Superlattice Reflections Research Articles

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 .

Dielectric Properties and Related Domain Structures in 0.75BiFeO3–0.25BaTiO3

We report temperature changes of dielectric susceptibility and related domain structures in 0.75BiFeO3–0.25BaTiO3 in the temperature window between 298 and 800 K. It was found that the dielectric susceptibility exhibits a broad peak at around 773 K. The transmission electron microscopy (TEM) observation revealed that complex ferroelectric domain structures of 20–30 nm widths are formed in the ferroelectric phase of 0.75BiFeO3–0.25BaTiO3. A rhombohedral-to-pseudocubic transition was observed at around 773 K, at which the superlattice reflection spots at 1/2 1/2 1/2-type reciprocal positions changed into diffuse spots with very low intensities. Temperature changes of ferroelectric domain structures were also revealed by in situ TEM experiment.

Direct Observation of Charge Order and an Orbital Glass State in MultiferroicLuFe2O4

Geometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe2+ and Fe3+. The polarization and azimuthal angle dependence of the superlattice reflections demonstrate the absence of differences in anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state.

Syntheses, Structures and Solid-State Properties of MMX Mixed-Valence Chains, [NiII/III2(RCS2)4I]∞ (R = Et, n-Pr and n-Bu): Evidence of a Spin-Peierls Transition

The partial oxidation of [Ni(II/II)(2)(RCS(2))(4)] (R = Et (1), n-Pr (2), and n-Bu (3)) with iodine affords the MMX chain compounds [Ni(II/III)(2)(RCS(2))(4)I](infinity) (R = Et (4), n-Pr (5), and n-Bu (6)), respectively. The crystal structures of 4-6 consist of neutral one-dimensional (1-D) chains with a repeating -Ni-Ni-I- unit. The room-temperature (RT) structure of 4 indicates a charge-polarization (CP) state {-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)- (delta << 0.5)} close to an averaged valence state judged by the Ni-I distances. In contrast, 5 and 6 exhibit a 3-fold periodicity of a -Ni-Ni-I- unit due to the disorder of the dithiocarboxylato ligands. Compounds 4-6 show typical semiconducting behavior and exhibit an intense sharp absorption band centered at 5400 cm(-1), which is attributed to a Mott-Hubbard gap due to a relatively large on-site Coulomb repulsion energy U of the nickel atoms. The high-temperature magnetic susceptibilities of 4-6 can be described by a 1-D Heisenberg antiferromagnetic chain model with |J|/k(B) ranging from 898(2) to 939(3) K. Compounds 4 and 5 undergo a spin-Peierls (SP) transition at relatively high T(sp) = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to a 2-fold -Ni-Ni-I- period below T(sp). By determining the superstructure of 4 at 26 K, we conclude that the valence-ordered state changes from the CP in the RT phase to the alternate charge-polarization (ACP) state of -Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5+delta)+)-Ni((2.5-delta)+)-I(-)- in the SP phase. Such a spin-Peierls transition could not be observed for 6.

Structural changes at the Verwey transition in Fe 3O 4

Investigations of the dynamic process of the appearance of the Verwey transition in magnetite are performed by simultaneous measurements of magnetic susceptibility χ AC and the intensity of the ( 1 1 1 2 2 ) superstructure peak, using the coherent X-ray diffraction (CXD) technique. The results suggest that while the volume of a new low-temperature monoclinic phase increases gradually across the transition, it is initially in the form of grains too small to form the well-developed superlattice reflection. Finally, at the end of the transition, those small grains coalesce into micrometer size structural domains, reflected in ( 1 1 1 2 2 ) superlattice peak.

Ordered Rock-Salt Related Nanoclusters in CaMnO2

Oxygen engineering techniques performed under adequate controlled atmosphere show that the CaMnO(3)-CaMnO(2) topotactic reduction-oxidation process proceeds via oxygen diffusion while the cationic sublattice remains almost unaltered. Extra superlattice reflections in selected area electron diffraction patterns indicate doubling of the CaMnO(2) rock-salt cell along the cubic directions of a distorted rhombohedral cell originated by ordering of Ca(2+) and Mn(2+) ions distributed in nanoclusters into a NaCl-type matrix, as evidenced by dark field electron microscope images. The local nature of the information provided by the transmission electron microscopy techniques used to characterize the rock-salt type Ca(1-x)Mn(x)O(2) solid solution clearly hints at the existence of subtle extra ordering in other upper oxides of the Ca-Mn-O system. The combination of local characterization techniques like electron microscopy with more average ones like powder X-ray and neutron diffraction allows a very complete characterization of the system.

