Intensities Of Superlattice Reflections Research Articles

Disordered Gd6UO12-δ with the cation antisite defects prepared by a combined mechanochemical−thermal method

• The rhombohedral Gd 6 UO 12-δ single phase is synthesized for the first time via mechanochemical − thermal route. • The as-prepared uranate exhibits a remarkable degree of cation antisite disorder and a relatively high concentration of oxygen vacancies . • The XRD simulations reveal a decrease of the intensity of superlattice reflections with increasing degree of cation antisite defects in Gd 6 UO 12-δ . • The nonequilibrium cation distribution in Gd 6 UO 12-δ does not influence the symmetrical geometry of polyhedra around the 3 a sites, whereas the octahedra around the 18 f sites are found to be distorted. • The six-fold coordination of cations at 18 f sites is a consequence of a relatively large oxygen deficiency in the as-prepared material. The synthesis of the rhombohedral Gd 6 UO 12-δ is reported via mechanochemical processing of stoichiometric Gd 2 O 3 /UO 2 mixtures and their subsequent annealing. Rietveld refinement of XRD data reveals that the as-prepared material exhibits a remarkable degree of cation antisite disorder and oxygen deficiency. The simulations of intensities of the selected XRD superlattice reflections are performed for limiting states of Gd 6 UO 12-δ with its most extreme degrees of the cation antisite disorder. On the basis of the estimated bond lengths it can be stated that distorted geometry of structural units in the material is a consequence of its relatively large oxygen deficiency. The cations in Gd 6 UO 12-δ prepared by mechanochemical − thermal route are distributed over the 3 a and 18 f sites of octahedral coordination with the symmetrical and distorted geometry, respectively.

Influence of substitution of Fe by Co on structural and magneto-mechanical properties of Fe-27Ga alloy

This paper investigates the influence of substitution of 18 at.% of Fe by Co in Fe-27Ga alloy as an effort to improve magneto-mechanical coupling. Study of phase transitions by neutron diffraction reveals that the formation of close packed phases is suppressed in the as-cast Fe-18Co-27Ga alloy. The degree of D03 ordering in the as-cast state for the Fe-18Co-27Ga alloy is lower than that for the Fe-27Ga alloy as estimated by intensities of super-lattice reflections of neutron diffraction spectra in these alloys. Investigation of magnetic properties by vibrating sample magnetometry and magnetic thermal gravimetry proved that the high-temperature B2 phase in the Fe-18Co-27Ga is ferromagnetic, whereas that of the Fe-27Ga is non-ferromagnetic. The Curie temperature (TC) here and later in the text in slowly-cooled Fe-18Co-27Ga is 726 °C, which is higher than that (650 °C) of Fe-27Ga. The mechanical and magneto-mechanical properties samples were studied through impulse excitation of vibration (IEV) method by the Resonance Frequency and Damping Analysis (RFDA) device. The results demonstrate that substitution of 18 at.%Fe by Co in Fe-27Ga alloy could enhance both the elastic, E0, and shear moduli, G0, at zero filed and the total (flexure mode) damping capacity, (ΔW/W)F0, under the measuring conditions.

Second-order structural transition in the superconductorLa3Co4Sn13

The quasi-skutterudite superconductor La$_3$Co$_4$Sn$_{13}$ undergoes a phase transition at $T^*=152$ K. By measuring the temperature dependence of heat capacity, electrical resistivity, and the superlattice reflection intensity using X-ray, we explore the character of the phase transition at $T^*$. Our lattice dynamic calculations found imaginary phonon frequencies around the ${\bf M}$ point, when the high temperature structure is used in the calculations, indicating that the structure is unstable at the zero temperature limit. The combined experimental and computational results establish that $T^*$ is associated with a second-order structural transition with $\bf{q}$=(0.5, 0.5, 0) (or the ${\bf M}$ point). Further electronic bandstructure calculations reveal Fermi surface sheets with low curvature segments, which allow us to draw qualitative comparison with both Sr$_3$Ir$_4$Sn$_{13}$ and Sr$_3$Rh$_4$Sn$_{13}$ in which similar physics has been discussed recently.

