Lattice Parameter Ratio Research Articles

Structural, morphological and magnetic analysis of Cd–Co–S dilute magnetic semiconductor nanofilms

Cd1−xCoxS dilute magnetic semiconductor nanofilms (0≤x≤0.08at%) deposited by chemical bath deposition have been investigated using grazing angle x-ray diffraction, atomic force microscopy and vibrating sample magnetometer. The introduction of Co2+ ions in CdS structure induces structural disorders and hence, results in degradation of crystallinity. The crystallite size, interplanar spacing and lattice parameter ratio decrease with increasing Co2+ concentration in CdS. The diamagnetic state of CdS disappears with increase in Co concentration and films with x>0.02 exhibit ferromagnetism. This may be explained in terms of the spin–orbit interactions and Co2+ ion induced the lattice defects and phase separation.

Influence of thickness on the structural properties of radio-frequency and direct-current magnetron sputtered TiO2 anatase thin films

Thin films of TiO2 were deposited by reactive sputtering of a Ti target on unheated substrates and post-heated at 300°C and 500°C. They exhibit a granular structure. Direct current-sputtered films are amorphous as-deposited and crystallize (to pure anatase) only at 500°C. Radio-frequency (rf)-sputtered films are already crystalline (pure anatase) as-deposited on unheated substrates. Above a thickness of 100nm, the crystallite size, as deduced from the half-width of X-ray diffraction (XRD) peaks, is constant at 35nm and decreases to zero when the thickness decreases to 25nm. Below a thickness of 25nm, the films are X-ray amorphous. Height and half-width of the XRD peaks of rf-sputtered films do not change upon post-heating at 300 or 500°C. A larger lattice parameter ratio c/a is observed with respect to the bulk value that decreases with increasing film thickness and is about 1% larger for a film thickness larger than 100nm.

Fe–Ni composition dependence of magnetic anisotropy in artificially fabricated L1 0-ordered FeNi films

We prepared L10-ordered FeNi alloy films by alternate deposition of Fe and Ni monatomic layers, and investigated their magnetic anisotropy. We employed a non-ferromagnetic Au–Cu–Ni buffer layer with a flat surface and good lattice matching to L10-FeNi. An L10-FeNi film grown on Au6Cu51Ni43 showed a large uniaxial magnetic anisotropy energy (Ku = 7.0 × 106 erg cm−3). Ku monotonically increased with the long-range order parameter (S) of the L10 phase. We investigated the Fe–Ni composition dependence by alternating the deposition of Fe 1 − x and Ni 1 + x monatomic layers (− 0.4 < x < 0.4). Saturation magnetization (Ms) and Ku showed maxima (Ms = 1470 emu cm−3, Ku = 9.3 × 106 erg cm−3) for Fe60Ni40 (x = −0.2) while S showed a maximum at the stoichiometric composition (x = 0). The change in the ratio of lattice parameters (c/a) was small for all compositions. We found that enrichment of Fe is very effective to enhance Ku. The large Ms and Ku of Fe60Ni40 indicate that Fe-rich L10-FeNi is promising as a rare-earth-free permanent magnet.

Effect of temperature on the electrical properties of Zn0.95M0.05O (M = Zn, Fe, Ni)

We report here the structural and electrical properties of Zn0.95M0.05O ceramic varistors, M = Zn, Ni and Fe. The samples were tested for phase purity and structural morphology by using X-Ray diffraction XRD and scanning electron microscope SEM techniques. The current-voltage characteristics J-E were obtained by dc electrical measurements in the temperature range of 300–500 K. Addition of doping did not influence the hexagonal wurtzite structure of ZnO ceramics. Furthermore, the lattice parameters ratio c/a for hexagonal distortion and the length of the bond parallel to the c axis, u were nearly unaffected. The average grain size was decreased from 1.57 μm for ZnO to 1.19 μm for Ni sample and to 1.22 μm for Fe sample. The breakdown field EB was decreased as the temperature increased, in the following order: Fe > Zn > Ni. The nonlinear region was clearly observed for all samples as the temperature increased up to 400 K and completely disappeared with further increase of temperature up to 500 K. The values of nonlinear coefficient, a were between 1.16 and 42 for all samples, in the following order: Fe > Zn > Ni. Moreover, the electrical conductivity s was gradually increased as the temperature increased up to 500 K, in the following order: Ni > Zn > Fe. On the other hand, the activation energies were 0.194 eV, 0.136 and 0.223 eV for all samples, in the following order: Fe, Zn and Ni. These results have been discussed in terms of valence states, magnetic moment and thermo-ionic emission, which were produced by the doping, and controlling the potential barrier of ZnO.