Structure analysis of complex metallic alloys in the Eu-Ag-In system by X-ray diffraction

In this work we investigate the complex atomic structures of two CMAs in the Eu-Ag-In system, these are quasicrystal approximants and their structures are similar to those of the 1/1 and 2/1 approximants found in the Yb-Cd system. The similarities between the Yb-Cd and Eu-Ag-In systems indicate that the RE-Ag-In systems in several ways can be treated as pseudo-binary. The investigated phases however also show some interesting differences; when Cd is replaced by the elements Ag and In partial chemical disorder is introduced into the system. As a response, the positional disorder which is usually observed in certain atomic positions for these types of approximants tends to disappear. The complete structures refined from single crystal data are described in detail and the differences with other related phases are highlighted. The 2/1 approximant in the Eu-Ag-In system is the first to exhibit such a degree of structural perfection in terms of positional order, a feature which makes this phase very suitable for further studies of chemical order and also can give insight into the local structures of related disordered phases. Electron diffraction studies on the 1/1 approximant phase have previously shown the presence of weak superlattice reflections, indicating a superstructure similar to that of the related phase Eu4Cd25. However, no superlattice reflections could be observed by single crystal diffraction and it is thus concluded that the phase has a disordered average structure with possible short to medium range superlattice ordering.

Structural and Dielectric Characteristics of Calcium Strontium Magnesium Niobate

The dielectric and structural characteristics of (Ca1-xSrx)(Mg1/3Nb2/3)O3, (x = 0–0.5) solid solutions are investigated by measuring and observing these properties by X-ray diffractometry, transmission electron microscopy, and Raman spectroscopy. The splitting of superlattice reflections shows that the (Ca1-xSrx)(Mg1/3Nb2/3)O3 solid solution has a 1:2 ordered structure distorted by the antiphase, the in-phase tilting of oxygen octahedra, and the antiparallel shift of cations at 0 ≤x ≤0.5. The strontium substitution causes a tolerance factor increase, which leads to higher structure symmetry. This study indicates that ionic polarizability, tolerance factor, cation substitution, and grain size have a significant effect on the microwave dielectric properties of this solid solution.

Non-cubic crystal symmetry of CaSiO 3 perovskite up to 18 GPa and 1600 K

In situ synchrotron X-ray diffraction experiments have been conducted on CaSiO 3 perovskite (CaPv) in a double-stage multianvil apparatus up to 18 GPa and 1600 K using a newly developed step-scan diffraction technique, which utilizes an energy-dispersive setup with a solid-state detector and collimator assemblies to collect angle-dispersive diffraction data over a wide range of photon energies. Superlattice reflections were resolved throughout the pressure ( P) and temperature ( T) range of the experiments, confirming that the crystal symmetry of CaPv is neither cubic nor tetragonal. A combination of analyses of the complete two-dimensional intensity datasets (photon energy from 20 to 160 keV and 2 θ angle from 3.0° to 9.0°) and Rietveld refinements at selected wavelengths revealed that the most likely space group of CaPv in the experimental P– T range, and therefore in the Earth's transition zone, is either Pbnm or Cmcm. The difference between these two space groups was too small to resolve with our technique.

Nd-doped BiFeO3 ceramics with antipolar order

Bi 1 − x Nd x FeO 3 (0≤x≤0.2) ceramics have been investigated using x-ray diffraction (XRD) and electron diffraction (ED). XRD patterns for x≤0.1 were consistent with rhombohedral BiFeO3, whereas those for x=0.15 and x=0.2 exhibited peak splitting and superlattice reflections representative of orthorhombic, antiferroelectric PbZrO3. ED for the latter samples confirmed the presence of 14(hk0) superlattice reflections typical of a PbZrO3-like structure but additional superlattice reflections were observed at 14(00l) yielding a √2a,2√2a,4a (where a is the pseudocubic lattice parameter) unit cell. The transition from rhombohedral BiFeO3 to the PbZrO3-like structure implies a modification from polar to antipolar behavior.