A monoclinic to tetragonal crossover in (Na0.5Bi0.5TiO3)(1−x)(BaZrO3)xceramic: a lead-free ferroelectric material

A single-phase solid solution of Na0.5Bi0.5TiO3(NBT) and BaZrO3(BZ),i.e.(NBT)(1−x)(BaZrO3)x(x= 0.00, 0.01, 0.03, 0.05 and 0.07), was synthesized using the solid-state reaction method. It is found that the temperature for single-phase formation increases with increasing BZ content. A detailed structural investigation was carried out as a function of composition using Rietveld analysis of powder X-ray diffraction data. The structural refinement reveals a crossover from monoclinic (Cc) to tetragonal (P4mm) symmetry in the composition range 0.03 <x< 0.05. The intensity of superlattice reflection due toa−a−c−tilting of oxygen octahedra in theCcphase was found to decrease with increasing BZ content. The unit-cell volume was also found to increase with BZ content, which was attributed to the higher ionic radii of Ba2+and Zr4+in comparison to Bi3+(Na+) and Ti4+. The structural phase boundary observed in (NBT)(1−x)(BaZrO3)xis concurrent with the observation of morphotropic phase boundary like behaviour.

Giant Magnetoresistance and Structure of Electrodeposited Co/Cu Multilayers: The Influence of Layer Thicknesses and Cu Deposition Potential

The giant magnetoresistance (GMR) and structure was investigated for electrodeposited Co/Cu multilayers prepared by a conventional galvanostatic/potentiostatic pulse combination from a pure sulfate electrolyte with various layer thicknesses, total multilayer thickness and Cu deposition potential. X-ray diffraction (XRD) measurements revealed superlattice satellite reflections for many of the multilayers having sufficiently large thickness (at least 2 nm) of both constituent layers. The bilayer repeats derived from the positions of the visible superlattice reflections were typically 10 – 20% higher than the nominal values. The observed GMR was found to be dominated by the multilayer-like ferromagnetic (FM) contribution even for multilayers without visible superlattice satellites. There was always also a modest superparamagnetic (SPM) contribution to the GMR and this term was the largest for multilayers with very thin (0.5 nm) magnetic layers containing apparently a small amount of magnetically decoupled SPM regions. No oscillatory GMR behavior with spacer thickness was observed at any magnetic layer thickness. The saturation of the coercivity as measured by the peak position of the MR(H) curves indicated a complete decoupling of magnetic layers for large spacer thicknesses. The GMR increased with total multilayer thickness which could be ascribed to an increasing SPM contribution to the GMR due to an increasing surface roughness, also indicated by the increasing coercivity. For multilayers with Cu layers deposited at more and more positive potentials, the GMRFM term increased and the GMRSPM term decreased. At the same time, a corresponding reduction of surface roughness measured with atomic force microscopy indicated an improvement of the multilayer structural quality which was, however, not accompanied by an increase of the superlattice reflection intensities. The present results underline that whereas the structural quality as characterized by the surface roughness generally correlates fairly well with the magnitude of the GMR, the microstructural features determining the amplitude of superlattice reflections apparently do not have a direct influence on the GMR.

Effect of carbon on formation of mixed solid solutions during mechanochemical synthesis of Ni-Al-Mo-C mixtures and ordering of solutions during heating

Solid solutions Ni(Al, Mo, C) are formed via milling the Ni2.8Al1Mo0.2 and Ni3Al0.8Mo0.2 and graphite-containing Ni2.8Al1Mo0.2C(0.25, 0.5) and Ni3Al0.8Mo0.2C(0.25, 0.5) mixtures. In this case, some amount of Mo remains beyond the solid solution. Graphite added to a starting mixture decreases the Mo solubility and favors the amorphization of solid solutions. The complete amorphization was found for the mixture with the 5 at % C and 5 at % Mo, which was added instead of Ni. The heating of mechanically synthesized (MS) powder alloys leads to the ordering of carbon-free and carbon-containing solid solutions with the formation of the L12 and E21 structure, respectively. In the course of the ordering of the Ni(Al, Mo, C) solid solutions, Mo and carbon precipitate in the form of the molybdenum carbide (Mo2C) second phase. The hardness of the MS three-phase Ni-Al-Mo-C solid solutions subjected to hot isostatic pressing is determined by the mass fraction of the formed Mo2C carbide. It is shown that the carbon content in the multicomponent antiperovskite can be estimated by analyzing the ratio of integral intensities of superlattice reflections I (100)/I (110).