Twist, tilt, and symmetric grain boundaries in hexagonal materials

Grain boundaries of characteristic geometry, e.g., twist, tilt, and symmetric boundaries are often used as reference boundaries in analyses of boundary networks in polycrystalline materials. This article deals with the issue of proper identification of characteristic boundaries in the case of materials with hexagonal D6h symmetry. To identify all boundaries of characteristic types, both analytical calculations and numerical searches are used. The first approach provides exact parameters of the characteristic boundaries, whereas the second one gives boundaries which can be classified as characteristic if some tolerance is allowed. In both methods, all symmetrically equivalent boundary representations are taken into consideration. The obtained sets of twist, tilt, symmetric, and 180°-tilt boundaries are presented in the form of two-dimensional maps containing stereographic projections of the corresponding boundary plane normals for selected grain misorientations. These diagrams facilitate interpretation of experimental distributions of grain boundaries; with the representation used, they can be directly linked to experimental distributions. Examples of such diagrams for lattice parameter ratios c/a of \(\sqrt{5/2}\) and \(\sqrt{20/21}\) are presented. They are compared to example boundary distributions in Ti alloy and distributions of WC/WC boundaries in WC–Co composites available in the literature.

Effect of solute atoms on fracture toughness in dilute magnesium alloys

The effect of alloying elements on the toughness and the fracture behaviour was investigated on seven kinds of Mg-0.3 at.% X (X = Ag, Al, Ca, Pb, Sn, Y and Zn) alloys with a grain size of 3–5 μm. The fracture toughness and fracture behaviour in magnesium alloys were closely related to the segregation energy. The Mg–Al and –Zn alloys that had small segregation energy showed high toughness and ductile fracture in most regions, while the Mg–Ca alloy with large segregation energy exhibited low toughness and intergranular fracture. These different tendencies resulted from solute segregation at grain boundaries (GBs). The change in the lattice parameter ratio was the influential material parameter regardless of whether the GB embrittlement was for enhancement or suppression.

Solid Solutions of Lead Metaniobate—Stabilization of the Ferroelectric Polymorph and the Effect on the Lattice Parameters, Dielectric, Ferroelectric, and Piezoelectric Properties

Ferroelectric orthorhombic lead metaniobate (PbNb2O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The high‐temperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2O6 by minor chemical substitution in the series [(1 − x) PbNb2O6–xBiTiNbO6], [(1 − x)PbNb2O6–xNa0.5Bi0.5Nb2O6], [(1 − x)PbNb2O6–xK0.5Bi0.5Nb2O6], and [(1 − x)PbNb2O6–xCaTiO3]. The high‐temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid‐state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2b/a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower Tc. However, some improvements seen with 2% CaTiO3 were not accompanied by Tc decrease.

Signatures of a Pressure-Induced Topological Quantum Phase Transition in BiTeI

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through p(z) band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition and are consistent with a recent theoretical proposal.

