Short-range Order Phase Research Articles

Direct Synthesis of CuPd Icosahedra Supercrystals Studied by In Situ X-Ray Scattering.

Nanocrystal self-assembly into supercrystals provides a versatile platform for creating novel materials and devices with tailored properties. While common self-assembly strategies imply the use of purified nanoparticles after synthesis, conversion of chemical precursors directly into nanocrystals and then supercrystals in simple procedures has been rarely reported. Here, the nucleation and growth of CuPd icosahedra and their consecutive assembly into large closed-packed face-centered cubic (fcc) supercrystals are studied. To this end, the study simultaneously and in situ measures X-ray total scattering with pair distribution function analysis (TS-PDF) and small-angle X-ray scattering (SAXS). It is found that the supercrystals' formation is preceded by an intermediate dense phase of nanocrystals displaying short-range order (SRO). It is further shown that the organization of oleic acid/oleylamine surfactants into lamellar structures likely drives the emergence of the SRO phase and later of the supercrystals by reducing the volume accessible to particle diffusion. The supercrystals' formation as well as their disassembly are triggered by temperature. The study demonstrates that ordering of solvent molecules can be crucial in the direct synthesis of supercrystals. The study also provides a general approach to investigate novel preparation routes of supercrystals in situ and across several length scales via X-ray scattering.

Organic carbon and short-range order minerals responsible for cementation of the spodic horizon (ortstein): a new proposal of chemical extractions in undisturbed samples

Context The cementation of spodic horizons (ortstein) has long been studied through chemical analysis in disturbed soil samples of <2 mm (soil structure destructuring). Aims Use of the undisturbed samples and selective chemical extractions to study the soil organic matter (SOM) and short-range order phase (SROP) cementing the spodic horizon. Methods The study was carried out in a Spodosol (ortstein) from southern Brazil in undisturbed cubes (1.5 cm × 1.5 cm × 1.5 cm). The undisturbed cubes were separated in two visual colour standards (yellow colour (YC – 10YR 6/8) and yellow dark colour (YDC – 10YR 4/3)) and were submitted to four SROP chemical extractions (pyrophosphate (PYR), ammonium oxalate (AO), NaOH 0.5 mol L−1, and water). Key results The choice of selective SROP extraction in two colour standards of undisturbed samples enabled the identification of cementation details of the ortstein : (1) SOM illuviation also increases Fe, Al, Mn, Si, Ca, and Mg contents; (2) the SRO-Fe and Al oxides cover the SOM and/or there is a larger proportion of mineral in relation to OC in the organo/mineral association; (3) SRO-Al oxides were the main compounds responsible for cementation; and (4) the illuvial SOM associated with SRO-Fe oxides did not contribute to ortstein physical stability. Conclusions and implications The best extractions to study the samples were AO for YC colour and NaOH for YDC colour. We recommend the use of undisturbed samples and the inclusion of NaOH 0.5 mol L−1 in the chemical protocol for analysis to better understand which organic and mineral phases clog the ortstein pores in different pedogenetic conditions around the world.

Correlation between critical magnetic behavior and griffiths phase in ferromagnetic La0.7Sr0.3Mn1−xFexO3 (x = 0.05) nanoparticle

The magnetic behavior of the La0.7Sr0.3Mn0.95Fe0.05O3 nanoparticles was investigated. X-ray diffraction analysis revealed that this sample exhibited rhombohedral symmetry with space group. The sample underwent a ferromagnetic-paramagnetic transition at approximately 290 K, characterized by a second-order phase transition. Interestingly, the state above the transition temperature is not purely paramagnetic but rather exhibits a short-range ferromagnetic order or Griffiths phase, which becomes more pronounced as the temperature increases up to the Griffiths transition temperature (345 K). The values of the critical exponents obtained in this study, β = 0.671, γ = 1.087, and δ = 2.612, with T C = 290.09 K, are distinct from those of any known universality class, and are confirmed by a single scaling equation of state. This unusual behavior is ascribed to the existence of the Griffiths phase, which reflects the unconventional critical behavior of magnetic susceptibility. In addition to conventional magnetic measurements, detailed studies of isothermal magnetization and magnetocaloric effects have been performed, and theoretical analyses have been utilized to further explore the Griffiths phase. Furthermore, a correlation between the Griffiths phase and the anomalous critical parameters was established.