Emergence of reentrant metal-nonmetal transition inPr0.85Ce0.15Ru4P12andPr(Ru0.95Rh0.05)4P12

The metal-nonmetal transition in Ce- and Rh-substituted ${\text{PrRu}}_{4}$${\text{P}}_{12}$ was investigated through magnetic susceptibility, x-ray superlattice reflection, and inelastic neutron scattering (INS) measurements. The saturation in the magnetic susceptibility at low temperatures suggests a singlet ground state for Pr sites in both compounds. This finding is in contrast to the staggered order of singlet and triplet ground states found in the ordered phase of ${\text{PrRu}}_{4}$${\text{P}}_{12}$, which leads to a diverging susceptibility. The intensities of x-ray superlattice reflections of both compounds develop below 45 K and decrease rapidly below 10 K with a simultaneous decrease in electrical resistivity. INS experiments reveal that 4$f$ electron states in both compounds change drastically from two distinct crystal field (CF) level schemes corresponding to an order parameter in the multipole ordered phase (10 $\text{K}<T<45$ K) to a nearly uniform CF level scheme below 10 K. The uniform CF ground state at the Pr sites carries no degree of freedom relevant to the staggered ordering pattern and electronic gap formation. Therefore, the substituted systems exhibit a reentrant metallic state at the lowest temperature. These phenomena may originate from effective electron doping due to atomic substitution.

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni70Al25Mo5 (composition 1) and Ni75Al20Mo5 (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ∼7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L12) at T ∼ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni3 Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.

Form factor dispersion at La M5,4 edges and average density of resonant atoms

Resonant soft x-ray scattering on complex oxide superlattices shows very large variations in the superlattice reflection position and intensity near La M5,4 edges. Resonant dispersion of the La x-ray form factor describes the observations well. We determine the average density of resonant La atoms and the thickness of superlattice layers.

Estimation of Order Parameter and Spin Moment of Fe<sub>3</sub>Pt by White X-Ray Diffraction Method

Diffraction intensities of a single crystal of Fe3Pt alloy have been measured by using white X-rays of synchrotron radiation. The observed intensities have been compared with the calculated ones for fundamental and superlattice reflections. By normalizing the calculated intensities of fundamental reflections to the observed ones and evaluating the ratio of the observed intensity of superlattice reflection to the calculated ones, we have estimated the order parameter as 0.82±0.03. The obtained order parameter has been applied to an analysis of the X-ray magnetic diffraction data to estimate the spin moment of Fe and Pt atoms in this alloy.

Quantification of octahedral rotations in strained LaAlO3films via synchrotron x-ray diffraction

In recent years, there has been an increased interest in octahedral rotations in perovskite materials, particularly on their response to strain in epitaxial thin films. The current theoretical framework assumes that rotations are affected primarily through the change in in-plane lattice parameters imposed by coherent heteroepitaxy on a substrate of different lattice constant. This model, which permits prediction of the thin-film rotational pattern using first-principles density functional theory, has not been tested quantitatively over a range of strain states. To assess the validity of this picture, coherent LaAlO${}_{3}$ thin films were grown on SrTiO${}_{3}$, NdGaO${}_{3}$, LaSrAlO${}_{4}$, NdAlO${}_{3}$, and YAlO${}_{3}$ substrates to achieve strain states ranging from $+$3.03$%$ to $\ensuremath{-}$2.35$%$. The out-of-plane and in-plane octahedral rotation angles were extracted from the intensity of superlattice reflections measured using synchrotron x-ray diffraction. Density functional calculations show that no measurable change in intrinsic defect concentration should occur throughout the range of accessible strain states. Thus, the measured rotation angles were compared with those calculated previously for defect-free films. [Hatt and Spaldin, Phys. Rev. B 82, 195402 (2010)]. Good agreement between theory and experiment was found, suggesting that the current framework correctly captures the appropriate physics in LaAlO${}_{3}$.

