High-resolution Neutron Diffraction Research Articles

Unexpected phase transition sequence in the ferroelectric Bi4Ti3O12.

The high-temperature phase behaviour of the ferroelectric layered perovskite Bi4Ti3O12 has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that Bi4Ti3O12 undergoes two separate structural transitions from the aristotype tetragonal phase (space group I4/mmm) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, B1a1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above T C with tetragonal symmetry, space group P4/mbm. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi3+ cations within the perovskite blocks.

Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10

The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

Structural evolution and phase diagram of the superconducting iron selenides Li x (C2H8N2) y Fe2Se2(x= 0 ~ 0.8).

Here we report on the structural and electronic phase diagram of lithium and ethylenediamine intercalated FeSe in a wide range of dopant concentration . Undoped crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transition occurs at , and the superconducting tetragonal phase persists until . Meanwhile, the is found dependent strongly on dopant concentration, raising rapidly from 30 K at to 45 K at . The crystal structures of are determined by using high-resolution neutron diffraction data at 5, 60, 150, and 295 K, respectively. The distortion of the FeSe tetrahedron is enhanced significantly from 150 to 295 K, meanwhile, the normal-state Hall resistivity changes sign from negative to positive in the same temperature range. The dominant hole carrier in electron-doped above 230 K suggests that the temperature-induced structure distortion may lead to a reconstruction of the Fermi surface topology and the appearance of hole pockets.

Strain partitioning and strain localization in medium manganese steels measured by in situ microscopic digital image correlation

In situ microscopic-digital image correlation (µ-DIC) is used to investigate the strain partitioning and strain localization behavior in a medium manganese steel. Continuous yielding results from strain partitioning with higher strain in the reverted austenite (γR) islands and less strain in the tempered martensite (αtemp′) matrix, both in hot and cold rolled material. µ-DIC experiments are performed to further understand the effects of texture and grain morphology on strain partitioning which cannot be locally resolved through high resolution x-ray or neutron diffraction experiments. Apart from strain partitioning, strain localization is observed in hot rolled samples within colonies of lamellar γR islands. This localization does not only depend on the crystallographic orientation, but also on the spatial alignment of an austenite island relative to the loading direction. The effects of texture, spatial and colony alignment are interpreted within the concept of a relative grain size effect resulting in different yield stresses in the hot and cold rolled samples showing continuous yielding. Strain partitioning and strain localization based on texture and spatial alignment can be extended to numerous dual phase morphologies with similar texture, colony and spatial alignment effects.

Neutron investigation of Nitinol stents and massive samples before and after PIRAC coating

Nitinol, a thermoelastic Ni-Ti Shape Memory Alloy (SMA) with approximately 50 at. % Ti, is adopted in a wide range of medical equipment and devices used in interventional radiology, orthopaedics, neurology and cardiology, in particular as a smart material for stents. In this work, NiTi real stents and massive samples before and after different Powder Immersion Reaction Assisted Coating (PIRAC) treatments have been investigated by using two neutron techniques: (1) Small and Ultra-Small Angle Neutron Scattering (SANS, USANS) for nano- and micro-scale characterization, obtaining information on structure and the effects due to the coating treatment; and (2) High-Resolution Neutron Diffraction (HRND), evaluating the macrostrain components resulting from angular shifts of diffraction peaks and the micro-strains in the plastically deformation region by means of profile-broadening analysis. The obtained results contribute: improving knowledge of defects and other key features of the materials complementary to those achieved by using traditional examination techniques; helping to better understand the functional characteristics of Nitinol parts and predict the material’s mechanical behaviour.

Crystal structures and electronic properties in 3d transition metal doped SrRuO3.

The synthesis of polycrystalline samples of B-site doped SrRu1-xMxO3 with x≤ 0.2 by solid state methods is described for a number of dopants (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) and the structures of these established using Synchrotron X-ray powder diffraction, and for SrRu0.8Cu0.2O3 high resolution neutron diffraction. With the exception of M = Cu, samples with x = 0.2 form an orthorhombic Pbnm type perovskite structure at room temperature and these exhibit a sequence of phase transitions upon heating associated with the gradual reduction in the cooperative tilting of the corner sharing octahedra. SrRu0.8Cu0.2O3 forms a unique monoclinic structure at low temperatures and this transforms to the cubic Pm3[combining macron]m structure via an I4/mcm intermediate upon heating. The magnetic and electronic properties of the samples have been studied. Doping results in a decrease in the Curie temperature and at x = 0.2 all the samples are insulators. This is a consequence of the partial oxidation of the Ru cation that narrows the Ru 4d bands coupled with the suppression of the itinerant nature of the Ru 4d electrons due to the random distribution of the dopant cations. Ru L3-edge X-ray absorption spectroscopy of the Cu doped samples reveal a gradual increase in the average Ru oxidation sate upon doping. Electrical resistivity measurements show that doping increases the resistivity of the samples, and the temperature dependence of the resistivities are consistent with Arrhenius-type charge conduction.

