Unit Cell Of Phase Research Articles

Structural and Raman characterization of nanogranular BaTiO3‐NiFe2O4 thin films deposited by laser ablation on Si/Pt substrates

AbstractThin films composed by (BaTiO3)1‐x‐(NiFe2O4)x with different nickel ferrite concentrations (x) have been deposited by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X‐ray diffraction and Raman spectroscopy. It was found that the NiFe2O4 phase unit cell was expanded along the growth direction of the films, with a lattice parameter that increased with increasing NiFe2O4 concentration. The opposite behavior was observed on the BaTiO3 phase, with an expansion of its unit cell that lowered with increasing x. The presence of the strain in the films induced a redshift of the Raman peaks of NiFe2O4 that decreased with increasing NiFe2O4 concentration. Cation disorder in the nickel ferrite was observed for lower x, where the nanograins are more isolated and subjected to more strain, which was progressively decreased for higher NiFe2O4 content in the films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synchrotron micro-diffraction analysis of the microstructure of cryogenically treated high performance tool steels prior to and after tempering

The phase transformation and strain changes within cryogenically (−196 °C) treated high performance tool steels (AISI H13) before and after tempering have been examined using both laboratory XRD and synchrotron micro-diffraction. The martensitic unit cell was found to have very low tetragonality as expected for low carbon steel. Tempering resulted in the diffusion of excess carbon out of the martensite phase and consequent unit cell shrinkage. In addition on tempering the martensite became more homogeneous as compared to the same samples prior to tempering. For cryogenically treated samples, the effect was most pronounced for the rapidly cooled sample which was the least homogenous sample prior to tempering but was the most homogenous sample after tempering. This suggests that the considerable degree of disorder resulting from rapid cryogenic cooling results in the beneficial release of micro-stresses on tempering thus possibly resulting in the improved wear resistance and durability observed for cryogenically treated tool steels.

Fourth-Order Curvature Energy Model for the Stability of Bicontinuous Inverted Cubic Phases in Amphiphile−Water Systems

The bicontinuous inverted cubic (Q(II)) phases of amphiphiles in water have many practical applications. It is necessary to understand the stability of these phases as a function of composition and ambient conditions in order to make the best use of them. Moreover, many biomembrane lipids and some biomembrane lipid extracts form Q(II) phases. The stability of Q(II) phases in a given lipid composition is closely related to the susceptibility of that composition to membrane fusion: changes in composition that stabilize Q(II) phases usually increase the rate of membrane fusion. However, the factors determining Q(II) phase stability are not fully understood. Previously, an expression was derived for the curvature free energy of Q(II) phases with respect to that of the lamellar (L(alpha)) phase using a model for the curvature energy with terms up to fourth order in curvature as formulated by Mitov. Here this model is extended to account for the effects of water content on Q(II) phase stability. It is shown that the observed L(alpha)/Q(II) phase-transition temperature, transition enthalpy, and transition kinetics are all sensitive to water content. The same observables also become sensitive to small noncurvature energy contributions to the total free-energy difference between the Q(II) and L(alpha) phases, especially the unbinding energy in the L(alpha) phase. These predictions rationalize earlier observations of Q(II) phase formation in N-monomethylated dioleoylphosphatidylethanolamine that otherwise appear to be inconsistent. The model also provides a fundamental explanation of the hysteresis typically observed in transitions between the L(alpha) and Q(II) phases. It is an accurate model of Q(II) phase stability when the ratio of the volume fraction of the lipid in the Q(II) phase unit cell is < or = 0.5.

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni 3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni 3Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ c , differ markedly from those of corresponding bulk alloys. The Ni 3Al ordered films form in a concentration range larger than the stability domain of the L1 2 Ni 3Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases. It also appears that the value of θ c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ c = 4.5 ML for a flat surface to θ c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.

Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates.