A temperature-induced reversible transformation between paratacamite and herbertsmithite

The crystal chemistry of paratacamite has been re-evaluated by studying a crystal from the holotype specimen BM86958 of composition Cu3.71Zn0.29(OH)6Cl2 using single-crystal X-ray diffraction at 100, 200, 300, 353, 393 and 423 K. At 300 K paratacamite has space group \(R\bar{3}\) with unit-cell parameters a 13.644 and c 14.035 A and exhibits a pronounced subcell, a′ = ½a and c′ = c, analogous to that of the closely related mineral herbertsmithite, Cu3Zn(OH)6Cl2. Between 353 and 393 K, paratacamite undergoes a reversible phase transformation to the herbertsmithite-like substructure, space group \(R\bar{3}m\), unit-cell parameters a 6.839 and c 14.072 A (393 K). The transformation is characterised by a gradual reduction in intensity of superlattice reflections, which are absent at 393 and 443 K. On cooling from 443 to 300 K at ~10 K min−1, the superlattice reflections reappear and the refined structures (\(R\bar{3}\)) of the initial and recovered 300 K states are almost identical. The complete reversibility of the transformation establishes that paratacamite of composition Cu3.71Zn0.29(OH)6Cl2 is thermodynamically stable at ambient temperatures. The nature of the rhombic distortion of the M(2)O6 octahedron is discussed by considering two possibilities that are dependent upon the nature of cation substitution in the interlayer sites.

Electron Density Distribution of the Commensurate Phase of TMATC-Mn

The superlattice structure (phase III) of {N(CH3)4}2MnCl4 was analyzed from the X-ray diffraction data by using Fourier synthesis method based on maximum entropy algorithm. The partially disordered MnCl4 was found without assuming a specified structural model. As the occupation probability of Cl changes, the mean position of the atom deviates continuously from the c-glide plane, which explains the temperature dependence of the superlattice reflection intensity with decreasing temperature in the commensurate phase III. The ordering of N(CH3)4 is also found to be similar with the behavior of MnCl4.

Phase stability and structural temperature dependence in sodium niobate: A high-resolution powder neutron diffraction study

We report investigation of structural phase transitions in technologically important material sodium niobate as a function of temperature on heating over 300-1075 K. Our high resolution powder neutron diffraction data show variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice parameters is found only at 633 K that indicates that the transition of orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm) phase is first order in nature, while other successive phase transitions are of second order. New superlattice reflections appear at 680 K (R phase) and 770 K (S phase) that could be indexed using an intermediate long-period modulated orthorhombic structure whose lattice parameter along <001> direction is 3 and 6 times that of the CaTiO3-like Pbnm structure respectively. The correlation of superlattice reflections with the phonon instability is discussed. The critical exponent ({\beta}) for the second order tetragonal to cubic phase transition at 950 K, corresponds to a value {\beta}$\approx 1/3$, as obtained from the temperature variation of order parameters (tilt angle and intensity of superlattice reflections). It is argued that this exponent is due to a second order phase transition close to a tricritical point. Based on our detailed temperature dependent neutron diffraction studies, the phase diagram of sodium niobate is presented that resolves existing ambiguities in the literature.

Component-specific electromechanical response in a ferroelectric/dielectric superlattice

The electronic and electromechanical properties of complex oxide superlattices are closely linked to the evolution of the structure and electrical polarization of the component layers in applied electric fields. Efforts to deduce the responses of the individual components of the superlattice to applied fields have focused on theoretical approaches because of the limitations of available experimental techniques. Time-resolved x-ray microdiffraction provides a precise crystallographic probe of each component using the shift in wave vector and change in intensity of superlattice satellite reflections. We report in detail the methods to measure and analyze the x-ray diffraction patterns in applied electric field and their application to a 2-unit-cell ${\text{BaTiO}}_{3}/4$-unit-cell ${\text{CaTiO}}_{3}$ superlattice. We find that the overall piezoelectric distortion is shared between the two components. Theoretical predictions of the electromechanical properties of a superlattice with the same composition constrained to tetragonal symmetry are in excellent agreement with the experiments. Lattice instability analysis, however, suggests that the low-temperature ground state could exhibit antiferrodistortive rotations of ${\text{TiO}}_{6}$ octahedra within and/or at the interfaces of the ${\text{CaTiO}}_{3}$ component.

