Lattice Constant Measurements Research Articles

Structural and Magnetic Studies of Nano-crystalline Ferrites MFe2O4 (M = Zn, Ni, Cu, and Co) Synthesized Via Citrate Gel Autocombustion Method

Nano-crystalline ferrites with the chemical formula MFe2O4 (M = Zn, Ni, Cu, and Co) were synthesized via autocombustion route with citric acid as fuel. The ratio of citric acid to metal nitrate was taken as 1:1. The synthesized samples were characterized by powder X-ray diffraction and far-IR spectroscopy. The measured lattice constants and observed characteristic IR absorption bands of all samples were in good agreement with the reported values and showed the formation of a cubic spinel structure. The crystallite sizes of all samples were determined using high-intensity peaks and a W-H plot. The crystallite sizes of all samples were observed to be in the range of 16–26 nm with the least crystallite size of 16.8 nm for copper ferrite. All structural parameters were calculated using an experimental lattice constant and an oxygen positional parameter and correlated with far-IR results. Zinc and copper owing to their large cation radii showed the expansion of the lattice in view of the oxygen positional parameter. Magnetic measurements were carried out using a vibrating sample magnetometer at room temperature, and parameters such as saturation magnetization, coercivity, remanence, squareness ratio, and Bohr magnetons were calculated. Among all synthesized nano-ferrites, cobalt nano-ferrite showed the highest saturation magnetization of 41 emu/g, whereas zinc ferrite displayed a low saturation magnetization value of 12.87 emu/g.

Lattice constant measurement from electron backscatter diffraction patterns.

Kikuchi bands in election backscattered diffraction patterns (EBSP) contain information about lattice constants of crystallographic samples that can be extracted via the Bragg equation. An advantage of lattice constant measurement from EBSPs over diffraction (XRD) is the ability to perform local analysis. In this study, lattice constants of cubic STN and cubic YSZ in the pure materials and in co-sintered composites were measured from their EBSPs acquired at 10 kV using a silicon single crystal as a calibration reference. The EBSP distortion was corrected by spherical back projection and Kikuchi band analysis was made using in-house software. The error of the lattice constant measurement was determined to be in the range of 0.09-1.12% compared to values determined by XRD and from literature. The confidence level of the method is indicated by the standard deviation of the measurement, which is approximately 0.04 Å. Studying Kikuchi band size dependence of the measurement precision shows that the measurement error decays with increasing band size (i.e. decreasing lattice constant). However, in practice, the sharpness of wide bands tends to be low due to their low intensity, thus limiting the measurement precision. Possible methods to improve measurement precision are suggested.

Accurate determination of lattice parameters based on Niggli reduced cell theory by using digitized electron diffraction micrograph

In this paper, we used Niggli reduced cell theory to determine lattice constants of a micro/nano crystal by using electron diffraction patterns. The Niggli reduced cell method enhanced the accuracy of lattice constant measurement obviously, because the lengths and the angles of lattice vectors of a primitive cell can be measured directly on the electron micrographs instead of a double tilt holder. With the aid of digitized algorithm and least square optimization by using three digitized micrographs, a valid reciprocal Niggli reduced cell number can be obtained. Thus a reciprocal and real Bravais lattices are acquired. The results of three examples, i.e., Mg4Zn7, an unknown phase (Precipitate phase in nickel-base superalloy) and Ba4Ti13O30 showed that the maximum errors are 1.6% for lengths and are 0.3% for angles.

How does pore-induced crack change as temperatures decrease from 293 K to 77 K?

The pore-induced cracks in pressureless-sintered SiC ceramics have been investigated at ambient and cryogenic temperatures. The Weibull plots of flexural strength were determined to compare the effects of pore-induced cracks on fracture behavior at 77K and 293K. The stress field near crack tip was investigated based on the microstructure of sintered SiC. The measurements of lattice constants of single crystal SiC were carried out by in situ X-ray diffraction. It was found that the effect of defect-induced crack on fracture was diminished at cryogenic temperatures, owing to the decreases of cracks׳ effective size and increases of propagate resistance. The improved fracture reliability and enhanced flexural strength of SiC ceramics facilitate their applications in space-based reflectors.