Short-range order and compositional phase stability in refractory high-entropy alloys via first-principles theory and atomistic modeling: NbMoTa, NbMoTaW, and VNbMoTaW

Using an all-electron, first-principles, Landau-type theory, we study the nature of short-range order and compositional phase stability in equiatomic refractory high-entropy alloys, NbMoTa, NbMoTaW, and VNbMoTaW. We also investigate selected binary subsystems to provide insight into the physical mechanisms driving order. Our approach examines the short-range order of the solid solutions directly, infers disorder/order transitions, and also extracts parameters suitable for atomistic modeling of diffusional phase transformations. We find a hierarchy of relationships between the chemical species in these materials which promote ordering tendencies. The most dominant is a relative atomic size difference between the $3d$ element, V, and the other $4d$ and $5d$ elements which drives a $B32$-like order. For systems where V is not present, ordering is dominated by the difference in filling of valence states; pairs of elements that are isoelectronic remain weakly correlated to low temperatures, while pairs with a valence difference present $B2$-like order. Our estimated order-disorder transition temperature in VNbMoTaW is sufficiently high for us to suggest that SRO in this material may be experimentally observable.

Quantification of amorphous Si, Al, and Fe in palagonitic Mars analogs by chemical extraction and X-ray spectroscopy

Data collected by orbiters and landers have consistently shown abundant amorphous materials on Mars. Developing analytical techniques that target amorphous materials in terrestrial analogs can help determine the environmental conditions under which the amorphous assemblages on Mars were formed. However, the inherent short-range order, chemical heterogeneity, and nanoscale phase interactions create challenges in characterizing these phases. This study is aimed to overcome these challenges by combining chemical dissolution rate analysis with spectroscopy-based mass balance calculations (MBC) to characterize different pools of amorphous materials in terrestrial analogs. The differences in dissolution rates between rapidly dissolving Si, Al, and Fe amorphous materials and the slowly dissolving crystalline minerals were modeled to predict amorphous composition in five palagonitic samples from Hawaii (Mauna Kea) and four hyaloclastite tuffs from the subglacial volcanoes in southwest Iceland. These samples were selected as potential analogs because of their spectral and compositional resemblance to Mars's surface materials.The amorphous Si, Al, and Fe compositions from both sites varied based on the degree of aqueous alteration, grain size, and location. The amorphous fractions of the unconsolidated palagonitic analogs from Hawaii are composed of alteration products such as opal CT and ferrihydrite with minor amounts of unaltered basaltic glass. In contrast, the amorphous fractions of the cemented Icelandic samples were composed mainly of unaltered glass and mixed Fe(II, III) iron phases. Amorphous compositions of the loose Hawaiian and the consolidated Icelandic palagonites are comparable with the Martian modern and ancient aeolian materials respectively. The amorphous compositions in the Hawaiian sample, HWMK101, closely resemble the amorphous materials in Rocknest, a modern aeolian material from the sand shadow, and the Icelandic samples are comparable to the ancient aeolian materials from the Stimson formation. Our analysis indicates the presence of hydrated secondary alteration products such as ferrihydrite and allophane in Rocknest, and a mixture of reduced iron phases and silicate glass outlined with finer altered opaline silicates in Greenhorn. The chemical extraction analysis combined with MBC can be used to characterize amorphous phases in terrestrial analogs to better constrain the formation and characterization of the abundant amorphous materials on Mars.