Field-Induced Magnetic Structures in Antiferroquadrupolar Ordering Compound DyB2C2

Neutron scattering experiments were performed on the single-crystalline DyB 2 C 2 under magnetic fields in order to elucidate the magnetic structures of field-induced phases. The measurements in the a * – c * reciprocal lattice plane under the magnetic fields along the [010] direction resulted in finding a new magnetic reflection of 1 0 3/4 in a field-induced phase (phase II ′′ ) in addition to the magnetic reflections that are common to those in the phase under a zero magnetic field (phase III). The magnetic unit cell in phase II ′′ has dimensions of a × a ×4 c , where all the Dy magnetic moments on the AFQ structure have advantageous directions to the magnetic field, and their AFQ couplings with the neighbouring moments along the c -axis are partly broken. This magnetic structure model for phase II ′′ well explains the magnetisation process at 5 K up to 14 T.

Study of superstructure II in multiferroic BiMnO3

The crystal structure of the minor phase, named superstructure II, existing in multiferroic compound BiMnO3 has been studied by electron diffraction and high-resolution transmission electron microscopy. Domains of major and minor phases coexisting in BiMnO3 were observed in high-resolution electron microscope images. The unit cell of minor phase was determined to be triclinic with the size 4×4×4 times as large as the distorted perovskite subcell. The [111] and [101̄] projected structure maps of the minor phase have been derived from the corresponding images by means of the image processing. A possible rough three-dimensional (3D) structure model was proposed based on the 3D structural information extracted from the two projected structure maps. Since there is no inversion centre in the proposed model, the minor phase may contribute to the ferroelectric property of BiMnO3.

Influence of the Lamellar Phase Unbinding Energy on the Relative Stability of Lamellar and Inverted Cubic Phases

Based on curvature energy considerations, nonbilayer phase-forming phospholipids in excess water should form stable bicontinuous inverted cubic (QII) phases at temperatures between the lamellar (Lα) and inverted hexagonal (HII) phase regions. However, the phosphatidylethanolamines (PEs), which are a common class of biomembrane phospholipids, typically display direct Lα/HII phase transitions and may form intermediate QII phases only after the temperature is cycled repeatedly across the Lα/HII phase transition temperature, TH, or when the HII phases are cooled from T>TH. This raises the question of whether models of inverted phase stability, which are based on curvature energy alone, accurately predict the relative free energy of these phases. Here we demonstrate the important role of a noncurvature energy contribution, the unbinding energy of the Lα phase bilayers, gu, that serves to stabilize the Lα phase relative to the nonlamellar phases. The planar Lα phase bilayers must separate for a QII phase to form and it turns out that the work of their unbinding can be larger than the curvature energy reduction on formation of QII phase from Lα at temperatures near the Lα/QII transition temperature (TQ). Using gu and elastic constant values typical of unsaturated PEs, we show that gu is sufficient to make TQ>TH for the latter lipids. Such systems would display direct Lα→HII transitions, and a QII phase might only form as a metastable phase upon cooling of the HII phase. The gu values for methylated PEs and PE/phosphatidylcholine mixtures are significantly smaller than those for PEs and increase TQ by only a few degrees, consistent with observations of these systems. This influence of gu also rationalizes the effect of some aqueous solutes to increase the rate of QII formation during temperature cycling of lipid dispersions. Finally, the results are relevant to protocols for determining the Gaussian curvature modulus, which substantially affects the energy of intermediates in membrane fusion and fission. Recently, two such methods were proposed based on measuring TQ and on measuring QII phase unit cell dimensions, respectively. In view of the effect of gu on TQ that we describe here, the latter method, which does not depend on the value of gu, is preferable.

Magneto transport characterization of the Sn-doped TbMnO3 manganites

Abstract The structural, magnetic and electrical transport properties of the Sn-doped TbMnO3 manganites are studied by X-ray diffraction, ac susceptibility, dc magnetization and electrical resistivity measurements. The Sn doping into the Tb and Mn sites of TbMnO3 compresses the unit cell and changes parameters of the antiferromagnetic phase whereas the magnetic moment of Mn are only weakly affected. The electrical resistivity of doped manganites is reduced and the activation energy EA is determined for the thermally activated conduction.