Asymmetric “melting” and “freezing” kinetics of the magnetostructural phase transition in B2-ordered FeRh epilayers

Synchrotron X-ray diffraction was used to study the phase transformation processes during the magnetostructural transition in a B2-ordered FeRh (001)-oriented epilayer grown on MgO by sputtering. Out-of-plane lattice constant measurements within the hysteretic regime of the transition reveal a microstructure consistent with the coexistence of lattice-expanded and contracted phases in spatially distinct regions. It was found that the phase separation is more pronounced during cooling than heating. Furthermore, whilst lattice-expanded domains that span the height of the film can be undercooled by several kelvins, there is no equivalent superheating. This asymmetry between the cooling and heating processes in FeRh is consistent with the difference in the kinetics of generic freezing and melting transitions.

High‐Throughput Fabrication of Au–Cu Nanoparticle Libraries by Combinatorial Sputtering in Ionic Liquids

Materials libraries of binary alloy nanoparticles (NPs) are synthesized by combinatorial co‐sputter deposition of Cu and Au into the ionic liquid (IL) 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C4im][Tf2N]), which is contained in a micromachined cavity array substrate. The resulting NPs and NP‐suspensions are investigated by transmission electron microscopy (TEM), X‐ray diffraction (XRD), UV‐Vis measurements (UV‐Vis), and attenuated total reflection Fourier transformed infrared (ATR‐FTIR) spectroscopy. Whereas the NPs can be directly observed in the IL using TEM, for XRD measurements the NP concentration is too low to lead to satisfactory results. Thus, a new NP isolation process involving capping agents is developed which enables separation of NPs from the IL without changing their size, morphology, composition, and state of aggregation. The results of the NP characterization show that next to the unary Cu and Au NPs, both stoichiometric and non‐stoichiometric Cu–Au NPs smaller than 7 nm can be readily obtained. Whereas the size and shape of the alloy NPs change with alloy composition, for a fixed composition the NPs have a small size distribution. The measured lattice constants of all capped NPs show unexpected increased values, which could be related to the NP/surfactant interactions.

In situ TEM study of deformation twinning in Ni–Mn–Ga non-modulated martensite

The straining of non-modulated (NM) Ni–Mn–Ga martensite was studied by in situ transmission electron microscopy (TEM). Initially, the self-accommodated NM martensitic structure consists of internally twinned domains. During straining, the detwinning process starts within these domains. The internal twin variant more favorably oriented to the stress grows at the expense of the other one. In the detwinned, single-variant domain, a new twin variant can form, gradually replacing the existing variant via the twinning process. Both processes—detwinning and new twinning—proceed by the same mechanism, namely by the movement of twinning dislocations along the twin boundary. Lattice dislocations are also created in the detwinning process. While the boundaries between the internal twins are coherent and mobile, the boundaries between the internally twinned domains are incoherent, strained and not mobile. The planes of the coherent twin boundary are {202) planes and the Burgers vectors of the twinning dislocations are parallel to the 〈101] direction. The magnitude of the Burgers vector determined from the TEM observations disagrees with the calculation from the lattice constant measurement by X-ray diffraction. Possible reasons for this discrepancy are discussed.

Characterization of CdCrTe Layers Obtained by Pulsed Laser Deposition

In this paper we have obtained CdCrTe thin layers by PLD method using the YAG:Nd3+ laser with pulse length 250μs. Synthesized CdCrTe solid solution with 50% at. of Cr has been taken as a target. The layers were deposited on KCl substrate. The target and films were analyzed using X-ray diffraction, TEM microscope and THEED electron diffraction. The morphology of the layers are homogenous. However, we have detected several crystallographic phases: cubic CdTe, hexagonal Cr and hexagonal Te. From the measurements of lattice constant of the layer, their composition was determined to be x=0,14

HREM analysis of graphite-encapsulated metallic nanoparticles for possible medical applications

High resolution electron microscopy has been applied to study the structure of metallic nanoparticles. These have sparked considerable interest as contrast agents in the field of biological imaging, including in magnetic resonance imaging (MRI) and computed tomography (CT). Here, we describe a method of synthesizing sub-10nm superparamagnetic metal and alloy nanoparticles by reduction of metallic salts. Annealing at 900°C in a methane/hydrogen environment forms a thin graphitic-carbon shell which is expected to improve stability, biocompatibility, and functionalization. Subsequent high resolution electron microscopy verifies graphitization and allows for crystallographic analysis. Most particles consist of single crystals in the phase predicted for the bulk material at the annealing temperature. Electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and lattice constant measurements show large variation in composition for alloy nanoparticles from a single synthesis. The magnetization relaxation time (T2) measurements demonstrate that Fe and AuFe nanoparticles compete with commercially available iron oxide MRI contrast agents. X-ray attenuation measurements of an AuFe alloy nanoparticle solution gave a relative radiodensity of 280 Hounsfield Units, demonstrating promise as a dual-purpose contrast agent in CT and MRI. Long term stability in an atmospheric environment was also tested, with no signs of corrosion or oxidation after several years of storage.