Magnetovolume effects and thermal expansion in chiral helical ferromagnets Fe-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-Si

Within the framework of the theory of band magnetism and the Heine model for the volume dependence of the electronic spectrum, an approach is being developed to study magneto-volume effects in chiral helical ferromagnets. Using Fe1-xCoxSi as an example, it was found that in the range of long-range order (at temperature T<Tc) the magnetovolume effect is determined by the amplitude of helicoidal spin spirals and leads to the experimentally observed negative volume thermal expansion coefficient. In the region of phase transitions of the first order (from Tc to Ts) prolonged in temperature, a new mechanism of the magnetovolume effect is established due to the spatial fluctuations of spin spirals arising due to the difference in the Hubbard potentials of iron and cobalt. It is shown that the considered volume effects lead not only to the experimentally observed negative volume thermal expansion coefficient (VCTE) in the chiral spin short-range order phase, but also to a noticeable increase in the transition temperature to the paramagnetic state (T>Ts). Keywords: helicoidal ferromagnetism, chirality, spin fluctuations, electronic and crystal structure, thermal expansion.

Магнитообъемные эффекты и тепловое расширение в киральных геликоидальных ферромагнетиках Fe-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-Si

Within the framework of the theory of band magnetism and the Heine model for the volume dependence of the electronic spectrum, an approach is being developed to study magneto-volume effects in chiral helical ferromagnets. Using Fe1-хCoxSi as an example, it was found that in the range of long-range order (at temperature Т<Tc) the magnetovolume effect is determined by the amplitude of helicoidal spin spirals and leads to the experimentally observed negative volume thermal expansion coefficient. In the region of phase transitions of the first order (from Тс to Тs) prolonged in temperature, a new mechanism of the magnetovolume effect is established due to the spatial fluctuations of spin spirals arising due to the difference in the Hubbard potentials of iron and cobalt. It is shown that the considered volume effects lead not only to the experimentally observed negative volume thermal expansion coefficient (VCTE) in the chiral spin short-range order phase, but also to a noticeable increase in the transition temperature to the paramagnetic state (Т>Ts).

First-principles analysis of precipitation in Mg-Zn alloys

Precipitation in Mg-Zn alloys was analyzed by means of first principles calculations. Formation energies of symmetrically distinct hcp Mg1-xZnx (0 < x < 1) configurations were determined and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (Mg4Zn7, MgZn2 cubic, MgZn2 hexagonal, Mg21Zn25 and Mg2Zn11) were calculated. The formation energies of Mg4Zn7, MgZn2 cubic, and MgZn2 hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that \beta_1^' precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high temperature stability. It was found that the MgZn2 hexagonal Laves phase is the most stable phase at high temperature and, thus, \beta_1^' precipitates tend to transform into the \beta_2^' (MgZn2 hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium \beta phase (Mg21Zn25) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase \beta_2^' to \beta .

Short-Range Order Evolution in Nanocrystalline Mechanically Activated Fe–Cr Alloys in the Process of Annealing

Nanocrystalline Fe–Cr alloys synthesized from pure components in a planetary ball mill with a chromium content of 20–48 at % were annealed for 4 h at temperatures of Tan = 400–700°C. Short-range order (SRO) evolution and phase separation dependent on Tan were studied using Mossbauer spectroscopy and X-ray diffraction. The precipitation of the σ-FeCr phase was observed only in the samples with 48 at % of Cr at Tan = 600 and 700°C. For all the samples, the grain growth began most intensively from 400°C, and the higher the Cr content in an alloy, the smaller the final size of grains after Tan = 700°C. The analysis of behavior of the mean hyperfine field on Fe nuclei and distribution of the hyperfine field width depending on Tan, as well as fitting of Mossbauer spectra with spectral components corresponding to the α (Cr depleted) and α' (Cr rich) phases, demonstrated that the mechanically alloyed Fe-Cr samples were characterized by weak short-range separation passing at Tan = 400°C to a nearly statistically uniform distribution of atoms. Heterogeneous short-range order with the α and α' regions was formed in the alloys with 30 at % of Cr and more after annealing at 500–700°C. An analysis with the material balance equation led to our conclusion about the existence of the grain boundary segregations of Cr atoms.