X‐ray diffraction and Raman study of nanogranular BaTiO3–CoFe2O4thin films deposited by laser ablation on Si/Pt substrates

AbstractNanocomposite thin films composed by (BaTiO3)1–x–(CoFe2O4)xwith different cobalt ferrite concentrations (x) have been deposited by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X‐ray diffraction and Raman spectroscopy. It was found that the CoFe2O4phase unit cell was compressed along the growth direction of the films, and it relaxed with increasingx. The opposite behavior was observed in the BaTiO3phase where the lattice parameters obtained from the X‐ray measurements presented a progressive distortion of its unit cell with increasingx. The presence of the strain in the films induced a blueshift of the Raman peaks of CoFe2O4that decreased with increasing CoFe2O4concentration. Cation disorder in the cobalt ferrite was observed for lowerx, where the nanograins are more isolated and subjected to more strain, which was progressively decreased for higher CoFe2O4content in the films. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structure and electrochemical properties of La 2− xMg xNi 7.0 ( x = 0.3–0.6) hydrogen storage alloys

The structure and electrochemical properties of La 2− x Mg x Ni 7.0 ( x = 0.3–0.6) alloys were investigated systematically. All alloys consist of a main phase with Ce 2Ni 7-type structure and a few impurities (mainly LaNi 5 and PuNi 3-type phase). As Mg content increases, the lattice parameters of unit cell of main phase represent a linear decrease. Equilibrium pressure of alloys during hydrogen absorption/desorption process rises and plateau becomes flatter and broader with the increase of Mg content. Measurements of electrochemical properties reveal that alloy electrode of La 1.5Mg 0.5Ni 7.0 exhibits the best overall electrochemical properties. Linear polarization and electrochemical impedance spectra indicate that the reaction rate of hydrogen for alloy electrode of x = 0.4, 0.5 is better and that charge-transfer resistance is smaller. The experiments of potential-step and anodic polarizations show that hydrogen diffusion in alloy bulk is more important to influence the kinetic character of alloy electrodes.

First-principle calculations of the cohesive energy and the electronic properties of PbTiO 3

The cohesive energy and electronic properties of PbTiO 3 have been studied using the full-potential-linearized augmented-plane wave method (FP-LAPW). The calculations have been made within the framework of density functional theory (DFT), with local density approximation (LDA) and generalized gradient approximation (GGA). The cohesive energy of PbTiO 3 with 19 core states for Pb, 4 for Ti and 1 for O atoms per unit cell for tetragonal phase lowers by 0.27 eV, since the Pb and O states hybridize, leading to a large strain that stabilizes this phase. In the tetragonal phase, it is found that the bond between Pb and O is ionic, while there is strong hybridization between Ti-3d and O-2p, which is necessary for ferroelectricity in PbTiO 3. It is also indicated that shallow Pb-6s semicore hybridize with the O-2p state. The results also show an indirect band gap of 1.7 eV at X–Γ points in the Brillouin zone for the cubic phase and an indirect band gap of 2.0 eV at X–Γ point for the tetragonal phase, in the absence of the scissors operation. These results are in better agreement with the published theoretical and experimental data.

Determining the Ratio of the Gaussian Curvature and Bending Elastic Moduli of Phospholipids from QII Phase Unit Cell Dimensions

A method is presented for measuring M, the ratio of the Gaussian (saddle splay) elastic modulus to the bending elastic modulus of a lipid monolayer. The ratio M is determined from measurements of the equilibrium bicontinuous inverted cubic ( Q II) phase unit cell size in excess water as a function of temperature. The analysis includes the effect of a curvature elastic term that is second-order in the Gaussian curvature, K. Preliminary results using data on DOPE-Me validate the method. The fitted value of M is within 8% of the value estimated in an earlier treatment. The method can be used to measure changes in M due to addition of exogenous lipids and peptides to a host lipid system. The Gaussian elastic modulus has a substantial effect on the stability of fusion intermediates (stalks, hemifusion diaphragms, and fusion pores). Studying the effects of peptides and different lipids on M via this method may yield insights into how fusion protein moieties stabilize intermediates in membrane fusion in vivo. The contribution of the K 2 curvature elastic term to the free energy of Q II phase and fusion pores explains some features of fusion pore stability and dynamics, and some peculiar observations concerning the mechanism of L α / Q II phase transitions.