Insights into the atomic and electronic structure triggered by ordered nitrogen vacancies in CrN

We report on the atomic and electronic structure of ordered nitrogen vacancies in CrN by using spherical aberration-corrected high-resolution transmission electron microscopy, electron energy-loss spectra, and ab initio calculations. The ordered nitrogen vacancies are identified to be distributed on ${111}$ atomic planes. The vacancy concentrations were evaluated by quantitative high-resolution transmission electron microscopy. A direct consequence of the ordered nitrogen vacancies is a lattice shrinking leading to a reduced lattice constant, displaying a distorted CrN. The experimental measured lattice constant is quantitatively compared to the ab initio calculations. A relationship between the lattice constant and nitrogen vacancy concentration is theoretically and experimentally established, and quantitatively compared. The presence of the ordered N vacancies further induces the electronic changes as reflected in a very small core-level shift as well as a shift of the volume plasmon energy. Moreover, the change of the ionicity in CrN with nitrogen vacancy concentration is revealed. A direct relation between the covalent-ionic level of the bonding and the nitrogen vacancy concentration is shown.

Epitaxy of GaN on Si(111) substrate by the hydride vapor phase epitaxy method

GaN epilayers were grown on Si(111) at 980°C by a hydride vapor phase epitaxy (HVPE) method. AlxGa1−xN with thickness of ∼900nm was inserted between Si substrate and GaN epilayer to act as a buffer layer. The investigation of the influence of different V/III ratios on GaN epilayer qualities reveals that high GaCl flow rate can increase the growth rate and improve the surface morphologies of GaN epilayers while high NH3 flow rate is not good for reducing dislocations in GaN epilayers, which can be proved from the analysis of X-ray diffraction rocking curve (XRC), cross-sectional image of scanning electron microscopy (SEM) and image of atomic force microscopy (AFM). Finally, 1.22μm GaN epilayer without cracks was successfully obtained with a three-step growth method. The surface roughness average (Ra) was measured to be 1.45nm and the FWHM of GaN(0002) was 599arcsec. The measured lattice constants for GaN epilayer were a=3.210Å and c=5.177Å, indicating the in-plane tensile stress and out-of-plane compressive stress of GaN epilayer.

Surface phase transitions in BiFeO3below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour.

High-pressure structural study of yttrium monochalcogenides from experiment and theory

High-pressure powder x-ray diffraction experiments using synchrotron radiation are performed on the yttrium monochalcogenides YS, YSe, and YTe up to a maximum pressure of 23 GPa. The ambient NaCl structure is stable throughout the pressure range covered. The bulk moduli are determined to be 93, 82, and 67 GPa for YS, YSe, and YTe, respectively. First-principles total energy calculations are carried out using the full-potential linear muffin-tin orbital method. The calculated and measured lattice constants and bulk moduli are in good agrement. Under applied pressure, the yttrium monochalcogenides are predicted to undergo a structural transition. Assuming that the high-pressure phase corresponds to the CsCl crystal structure, transition pressures of 53, 36, and 14 GPa are found for YS, YSe, and YTe, respectively.

Density functional study of weak ferromagnetism in a thick BiCrO3 film

Very thick films of BiCrO3 have been grown on a SrTiO3 substrate, maintaining a tetragonal lattice up to thicknesses of 14,000 Å. Assuming that we can treat films of this thickness as bulk crystals, we first calculated the experimentally undetermined atomic positions within the unit cell with the measured lattice constant of the film, and then relaxed the lattice constants along with the atomic positions. All of the calculations result in an antiferroelectric crystal with the Pnma space group with noncollinear Bi displacements. We find G-type antiferromagnetism with a spin-orbit induced weak ferromagnetic component; however, the weak ferromagnetic component can be canceled if the antiferromagnetic spins are oriented along a particular one of the three twofold rotation axes.