The aging-effect-modulated mechanical behavior in U-Nb shape memory alloys through the modified twinning-detwinning process of the α″ phase

The deformation behavior in the U-Nb shape memory alloys (SMAs) is significantly sensitive to the aging effect, where the underlying mechanism remains unclear. In the present work, in-situ neutron diffraction is used to investigate simultaneously the macroscopic and microscopic stress-strain behaviors as well as the microstructure evolution in α″-martensitic U-Nb SMAs. The aging-effect-modulated mechanical properties are achieved through the modified twinning-detwinning process. The initial yielding strength is significantly increased from about 330 to 650 MPa at room temperature, while from approximately 440 to 700 MPa at 100 °C. The in-situ observations show that the strength enhancement arises from the aging-induced resistance during the twinning-detwinning process. High resolution neutron diffraction patterns indicate that the α″ phase structure is unchanged after aging. The aging effect is therefore understood by employing the defect migration mechanism, which explains well the enhanced yielding strength and twinning-detwinning resistance. It also suggests that the controllable properties could be realized by engineering the defect migration process and thus tuning the aging effect. These findings might shed some light on materials design and properties controlling for α″-martensitic U-Nb and other SMAs (e.g., Ni-Ti).

In situ studies of atomic ordering in Fe-19Ga type alloys

Structure of binary Fe-19Ga and ternary Fe-19Ga-Tb alloys at room temperature are investigated by TEM and high-resolution neutron diffraction (ND). In contrast with as cast or water quenched samples with A2 structure, the D03 ordering was detected in slowly cooled or annealed samples by ND. According to TEM, the D03 ordering takes place in the form of ordered clusters with the size of about 5 nm embedded into an A2 matrix. Disordering of the D03 clusters at heating and ordering at cooling were investigated by three in situ techniques: ND, vibrating sample magnetometry (VSM), and internal friction (IF). Temperatures of ordering-disordering were estimated by VSM and IF to be close to 500 °C. The effect of Fe-Ga alloys doping with Tb is also analyzed.

Insights into cation disorder and phase transitions in CZTS from a first-principles approach

${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ (CZTS) is a promising thin-film photovoltaic absorber material consisting of earth-abundant and nontoxic elements, though the efficiency achieved in actual devices is well below the theoretical one. Cation disorder has been proposed as a possible explanation; it is, however, still badly understood. A recent investigation based on high-resolution neutron diffraction experiments by Bosson et al. [J. Mater. Chem. A 5, 16672 (2017)] contradicts previous reports and indicates Cu-Zn disorder, not just in the $2c$ and $2d$ Wyckoff sites but also in the $2a$ site of Cu. To clear up this confusion on the exact nature of the disorder, we employ a ternary cluster expansion in combination with Monte Carlo simulations to analyze the phase characteristics of CZTS while undergoing order-disorder transitions. Our calculations show that, in excellent agreement with experiments, a low-temperature phase transition takes place close to 545 K and that the nature of the corresponding disorder is the same as the one recently observed experimentally by Bosson et al.

Complex octahedral tilt phases in the ferroelectric perovskite system LixNa1−xNbO3

High-temperature phase behavior in the system LixNa1-xNbO3 has been studied by using high-resolution powder neutron diffraction. Each of the three compositions studied in the Na-rich part of the phase diagram (x = 0.03, 0.08 and 0.12) shows evidence for distinct and complex structural modulations based on different tilting schemes of NbO6 octahedral units. Whilst octahedral tilting is prevalent in the structural chemistry of perovskites the nature of complexity of the phases observed here is unprecedented. Neither of the long-range tilt phases observed in NaNbO3 itself occurs here; instead a novel phase with a well-defined 4-fold superlattice is observed for the composition Li0.12Na0.88NbO3, and yet more complex phases with modulations based on 20-fold and 30-fold repeats are observed for Li0.03Na0.97NbO3 and Li0.08Na0.92NbO3, respectively. This peculiar structural behavior makes the system LixNa1-xNbO3 the most structurally complex 'simple' perovskite known.

Substitution-Induced Structure Evolution and Zn2+/Ga3+ Ordering in "114" Oxides MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+).