Composition dependent properties of Fe3Si films grown on GaAs(113)A substrates

Structural, electrical, and magnetic properties of Fe3Si/GaAs(113)A hybrid structures are studied, dependent on the layer composition varying from 15 to 26 at. % Si. The presence of superlattice reflections in x-ray diffraction and lower resistivity confirms the long-range atomic ordering in the stoichiometric Fe3Si films, reflecting the D03 crystal structure. The observed atomic ordering is also found to influence the sign and magnitude of the antisymmetric component of the planar Hall effect observed in this orientation. However a finite disorder is observed even in nearly stoichiometric samples.

100ps time-resolved X-ray diffraction study on Nd0.5Sr0.5MnO3 thin film

We report the lattice dynamics in the photoinduced phase of Nd0.5Sr0.5MnO3 thin film by means of time-resolved x-ray diffraction measurements at the NW14A beamline in the Photon Factory Advanced Ring. We observed the decrease of intensity after the photoirradiation for a superlattice reflection which is associated with Jahn-Teller distortion.

Evidence for charge-density-wave nature of the charge-ordered phase inLa1/3Sr2/3FeO3

We have investigated the threefold-charge ordering in ${\text{La}}_{1/3}{\text{Sr}}_{2/3}{\text{FeO}}_{3}$ sample below ${T}_{\text{CO}}$ $\ensuremath{\sim}200\text{ }\text{K}$. We have measured superlattice reflections, $q={a}^{\ensuremath{\ast}}(1/3\text{ }1/3\text{ }1/3)$, using the resonant x-ray scattering technique. The nonresonant intensities are consistent with a periodic structural modulation of both Fe and $\text{Sr}(\text{La}){\text{O}}_{3}$ atomic planes. These displacements give rise to two nonequivalent Fe sites in the ratio of 2:1. Strong resonances are observed at the $\text{Fe}\text{ }K$ edge in the $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{'}}$ geometry. These resonances did not exhibit azimuthal dependence. A charge disproportionation of 0.6 electrons is determined between the two Fe sites. Hence, the charge-ordered phase can be better understood as a charge-density-wave with a $\dots{}{\text{Fe}}^{3.3+}{\text{Fe}}^{3.9+}{\text{Fe}}^{3.9+}\dots{}$ sequence along the cubic [111] direction.

Phase Formation and Hydrogen Ordering in Yttrium-Hydrogen System

AbstractPhase transformations in epitaxial yttrium films grown on (0001)Ti//(0001)Al2O3 Ti-buffered sapphire substrates and hydrogenated for 10 min were characterized using transmission electron microscopy. After hydrogen charging, dense twin lamellae form during α(Y(H))-to-β(YH2) phase transition with twin boundaries predominately parallel to the interface between Y and a substrate. High densities of Shockley partial dislocations are present at the twin boundaries, their glides during phase transformation are responsible for the formation of twin lamellae. Electron diffraction from YH2 phase shows superlattice reflections, which suggests a new type of ordering on octahedral interstitial sites.

La2MnVO6 double perovskite: a structural, magnetic and X-ray absorption investigation

The synthesis, electron diffraction (ED), synchrotron X-ray and neutron structure, X-ray absorption spectroscopy (XAS) and magnetic property studies of La2MnVO6 double perovskite are described. Analysis of the synchrotron powder X-ray diffraction data for La2MnVO6 indicates a disordered arrangement of Mn and V at the B-site of the perovskite structure. Absence of super-lattice reflections in the ED patterns for La2MnVO6 supports the disordered cation arrangement. Room temperature time-of-flight (TOF) neutron powder diffraction (NPD) data show no evidence of cation ordering, in corroboration with the ED and synchrotron studies (orthorhombic Pnma, a = 5.6097(3), b = 7.8837(5) and c = 5.5668(3) A; 295 K, NPD). A comparison of XAS analyses of La2TVO6 with T = Ni and Co shows T2+ formal oxidation state while the T = Mn material evidences a Mn3+ admixture into a dominantly Mn2+ ground state. V-K edge measurements manifest a mirror image behavior with a V4+ state for T = Ni and Co with a V3+ admixture arising in the T = Mn material. The magnetic susceptibility data for La2MnVO6 show ferromagnetic correlations; the observed effective moment, µeff (5.72 µB) is much smaller than the calculated moment (6.16 µB) based on the spin-only formula for Mn2+ (d5, HS) /V4+ (d1), supportive of the partly oxidized Mn and reduced V scenario (Mn3+/V3+).