Magnetic octupole order in Ce0.7La0.3B6: A polarized neutron diffraction study

Abstract Recently, in phase IV of Ce x La 1 - x B 6 , weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce 0.7 La 0.3 B 6 . The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in Ce x La 1 - x B 6 .

Order-Disorder Transformation in L1&lt;SUB&gt;0&lt;/SUB&gt;-FePd Nanoparticles Studied by Electron Diffraction

Disappearance of long-range atomic order in 10-nm-sized L10-FePd (Fe–58 at%Pd) nanoparticles has been studied by electron diffraction using a specimen heating stage attached to a transmission electron microscope. Ordered structure is kept at lowest up to 949 K, while the intensity of superlattice reflection abruptly drops at 982 K. The transformation temperature decreases by about 80 K compared to that of the bulk alloy of Fe–58 at%Pd. Nanobeam electron diffraction from a particle shows a particle size dependence of the transformation temperature and the intensity of 110 superlattice reflection largely decreases below about 15 nm in diameter at 983 K. However, a weak intensity remains in the 110 superlattice reflection even at 983 K, which is attributed to short-range order in the nanoparticles.

Raman spectroscopic study of (Pb1−xBax)(Yb1∕2Ta1∕2)O3 ceramics

The ordered complex perovskite lead ytterbium tantalate undergoes a change in structural and dielectric properties as the concentration of Ba increases. The x-ray diffraction patterns show a decrease in the intensity of superlattice reflections due to Pb-antiparallel displacement as well as to B-site ordering. The structure approaches cubic symmetry with increase in Ba content. The dielectric response shows a gradual change from antiferroelectric to relaxor ferroelectric and then the relaxor behavior starts decreasing as Ba content increases. Raman-scattering study of the samples shows that some of the modes seen for Pb(Yb0.5Ta0.5)O3 disappear and an additional mode appears at 420cm−1 with increase in Ba concentration. A qualitative analysis of various modes suggests that the substitution of Ba for Pb leads to a decrease in the degree of ordering and occurrence of local distortions in the lattice as evidenced by the broadening of certain peaks in the Raman spectra.

Studies of unusual magnetic and electronic properties of the low-carrier system CeP by synchrotron X-rays

We have observed superlattice reflections from the low-carrier system CeP under high pressures up to 2 GPa and also under magnetic field up to 12.5 T by means of synchrotron X-ray diffraction. The observed intensities of superlattice reflections are well explained by a model that takes into account both the lattice and charge modulation due to the periodic arrangement of ferromagnetic double layers of Γ 8-like Ce ions in the lattice of Γ 7 Ce ions. A new magnetic phase with the period of nine Ce planes was found in the experiment at the magnetic field of 12.5 T.

The P21/m? C2/m phase transition in synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: thermodynamic and crystal-chemical evaluation

The P21/m↔C2/m displacive phase transition in the synthetic end-member amphibole Na NaMg Mg5 Si8 O22 (OH)2 has been studied by monitoring changes in unit-cell parameters and the intensities of superlattice reflections at 25–400 °C. This amphibole allows investigation of the effects of compositional variations at the A- and B-group sites upon the transition. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a T c of 257 ± 3 °C, and Landau coefficients compatible with a second-order transition. Structure refinement of single-crystal data collected at 25, 140, 270 and 370 °C allowed modelling of the structural changes as a function of T and symmetry. Crystal-chemical analysis suggests that differences in T c in cummingtonites and in the crystal of this work depend mainly upon the relations between the aggregate ionic radii of the B- and C-group cations.