Epitaxial growth of LaFeAs(O,F) thin films by molecular beam epitaxy

LaFeAs(O,F) epitaxial thin films were grown on GaAs substrates by molecular beam epitaxy. Two films were grown with different growth time. The film grown for 1h was single phase, but did not show a superconducting transition because of the absence of doped fluorine. On the other hand, the film grown for 6h contained LaOF as an impurity, but showed a clear superconducting transition without the need of an ex situ heat treatment. The results of Hall coefficient and lattice constant measurements suggest that the superconducting film was sufficiently doped with fluorine although the transition temperature (Tc) was rather low, implying that a further improvement in the film quality would increase Tc.

Effects of Vanadium Doping on Resistive Switching Characteristics and Mechanisms of $\hbox{SrZrO}_{3}$-Based Memory Films

The effects of vanadium doping on resistive switching (RS) characteristics and mechanisms of RF-sputtered SrZrO3 (SZO)-based thin films are investigated in this paper. The physical and electrical properties of SZO-based thin films are modulated by vanadium doping due to the Zr4+ ion replaced by V5+, further affecting the RS parameters of SZO-based thin films. The conduction mechanisms of SZO-based thin films are dominated by ohmic conduction (hoping conduction) and Frenkel-Poole emission for the low resistance state (LRS) and the high resistance state (HRS), respectively. The turn-on process might be attributed to the formation of conducting filaments consisting of oxygen vacancies with the effective barrier height (φB,eff) in the range of 0.10-0.13 eV, whereas the turn-off process might result from thermally assisted oxidation of oxygen vacancies by the Joule heating effect. Furthermore, the introduction of the high valence cation (V5+) in a Zr4+ site of SZO crystalline structure can suppress the formation of oxygen vacancies due to the charge neutrality restriction. Such suppression leads to the changes in the forming voltage, turn-on voltage, HRS resistance, dielectric constant, and φB,eff with vanadium doping concentration up to 0.2 mol%, which is within the solid solubility limit based on our measured lattice constants and Vegard's law.

Invited Article: High-pressure techniques for condensed matter physics at low temperature

Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent DeltaP/P per unit area of +/-1.8 %/(10(4) microm(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021+/-0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.

Lattice Constants of Tubular Zeolite NaA Membranes by Parallel Beam X-Ray Diffraction

A parallel beam X-ray diffraction analysis (XRD) machine equipped with a parabolic multilayer, Soller slit and long horizontal slit has been applied to tubular zeolite NaA (LTA) membranes supported by a porous alumina tube, prepared by hydrothermal synthesis for the dehydration of ethanol. The present XRD is a suitable method used to analyze tubular LTA membranes and give a sharp XRD pattern, because the sample form does not affect the diffraction angles. For lattice-constant measurements, six diffraction lines in the 2θ region of 40∼60° were used, and their diffraction angles were corrected using 220 and 311 lines of silicon powder (NIST SRM640c) as an external standard. The XRD measurements were carried out for LTA membranes after pervaporation experiments of an ethanol/water (90/10) mixture, and the observed lattice constants were 24.573∼24.599 A, corresponding to the hydrothermal synthesis condition. Thus, the lattice constants of the tubular LTA membranes were first reported.

Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Reversibility of micro-yielding and critical current in a YBCO-coated conductor caused bya uniaxial tensile load

The deformation behaviour of aYBa2Cu3Ox (YBCO)-coated conductor based on Ni–W substrate and its influence on the criticalcurrent were precisely investigated. From a room temperature tensile test, the change ofpermanent strain as a whole of the coated conductor was divided into two regions ofmicro- and macroscopic yielding. The precise lattice constant measurement by synchrotronradiation at Spring8 revealed that the permanent strain in the YBCO layer wascharacterized by two regimes related just to the two regions of yielding mentioned above. Inthe micro-yielding region, the permanent strain in the YBCO layer was identical with thatin the metallic components, suggesting that the YBCO layer behaves elastically andany damage-like cracks were not introduced. In the macroscopic yielding region,the smaller permanent strain in the YBCO layer compared with the metalliccomponents suggested that damage-like cracks were introduced in the YBCOlayer during loading and remains under zero external load. There the invariantof total length is accommodated by the existence of damage-like cracks. Thereversibility of the critical current was evaluated by loading/unloading experiments. Thepermanent strain ensuring the reversible limit and the corresponding applied strainwere found to be 0.17% and 0.52%, respectively. This reversible limit coincidedwith the limit of micro-yielding. Furthermore the irreversible degradation of thecritical current as a function of strain is suggested to relate to the macroscopicyielding.