Quantum phase diagram of a frustrated spin- 12 system on a trellis ladder

We study an isotropic Heisenberg spin-1/2 model on a trellis ladder which is composed of two $J_1-J_2$ zigzag ladders interacting through anti-ferromagnetic rung couplings $J_3$. The $J_1$ and $J_2$ are ferromagnetic zigzag spin interaction between two legs and anti-ferromagnetic interaction along each leg of a zigzag ladder. A quantum phase diagram of this model is constructed using the density matrix renormalization group (DMRG) method and linearized spin wave analysis. In small $J_2$ limit a short range stripe collinear phase is found in the presence of $J_3$, whereas, in the large $J_2/J_3$ limit non-collinear quasi-long range phase is found. The system shows a short range non-collinear state in large $J_3$ limit. The short range order phase is the dominant feature of this phase diagram. We also show that the results obtained by DMRG and linearized spin wave analysis show similar phase boundary between stripe collinear and non-collinear short range phases, and the collinear phase region shrinks with increasing $J_3$. We apply this model to understand the magnetic properties of CaV$_2$O$_5$ and also fit the experimental data of susceptibility and magnetization. The variation of magnetic specific heat capacity as function of external magnetic field is also predicted. We note that $J_3$ is a dominant interaction in this system, whereas $J_1$ and $J_2$ are approximately half of $J_3$.

Persistent Paramagnons Deep in the Metallic Phase of Sr_{2-x}La_{x}IrO_{4}.

We have studied the magnetic excitations of electron-doped Sr_{2-x}La_{x}IrO_{4} (0≤x≤0.10) using resonant inelastic x-ray scattering at the Ir L_{3} edge. The long-range magnetic order is rapidly lost with increasing x, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.

Structural and magnetic properties of Fe3Ga alloy nanowires: Effect of post annealing treatment

We report on the influence of the annealing temperature on the crystal structure phase change, effective anisotropy energy and magnetization reversal of electrodeposited Fe3Ga nanowires (NWs) with diameter of 80 nm. Scanning transmission electron microscopy (STEM) energy-dispersive X-ray (EDX) spectroscopy technique for the elemental mapping of Fe3Ga NWs showed clearly the homogeneous distribution of Fe and Ga. These NWs are highly aligned to each other and exhibits high geometrical aspect ratio. The crystal structure of these nanowires shows strong dependence on the sintering temperature. X-ray diffraction reflects the highly textured A2-disorderd phase with precipitates of DO3 and L12 phase for as-synthesized Fe3Ga NWs. As the annealing temperature increases, the intensity of strong disordered phase A2 or short range order phase DO3 suddenly decreases and grains are textured along the stable L12 phase with small precipitates of DO3 phase. As a result of these structural changes, magnetization reversal mode switches from transverse to a combination of transverse and vortex modes. Angular magnetic response at room temperature is employed to identify the magnetization reversal modes for Fe3Ga NWs. Temperature dependent coercivity response can be understood in terms of thermal activation over an energy barrier with a 32 – power dependence on the field.

Incommensurate Short-Range Order in S = 1 Triangular Lattice Ising Antiferromagnet

We study the $S = 1$ triangular lattice Ising antiferromagnet by Monte Carlo simulations. Frustrations between a major antiferromagnetic third-neighbor interaction $J_3$ and a minor ferromagnetic nearest-neighbor interaction $J_1$ cause incommensurate short-range orders at intermediate temperatures. At low temperatures (below $T/J_3 \lesssim 0.2$ for $J_1/J_3 = -1/3$), the system exhibits fourfold periodic ordered state. In the short-range order phase, the system shows a glassy two-step relaxation. We demonstrate that the features of the short-range order are attributed to the cooperation between the frustrations and the nonmagnetic Ising spin states which is a particular feature of the integer spin systems.