Mixed Charge-Ordering State of MMX-Type Quasi-One-Dimensional Iodide-Bridged Platinum Complexes with Binary Countercations

A new charge-ordering (CO) state in quasi-1D iodide-bridged dinuclear Pt complexes (MMX-chains) was developed by introducing two different kinds of countercations. From crystal structure analysis, K(2)(H(3)NCH(2)CHMeCH(2)NH(3))[Pt(2)(pop)(4)I].4H(2)O (pop = P(2)H(2)O(5)(2-)) had structural features of both ACP (alternating charge-polarization) and CDW (charge-density-wave) states. Therefore, this new CO state was named ACP+CDW state. Remarkably, the structure was three dimensionally ordered, which was verified by the strong superlattice reflections observed in an X-ray oscillation photograph. Using polarized Raman and (31)P MAS NMR spectroscopies, it was shown that there were two different Pt(2) units and four inequivalent P atoms, respectively. These results agreed with the crystal structure analysis and supported the presence of the ACP+CDW state.

Charge and spin ordering in La 2−xSr xCoO 4 (0.4< x

Neutron scattering experiments were performed on the single crystals of layered-perovskite cobaltate La 2− x Sr x CoO 4. For a commensurate value of x=0.5, magnetic reflections due to high-spin (HS) state of Co 2+ are observed in (H, 0, L) plane. Further, in (H, H, L) plane, we found super-lattice reflections with wave vector q m=(1/4, 1/4, 1). The latter magnetic peaks cannot be understood by the Co 2+ spin configurations, indicating that these peaks are originated from Co 3+ spin ordering. From the hole-doping dependence, short ranged charge and spin orderings were observed below x=0.5, while those of peaks were suppressed at x=0.6. Our results suggest that double exchange interaction between the HS state of Co 2+ and the intermediate spin (IS) state of Co 3+ mediate the IS state in this system.

High-pressure behaviour of KMF3 perovskites

The structural stability of cubic KMF3 (M: Mg, Zn, Co, Ni) perovskites has been studied by powder X-ray diffraction under pressure. Neither superlattice reflections nor peak splitting associated to a phase transition were detected in the 0–10 GPa pressure range. Furthermore, KMgF3 showed no structural changes up to 50 GPa. The results were compared with previous reports on isostructural KMnF3. Tolerance factor and site parameters ratio have been analysed as high-pressure stability indexes for this family of perovskites.

Lattice-parameter misfits between the γ and γ′ phases in single crystals of nickel superalloys

X-ray diffractometry has been used to study the transformation of the profiles of X-ray diffraction lines (222), (004), (003) and to determine the values of the lattice misfit between the γ and γ′ phases (γ/γ′ misfit) for single crystals of nickel superalloys ZhS6U, ZhS30, ZhS32, VZhM1, and VZhM4 in the as-cast state and after heat treatment. It has been established that with an increase in the γ/γ′ misfit there increase tetragonal distortions of the fcc lattice of the γ solid solution. For the single crystals of VZhM4 alloyed with rhenium and ruthenium, splitting of the superlattice reflection of the strengthening γ′ phase has been revealed. The structural regularities revealed testify to qualitative changes in the factors that govern the redistribution of refractory alloying elements between the lattices of the γ and γ′ phases, for example, of Mo in the presence of Re and Ru.

Correlation between structural and magnetic properties of half‐metallic spinel CuV2S4 due to the order–disorder transition of 3d2(t2g) orbitals of V3+ ions

AbstractStructural and magnetic properties of the spinel CuV2S4 have been studied by means of X‐ray diffraction using synchrotron radiation and measurements of magnetic susceptibility. The structural phase transitions occur at about 55 K and 90 K, and the superlattice reflections (h ± qo, k ± qo, l) having an orthorhombic reduced wavevector qo = (1/4, 1/4, 0) are observed at 10 K. This suggests that the structure below 55 K is a commensurate unit cell of 4ap × 4bp × cp, where the subscript ‘p’ means a pseudo‐spinel structure, and that the structural phase transitions are induced by the cooperative displacement of S ions in one‐dimensional VS6 octahedral chains, which are aligned along an orthorhombic [110] direction. The susceptibility also shows that the weak ferromagnetic transition occurs at about 90 K, and the spike‐like susceptibility is observed in the region of about 40 K and 55 K. The unusual electronic and magnetic properties of CuV2S4 in the low‐temperature phases below about 90 K are interpreted by order and disorder states of (3dxy, 3dyz, 3dzx) for the V3+ 3d2(t2g) orbitals in the VS6 octahedral chains. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)