Method of X-Ray Anomalous Diffraction for Lipid Structures

The structures of the unit cells of lipid phases that exhibit long-range crystalline order but short-range liquid-like disorder are of biological interests. In particular, the recently discovered rhombohedral phase has a unit cell containing either the structure of a membrane fusion intermediate state or that of a peptide-induced transmembrane pore, depending on the lipid composition and participating peptides. Diffraction from such systems generally presents a difficult phase problem. The existing methods of phase determination all have their limitations. Therefore it is of general interest to develop a new phasing method. The method of multi-wavelength anomalous dispersion is routinely used in protein crystallography, but the same method is difficult for lipid systems for the practical reason that the commonly used lipid samples for diffraction do not have a well-defined thickness. Here we describe a practical approach to use the multi-wavelength anomalous dispersion method for lipid structures. The procedure is demonstrated with the lamellar phase of a brominated lipid. The method is general to all phases as long as anomalous diffraction is applicable.

Structural Investigation of δ-stabilized Plutonium Alloys Under Pressure

AbstractThe mechanism responsible for the stability of the delta-phase in plutonium alloys remains poorly understood despite previous extensive studies. In the present experiments, three alloys (Pu-8at%Am, Pu-15at%Am and Pu-2at%Ga) were studied under pressure by X-ray diffraction in diamond anvil cells and revealed i) a pressure-induced softening in the compressibility of the delta-phase and ii) the presence of an intermediate γ’-phase across the δ → α’ phase transition. The softening, an anomalous behaviour, is shown to be coherent with other known unusual properties found for these alloys like NTE (Negative Thermal Expansion). Furthermore, it has been observed that the cell volume for the α’-phase formed under pressure depends on the rate of the pressure increase. This behaviour may be related to the diffusion of solute atoms to specific sites within the α’-phase unit cell. Such diffusion may also explain the α’-phase unit cell’s volume shrinkage versus time in a Pu-2at%Ga sample kept at a constant pressure.These XRD results, obtained under pressure, may be used to test first-principle calculations or other general modelling which aim at predicting the solute’s diffusion and/or self-irradiation effects in plutonium alloys.

Ceramics Materials Based on (Ba, Sr)TiO3 Solid Solutions for Tunable Microwave Devices

Structure and electrical properties at radio frequencies as well as within the 3.5–35 GHz frequency range have been investigated for ceramic samples of the (1−y)(BaxSr1 − x)TiO3 · yMgO (BSM) system where x = 0.4–0.6; y = 0.15–0.30. For the compositions studied the bulk ferroelectrics were synthesized with the dielectric constant of 400–600 and high tunability coefficient. We indicated that the quality factor of the samples was in the range of 100–1000 within the frequency band of 3.5–35 GHz. The phase correlations and unit cell constants of the perovskite phase of the BSM samples were studied. The low loss factor and high tunability of the bulk material allowed us using the BSM ferroelectric ceramic layer for tunable accelerating structures of the Argonne Dielectric Wakefield Accelerator and for high power switches design and development for the future linear colliders.

Structural analysis of several W(VI) and Mo(VI) complex perovskites prepared by the polymeric precursors method

We describe in this work the synthesis by the Pechini method of five Mo(VI)- and W(VI)-containing complex perovskites and their structural characterisation by HREM and XRD. The compounds studied, Ba( B 2/3′ B 1/3″)O 3 ( B′=In and Y; B″=W and Mo) and Sr(In 2/3W 1/3)O 3, were obtained after firing the precursor powders for 8 h at 1200°C. Thermal analysis showed that the formation mechanism of the five perovskites is similar and implies the formation of barium carbonate and barium tungstates or molybdates of different stoichiometries as intermediate phases. Interesting enough, these similar mechanisms yield to materials of a quite different structure. Indeed, Ba(In 2/3Mo 1/3)O 3 and Sr(In 2/3W 1/3)O 3 were found to be disordered perovskites (unit cells: a p× a p× a p and 2 a p × 2 a p ×2a p , respectively); on the other hand, two phases coexist in the sample Ba(In 2/3W 1/3)O 3 at the synthesis conditions: an ordered predominant phase (unit cell: 2 a p×2 a p×2 a p) and, as a minor phase, a disordered perovskite (unit cell: a p× a p× a p). Finally, the two yttrium-containing compounds were found to be ordered perovskites (2 a p×2 a p×2 a p).

