LID in SMAs.

A two-dimensional analytical method for the transformation crystallography based on vector analysis

A simple 2-D analytical method has been developed for calculating the crystallographic features, including the orientation relationship (OR), habit plane (HP) and the direction and spacing of dislocations in the HP, under the condition that a pair of Burgers vectors from both phases are parallel to each other lying in the HP. In contrast to previous 2-D models for the OR and HP, which are on the plane normal to the parallel Burgers vectors, the present 2-D model is based on vector analysis in the HP. Closed-form expressions for the crystallographic features have been derived as functions of lattice parameter ratios. While the results are in agreement with those from a 2-D method based on the O-line model, the present method is simpler and improves our understanding of crystallographic features.

First-principles study of structural distortions in frustrated antiferromagnetα-NaMnO2

First-principles study on structural distortions of the frustrated antiferromagnet $\ensuremath{\alpha}{\text{-NaMnO}}_{2}$ has been performed by confining the distortions within $ab$ plane of the unit cell. The calculated ratios of lattice parameters, $a/b$, for both triclinic and monoclinic phases are quite close to the experimental values. The stability of antiferromagnetic triclinic phase compared to the monoclinic phase, which cannot be interpreted by the total energy and density of states, is clearly illustrated by the evolution of total energy with triclinic-monoclinic structural distortion, in good agreement with recent neutron-diffraction experiments. Magnetoelastic effect and anisotropic exchange interactions of this material are also discussed.

Short time and low temperature mineralization of ZnO nanorod-bunches from solution using cetyl trimethyl ammonium bromide

The low temperature mineralization of zinc oxide nanorod bunches from zinc precursor solution mixed with the cationic surfactant Cetyl Trimethyl Ammonium Bromide is reported. Five different combinations of zinc precursor and surfactant ratios are chosen and their properties are compared using the average particle size, lattice parameter ratio ( c / a ) of hexagonal ZnO from X-ray diffraction, the morphologies from SEM, TEM and their optical characteristics by using optical absorption spectroscopy spectrometry. A surfactant lowers the surface tension of the solution, and forms a thermodynamically ordered and a disordered phase wherein, ordered phase seeds the growth of the ZnO nanostructures. A peculiar circular structure called ‘micelle’ is formed by the surfactant. The heads, located on the peripheries of the micelle, are the ordered phase in the solution, and are the original nucleation sites of the Zinc Oxide nanostructures. This fashion of nucleation is the reason for the ‘arms of the wheel-like’ morphology consisting of nanorod bunches. Out of the five combinations experimented, the 3:1, 3:2, 1:1 ratios show nearness in the standard c / a value. The photoluminescence spectrum shows UV emission in the region of 380 nm and weak blue emission. The UV–VIS–NIR spectrum has exhibited a characteristic UV-absorption.

A study of a new type of deviation from the Kurdjumov –Sachs orientation relationship in face-centered-cubic/body-centered-cubic transformation system

Slight deviations from the Kurdjumov –Sachs orientation relationship (KS OR) are often observed in the face-centered-cubic/body-centered-cubic (fcc/bcc) transformation systems, and some can be explained on the premise of the parallelism of either the close-packed planes (CPPs) or the close-packed directions (CPDs), as suggested in many models. This work investigates a new OR which deviates from the KS OR in both the CPPs and the CPDs. This OR is determined by requiring the habit plane to contain a single set of periodic dislocations along the invariant line lying in the CPPs. The angles of the double deviation, as a function of the lattice parameter ratio, are proved to be identical. The calculated results for a stainless steel are in good agreement with the experimental observations. The habit plane for the new OR is associated with slightly lower interfacial energy than those with the ORs of single deviation, which might explain the preference by nature in the present case.

Formula omitted] Twinning nucleation mechanisms in hexagonal-close-packed crystals

Mechanisms for ( 1 ¯ 0 1 2 ) twinning in hexagonal-close-packed crystals at an atomic scale were studied using topological analysis and atomistic simulations. Two twinning mechanisms were found: a normal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of multiple twinning dislocations; and a zonal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of a partial dislocation and multiple twinning dislocations. The twinning direction, dependent on the ratio of lattice parameters c/ a, is along [ 1 0 1 ¯ 1 ] when c / a < 3 , but along the opposite direction when c / a > 3 . Atomistic simulations, using density function theory for Mg, Zr and Zn and an empirical potential for Mg, were performed to study the kinetics and energetics associated with the two twinning mechanisms. The results show that the zonal-twinning mechanism is energetically favorable relative to the normal-twinning mechanism, because the zonal dislocation has a smaller Burgers vector.

Substitution-induced superconductivity inSrFe2−xRuxAs2(0≤x≤2)

The magnetism in ${\text{SrFe}}_{2}{\text{As}}_{2}$ can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ and leads to bulk superconductivity for $0.6\ensuremath{\le}x\ensuremath{\le}0.8$. Magnetization, electrical resistivity, and specific-heat data show ${T}_{c}$ up to $\ensuremath{\approx}20\text{ }\text{K}$. Detailed structural investigations reveal a strong decrease in the lattice parameter ratio $c/a$ with increasing $x$. Density functional theory band-structure calculations are in line with the observation that the magnetic order in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ is only destabilized for large $x$.

Superconductivity and the effects of pressure and structure in single-crystallineSrNi2P2

Heat capacity, magnetic susceptibility, NMR, and resistivity of ${\text{SrNi}}_{2}{\text{P}}_{2}$ single crystals are presented, illustrating a purely structural transition at 325 K with no magnetism. Bulk superconductivity is found at 1.4 K. The magnitude of the transition temperature ${T}_{c}$, fits to the heat-capacity data, the small upper critical field ${H}_{c2}=390\text{ }\text{Oe}$, and Ginzburg-Landau parameter $\ensuremath{\kappa}=2.1$ suggest a conventional fully gapped superconductor. With applied pressure a second structural phase transition occurs which results in an 8% reduction in the $c/a$ ratio of lattice parameters. We find that superconductivity persists into this high-pressure phase, although the transition temperature is monotonically suppressed with increasing pressure. Comparison of these Ni-P data as well as layered Fe-As and Ni-As superconductor indicates that reduced dimensionality can be a mechanism for increasing the transition temperature.

Magnetostriction properties of FePd thin films: Dependence on microstructure

FePd-alloys as thin films are potential actuator materials due to their large magnetostriction. In this paper experiments and simulations of TEM and XRD diffraction patterns showed, that the L1 0-ordering has no influence on the lattice parameter ratio c/ a = 1.37. The degree of long-range order is higher for the sputtering temperature of 573 K than of 423 K, but the magnetostriction is higher for thin films produced at 423 K.

Mechanism of the fcc-bcc martensitic transformation with the fastest transformation of close-packed planes. I. The lattice parameter ratio and habitus planes

In terms of the concepts of heterogeneous nucleation and the related driving wave process, a version of the dynamic theory of the formation of martensite crystals is stated in which the wave process initiates the fastest transformation of close-packed {ie659-01} atomic planes of a parent fcc phase into {ie659-02} planes of a bcc phase. The lattice parameter ratio and the orientations of habit planes are analytically related with the elastic properties of the γ-phase. Quantitative estimation performed with the use of elastic moduli for an iron-nickel alloy yields habitus orientations close to {ie659-03}. An experiment is proposed to verify the theoretical predictions. A new pattern of the short-wave correction that finishes the fcc-bcc rearrangement is set forth.