Highlights on the Anisotropic Oxygen Transport Properties of Nickelates with K2NiF4-Type Structure: Links with the Electrochemical Properties of the Corresponding IT-SOFC’s Cathodes

Understanding fundamental aspects of oxygen self-diffusion in MIEC materials is crucial for the discovery of next-generation of cathode materials. Perovskite-type oxides are the most studied material, the oxide ion diffusion being based on the vacancy migration in a cubic-like disordered network. According to results of predictive atomistic simulations, a current trend concerns experimental works related to perovskite-derived phases with anisotropic structures where oxide ion transport is no longer based on oxygen vacancies but on oxygen interstitials. Then a particular attention is here paid to the nickelates Ln2NiO4+δ, Ln=La, Nd, Pr, with the orthorhombic K2NiF4-type structure characterized by a high c/a lattice parameters ratio. We have completed our previous work performed on single crystals and thin films aiming at experimentally determining the oxygen diffusion and surface exchange coefficients using the so-called IEDP method, as a function of the crystallographic orientation as well as the rare-earth cation. Highly anisotropic oxygen diffusion properties have been evidenced and the activation energies have been compared. Moreover, the electrochemical performances (polarization resistance measurements) of the corresponding porous cathode materials were previously determined in the literature in the same T-range, using symmetrical half cells including YSZ as electrolyte. The results are discussed with regards to the oxygen diffusion properties of the electrodes.

Evidence for Short-Range-Ordered Charge Stripes Far above the Charge-Ordering Transition inLa1.67Sr0.33NiO4

The temperature evolution of structural effects associated with charge order (CO) and spin order in La1.67Sr0.33NiO4 has been investigated using neutron powder diffraction. We report an anomalous shrinking of the c/a lattice parameter ratio that correlates with T(CO). The sign of this change can be explained by the change in interlayer Coulomb energy between the static-stripe-ordered state and the fluctuating-stripe-ordered state or the charge-disordered state. In addition, we identify a contribution to the mean-square displacements of Ni and in-plane O atoms whose width correlates quite well with the size of the pseudogap extracted from the reported optical conductivity, with a non-Debye-like component that persists below and well above T(CO). We infer that dynamic charge-stripe correlations survive to T∼2T(CO).

Influence of normal and shear strain on magnetic anisotropy energy of hcp cobalt: An ab initio study

Abstract

Element-specific quantitative determination of the local atomic order in CoPt alloy nanoparticles: Experiment and theory

An annealing-driven transition from a chemically disordered $A1$-like structure to a chemically ordered $L{1}_{0}$-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios $c/a$ are inferred. This ratio is found to be remarkably different depending on the approach used ($c/a&gt;1$ from x-ray absorption at the Co $K$ edge and $c/a\ensuremath{\simeq}0.94$ from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.

Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition.

We discover that hcp phases of Fe and Fe(0.9)Ni(0.1) undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mössbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.

Crystal structure of dense pseudo-cubic boron allotrope, pc-B52, by powder X-ray diffraction

During past years, a number of reports have been published on synthesis of tetragonal allotrope of boron, t-B52 phase. However, no unambiguous characterization of the crystal structure has been performed to the present time, while remarkable variation of the a/c lattice-parameter ratio raises strong doubts about its uniqueness. Here the Rietveld refinement of the crystal structure of the high pressure-high temperature boron phase synthesized by a direct solid-state transformation of rhombohedral β-B106 at 20 GPa and 2500 K has been reported for the first time. Although this boron allotrope belongs to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c ratio of $\sqrt 2 $ .

12% magnetic field-induced strain in Ni-Mn-Ga-based non-modulated martensite

Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni46Mn24Ga22Co4Cu4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c/a&amp;gt;1. The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σTW and the magnetic stress σMAG were also measured and the condition for a giant MFIS observation σTW&amp;lt;σMAG was confirmed. The MFIS was achieved in NM Ni46Mn24Ga22Co4Cu4 martensite by considerable lowering of the σTW value as compared to the values for NM martensites in ternary Ni-Mn-Ga system.

Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu2Zn1–xFexGeSe4

Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu(2)Zn(1-x)Fe(x)GeSe(4). Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials.