The Ba1-x R x F2 + x (R= Gd-Lu) phases with the distorted fluorite structure as products of crystallization of incongruent melts in BaF2-RF3 systems. II. Crystal structures of two new forms of Ba1-x YbxF2 +x fluorite phases and some features of the anionic packing and the cation distribution

Two fluorite-type phases (P and F) possessing different defect structures were isolated from products of double recrystallization of a melt of the nominal composition (with respect to the charge)Ba0.8Yb0.2F2.2. The P phase crystallized in the space group Pm3m with a = 6.096(1) A. The structure was refined to R = 2.19% and Rw = 2.14% (142 independent reflections; λAgKα radiation, sinθ /2≤1.2A−1; 20 parameters were refined). In the primitive cubic unit cell of theP phase, two crystallographic positions are differentiated by the type of the cations occupying these positions. Thus, the position 1 a (0, 0, 0) is occupied predominantly by Yb3+ cations (0.8Yb + 0.2Ba), whereas the position 3c (0, 1/2, 1/2) is completely occupied by Ba2+ cations. The Fphase crystallized in the space group Fm3m with a = 6.075(1) A; the structure was refined to R= 0.69% and Rw = 0.85% (72 independent reflections; λMoKα radiation, sinθ/λ≤1.0A−1; 15 parameters were refined). In the F phase, all Yb 3+ ions are shifted with respect to Ba2+ ions by 0.34 A in the [111] direction.

A M�ssbauer Study of DyFe2, YFe2 and HoFe2 Thin Films and Multilayers

We have performed room temperature Conversion Electron Mössbauer Spectroscopy (CEMS) on thin films and multilayers of DyFe2, YFe2 and HoFe2 under applied magnetic fields of 0 T and 0.25 T in plane. The samples were constructed by M.R. Wells and R.C.C. Ward using Molecular Beam Epitaxy on sapphire substrates giving single crystals with the [110] plane parallel to the surface. The samples produced and studied are: thin films of DyFe2, YFe2 and HoFe2; multilayers of DyFe2/YFe2 with respective layer thicknesses of 50 Å/50 Å, 40 Å/20 Å, 20 Å/40 Å and 20 Å/20 Å. In all spectra the fits obtained are different to those found in bulk samples: all the iron sites in the cubic Laves Phase unit cell are magnetically inequivalent and the spins on the iron atoms always have a component perpendicular to the layers. With a field of 0.25 T applied in the plane of the sample the iron spins in the YFe2 thin film are rotated into the plane by a large amount (8.5°) compared to only a minor change in the DyFe2 thin film (1°). In multilayer systems the iron atom spins in the YFe2 layers appear to be locked to those in the DyFe2 layers.

35Cl NMR studies of the domain structure of tetramethylammoniumcadmium chloride (TMCC) at lower temperatures

Quadrupolar perturbed 35Cl NMR and 35Cl NQR investigations are performed onsingle crystals of tetramethylammonium cadmium chloride, (CH3)4NCdCl3 (TMCC),in order to study the formation of ferroelastic domains in the ferroelasticphase II below 118 K in which the hexagonal symmetry of the high temperaturephase I is lost. The experimental results cannot simply be explained in termsof the well known monoclinic domain structure but additionally a twin domainstructure is found to exist in phase II: six orientational domains areverified from the NMR rotational pattern at 113 K in contrast to the expectednumber of three for a classical 6/mF2/m transition. The rotational angleΘ between the epitaxially grown twins is calculated from the strain tensors ofthe three orientation states, applying an algebraic approach. The theoreticalvalue Θ = 3.5° is in very good correspondence with the experimental result fromthe 35Cl NMR studies (~4°). Similar results were also derived in recentx-ray studies in phase II. In the low temperature phase III below 104 K thenumber of non-equivalent Cl positions is consistent with both the space groupof phase III and the sixfold enlarged size of the unit cell in phase III withrespect to that in phase I as observed in XRD studies.