The "114" oxides LnBa(Co/Fe)4O7+δ represent a new family of materials that exhibits intriguing physical properties, including geometrically frustrated magnetism, oxygen storage, and magnetoelectric couplings. Various chemical substitutions have been conducted to modify their crystal and magnetic structures as well as physical properties. However, the principles beneath the substitution-induced structural evolution and charge/cationic ordering have not yet been understood. Thus, in this contribution, two complete solid solutions of MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+) were designed, synthesized, and characterized by Rietveld refinements based on high-resolution X-ray diffraction (XRD) and neutron diffraction (ND) data. The structure symmetry of MAZn2Ga2O7 is determined by the cationic size mismatch between M and A cations that can be defined by the tolerance factor t, i.e., symmetry transitions from P63 mc ( t > 0.87) to P31 c (0.87 > t > 0.75) and to Pna21 ( t < 0.75) were observed for MAZn2Ga2O7, associated with the rotation of T1O4 tetrahedra in the triangular layers. The Zn2+/Ga3+ ordering at T sites is also a consequence of the increase or decrease of the average sizes of M and A cations. A small concentration of interstitial oxygen ions can be obtained in Sr2Zn2- xGa2+ xO7+ x/2 ( x = 0.1, 0.2); however, no oxygen ionic conduction was observed at high temperatures, indicating the migration ability of the interstitial oxygen was very limited.

Structure and conductivity in iron-doped Bi26Mo10O69-δ

A series of complex oxides of general formula Bi26Mo10-2xFe2xO69-3x was synthesized and studied. The solid solution formation ranges and polymorphic modification limits were determined. SEM investigations and chemical analysis showed homogenous samples and confirmed the theoretical concentrations of the dopant. High-resolution neutron and X-ray diffraction were used to describe the structural features of the solid solution at ambient and high temperatures (30–800 °C). Structure refinement was carried out by Rietveld analysis using a combination of X-ray and backscattered neutron data. Close inspection of the fit at room temperature revealed a non-centrosymmeric P1 space group and additional peaks associated with oxygen and/or magnetic ordering. Magnetic measurements showed the samples had low magnetic susceptibility and a magnetic transition at ~ − 20 °C. A.C. impedance spectroscopy was used to determine the electroconductive properties of Bi26Mo10-2xFe2xO69-3x. Two basic types of impedance curves were detected at low and high temperatures, correlated with a change from triclinic to monoclinic symmetry. Details of both the monoclinic and triclinic structures are discussed, with the variation of electroconductive properties explained by structural changes.

Influence of microstructure on symmetry determination of piezoceramics

The origin of the complex reflection splitting in potassium sodium niobate doped with lithium and manganese was investigated using temperature-dependent high-resolution X-ray and neutron diffraction as well as electron probe microanalysis and scanning electron microscopy. Two structural models were developed from the diffraction data. A single-phase monoclinicPmmodel is known from the literature and is able to reproduce the diffraction patterns perfectly. However, a model with phase coexistence of two classical orthorhombicAmm2 phases can also reproduce the diffraction data with equal accuracy. Scanning electron microscopy in combination with electron probe microanalysis revealed segregation of theA-site substituents potassium and sodium. This favours the model with phase coexistence and confirms the need for comprehensive analyses with complementary methods to cover a broad range of length scales as well as to assess both average and local structure.

Nonmonotonic particle-size-dependence of magnetoelectric coupling in strained nanosized particles of BiFeO3

Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO3 (≈20–200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (TN). The ions, in fact, exhibit large anomalous displacement around the TN which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.

Theoretical and Experimental Investigations into Novel Oxynitride Discovery in the GaN-TiO2 System at High Pressure

We employed ab initio evolutionary algorithm USPEX to speed up the discovery of a novel oxynitride in the binary system of GaN-TiO2 using high-pressure synthesis. A 1:2 mixture of GaN and nanocrystalline TiO2 (anatase) was reacted under 1 GPa of pressure and at 1200 °C in a piston cylinder apparatus to produce a mixture of TiO2 (rutile) and an unknown phase. From the initial analysis of high resolution neutron and X-ray diffraction data, it is isomorphic with monoclinic V2GaO5 with a unit cell composition of Ga10Ti8O28N2 with the following parameters: monoclinic, space group C2/m, a = 17.823(1) A, b = 2.9970(1) A, c = 9.4205(5) A, β = 98.446(3)°; Volume = 497.74(3) A3. Further, a joint rietveld refinement revealed two distinct regimes—A Ti-rich block and a Ga-rich block. The Ti-rich block consists of four edge-shared octahedra and contains a site which is about 60% occupied by N; this site is bonded to four Ti. The remainder of the block consists of edge linked Ti-octahedral chains linked to the TiN/TiO fragments at octahedral corners partially occupied by nitrogen. The Ga-block contains two symmetry independent octahedral sites, occupied mostly by Ga, and a pure Ga-centered tetrahedral site bonded mostly to oxygen.