Lattice crossover and phase transitions in NdAlO3–GdAlO3 system

Phase and structural behaviour in the (1−x)NdAlO3–xGdAlO3 system in a whole concentration range has been studied by means of in situ high-resolution X-ray synchrotron powder diffraction technique and differential thermal analysis. Two kinds of solid solutions Nd1−xGdxAlO3 have been found at room temperature: one with rhombohedral (x<0.15) and one with orthorhombic (x≥0.20) symmetry. A morphotropic phase transition occurs at x≈0.15, where the co-existence of both phases was observed. Peculiarity of the orthorhombic solid solution is the lattice parameter crossover at the compositions with x=0.33, 0.49 and 0.62. First-order structural transition Pbnm↔R3¯с has been detected both from in situ powder diffraction and thermal analysis data. Continuous phase transformation R3¯с↔Pm3¯m above 2140K has been predicted for Nd-rich sample Nd0.85Gd0.15AlO3 from the extrapolation of high-temperature behaviour of the lattice parameter ratio of the rhombohedral phase. Based on the experimental data, the phase diagram of the pseudo-binary system NdAlO3–GdAlO3 has been constructed.

Film Thickness Dependence of Crystal Structure in 100-Oriented Epitaxial Pb(Zr0.65Ti0.35)O3 Films Grown on Single-Crystal Substrates with Different Thermal Expansion Coefficients

100-oriented epitaxial Pb(Zr0.65Ti0.35)O3 films with various film thicknesses from 0.1 to 3 µm were grown on (100)cSrRuO3 ∥ (100)SrTiO3 and (100)cSrRuO3 ∥ (100)LaNiO3 ∥ (001)CaF2 substrates. The out-of-plane/in-plane lattice parameter ratio of the films on the CaF2 substrates was larger than that on the SrTiO3 substrates up to 1.1 µm film thickness, while (90°-α) (α was defined as the internal tilt angle) was almost 0°. Results of analysis of Raman spectra and piezoresponse images suggest that the 1.1-µm-thick film grown on the (100)cSrRuO3 ∥ (100)LaNiO3 ∥ (001)CaF2 substrate had tetragonal symmetry with a polar-axis orientation. Moreover, the saturation polarization values of the films measured from P–E hysteresis loops correspond to the two Ps values estimated from the thermodynamic theory, assuming the change in the polar direction due to the symmetry change to tetragonal, and from the crystal distortion in tetragonal symmetry. This can be explained by the large compressive stress from the CaF2 substrate having a large thermal expansion coefficient.

Synthesis of Nanocrystalline β-Tricalcium Phosphate from Chicken Eggshells by Precipitation Method

Nanocrystalline β-TCP was successfully synthesized by simple precipitation method using calcium nitrate obtained eggshell as the calcium source, ammonium phosphate as the phosphate source and ammonia solution as agents for pH adjustment via our synthesis condition. Also, the effect of calcination temperature in range of 200-1200°C on phase transformation was investigated. The synthesized powders were characterized by X-Ray Diffraction (XRD), Rietveld refinements and Fourier transform infrared (FT-IR) techniques. The results indicate that β-TCP was observed at the calcination temperature of 700 °C and above. Furthermore, their crystallinity and crystalline size increase with increasing temperature. The a/c ratio of lattice parameter at temperature of 700-1200°C is similar to the standard phase of β-TCP. The chemical structure confirmed via FT-IR showed the band positions and functional groups, which are similar to that of stoichiometric β-TCP. Finally, eggshells are the potential material for synthesizing nanostructured BCP and may be possible in a low-cost production.

Electronic property of SrFe2As2 under high pressure studied by 57Fe Mössbauer spectroscopy

We have studied the electronic state of Fe atoms in SrFe2As2 under pressure at room temperature by 57Fe Mossbauer spectroscopy using a single crystalline sample. A center shift δc.s. and an electric quadrupole interaction parameter e2qQ/2 show the discontinuous increase and decrease at around 7 GPa,respectively, implying the pressure-induced structural phase transition. Furthermore, δc.s. deviates from the linear pressure dependence above 4 GPa without any anomaly in the pressure dependence of e2qQ/2. The anomaly corresponds to a change of the pressure dependence of lattice parameter ratio c/a, where volume of the unit cell decreases monotonously with increasing pressure.