THE INVESTIGATION OF CRYSTAL AND MAGNETIC STRUCTURES OF SOLID SOLUTIONS BaFe12-xDxO19 (D= In AND Ga; x= 0.1 – 1.2)

The influence of partially substituted of barium ferrites by diamagnetic In and Ga ions with neutron diffraction method was studied. The substitution of Fe by In ions leads to increase of volume of unit cell while replacing with Ga ions insignificantly change the lattice parameters. The concentration dependence of the Tc Curie temperature as well as the magnetization is constructed. The refinement of the magnetic structure was carried out within the framework of Gorter’s model. The microstructural parameters of solid solutions BaFe12-xDxO19 (D= In and Ga; x = 0.1–1.2) were determined by the high-resolution neutron diffraction method.

The crystal structure of compositionally homogeneous mixed ceria-zirconia oxides by high resolution X-ray and neutron diffraction methods

AbstractThe real/atomic structure of single phase homogeneous nanocrystalline Ce0.5Zr0.5O2±δoxides prepared by a modified Pechini route and Ni-loaded catalysts of methane dry reforming on their bases was studied by a combination of neutron diffraction, synchrotron X-ray diffraction, total X-ray scattering and X-ray absorption spectroscopy. The effects of sintering temperature and pretreatment in H2were elucidated. The structure of the mixed oxides corresponds to a tetragonal space group indicating a homogeneous distribution of Ce and Zr cations in the lattice. A pronounced disordering of the oxygen sublattice was revealed by neutron diffraction, supposedly due to incorporation of water into the structure when in contact with air promoted by the generation of anion vacancies in the lattice after reduction or calcination at high temperatures. However, such disordering has not resulted in any occupation of the oxygen interstitial positions in the bulk of the nanodomains.

Low-temperature structure and the ferroelectric phase transitions in the CdTiO3 perovskite

The paraelectric-ferroelectric transition in $\mathrm{CdTi}{\mathrm{O}}_{3}$ has been monitored using high-resolution neutron diffraction data. This necessitated preparing a sample enriched in $^{114}\mathrm{Cd}$. A subtle, but significant, anisotropy in the thermal expansion of the lattice parameters for $\mathrm{CdTi}{\mathrm{O}}_{3}$ associated with the transition to the polar structure was observed. First-principles calculations are presented to understand energies, phonon dispersion, and structures of possible phases with different symmetries.

Comprehensive Study of Oxygen Storage in YbFe2O4+x (x ≤ 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase.

The multiferroic LuFe2.5+2O4 was recently proposed as a promising material for oxygen storage due to its easy reversible oxidation into LuFe3+2O4.5. We have investigated the similar scenario in YbFe2O4+x, leading to a slightly greater oxygen storage (OSC) capacity of 1434 μmol O/g. For the first time, the structural model of LnFe2O4.5 was fully understood by high-resolution microscopy images, and synchrotron and neutron diffraction experiments, as well as maximum entropy method. The oxygen uptake promotes a reconstructive shearing of the [YbO2] sub-units controlled by the adaptive Ln/Fe oxygen coordination and the Fe2/3+ redox. After oxidation, the rearrangement of the Fe coordination polyhedra is unique such that all available FeOn units (n = 6, 5, 4 in octahedra, square pyramids, trigonal bipyramids, tetrahedra) were identified in modulated rows growing in plane. This complex pseudo-ordering gives rise to short-range antiferromagnetic correlation within an insulating state.

Crossover between Tilt Families and Zero Area Thermal Expansion in Hybrid Prussian Blue Analogues

Materials in the family of Prussian blue analogues (C3 H5 N2 )2 K[M(CN)6 ], where C3 H5 N2 is the imidazolium ion and M=Fe, Co, undergo two phase transitions with temperature; at low temperatures the imidazolium cations have an ordered configuration (C2/c), while in the intermediate- and high-temperature phases (both previously reported as R3‾m ) they are dynamically disordered. We show from high-resolution powder neutron diffraction data that the high-temperature phase has zero area thermal expansion in the ab-plane. Supported by Landau theory and single-crystal X-ray diffraction data, we re-evaluate the space group symmetry of the intermediate-temperature phase to R3‾ . This reveals that the low-to-intermediate temperature transition is due to competition between two different tilt patterns of the [M(CN)6 ]3- ions. Controlling the relative stabilities of these tilt patterns offers a potential means to tune the exploitable electric behaviour that arises from motion of the imidazolium guest.