Quantum bicriticality in Mn1 − x Fe x Si solid solutions: Exchange and percolation effects

The T-x magnetic phase diagram of Mn1 − x Fe x Si solid solutions is probed by magnetic susceptibility, magnetization and resistivity measurements. The boundary limiting phase with short-range magnetic order (analogue of the chiral liquid) is defined experimentally and described analytically within simple model accounting both classical and quantum magnetic fluctuations together with effects of disorder. It is shown that Mn1 − x Fe x Si system undergoes a sequence of two quantum phase transitions. The first “underlying” quantum critical (QC) point x* ∼ 0.11 corresponds to disappearance of the long-range magnetic order. This quantum phase transition is masked by short-range order phase, however, it manifests itself at finite temperatures by crossover between classical and quantum fluctuations, which is predicted and observed in the paramagnetic phase. The second QC point x c ∼ 0.24 may have topological nature and corresponds to percolation threshold in the magnetic subsystem of Mn1 − x Fe x Si. Above x c the short-range ordered phase is suppressed and magnetic subsystem becomes separated into spin clusters resulting in observation of the disorder-driven QC Griffiths-type phase characterized by an anomalously divergent magnetic susceptibility χ ∼ 1/T ξ with the exponents ξ ∼ 0.5–0.6.

One-pot synthesis of POSS-containing alternating copolymers by RAFT polymerization and their microphase-separated nanostructures

The poly(styrene-alt-maleimide isobutyl POSS) (PSMIPOSS) alternating copolymer is one-pot synthesized via reversible addition-fragmentation transfer (RAFT) polymerization. The results show that a POSS-containing alternating copolymer with a high degree of polymerization (DP) and a low polydispersity index (PDI) is successfully constructed by alternating copolymerization of maleimide isobutyl POSS with styrene, and the molecular weight of the PSMIPOSS alternating copolymer can be well controlled by the amount of RAFT agent. Poly(styrene-alt-maleimide isobutyl POSS)-block-polystyrene (PSMIPOSS-b-PS) block copolymers are also one-pot prepared by RAFT polymerization, and their self-assembly behavior in the bulk is investigated by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), which exhibit a series of short-range order phase transitions from the POSS sphere, POSS cylinder to lamella structures with the weight fraction of MIPOSS ranging from 13% to 64%. The thermal properties of POSS-containing copolymers are evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. The results display that MIPOSS units can enhance the thermal properties of copolymers effectively.

Special features of the long-range order – short-range order phase transition in FCC-based alloys

Results of x-ray analysis of the long-range order in FCC-based alloys with L12, L12(M), L12(MM), and D1a superstructures are generalized. Regularities of the behavior of the long-range order parameter and average size of the thermal and periodic antiphase boundaries (APB) are traced, main mechanisms of temperature order-disorder phase transition are elucidated, and influence of the thermal and periodic APB on the special features of the order-disorder phase transition is revealed. The character of change of the periodic antiphase boundary density determines the special features of the order-disorder phase transition in alloys with longperiod superstructures.

Synthesis characterization of magnetic properties in La 0.7− x Ln x Pb 0.3 MnO 3 ( [formula omitted], Nd, Gd, Dy, Sm and Y) perovskite compounds

The influence of La substitution by rare-earth elements on the magnetization behaviors of perovskite oxides, La 0.7− x Ln x Pb 0.3 MnO 3 ( Ln = Pr , Nd, Gd, Dy, Sm and Y), is investigated. The replacement of La ion by Pr, Nd, Gd, Dy, Sm or Y results in a considerable decrease in the ferromagnetic ordering temperature T C and clearly irreversible behavior in the FC–ZFC curves showing a short-range spin order phase. The fact is in agreement with the smaller ionic radii of Pr, Nd, Gd, Dy, Sm and Y ions in contrast to La ion and the corresponding larger distortion of perovskite structures. The saturation magnetization M S increases as Pr or Nd content increases while M S decreases as Sm or Y content increases. Moreover, the saturation magnetization M S increases and then decreases as Gd or Dy content increases. These results can be explained in terms of the competition between the increase of ferromagnetically interacting spins due to the introduction of magnetic ions with f-shell electrons and suppression of ferromagnetism due to structure tuning induced by the small ionic radius of the interpolated cation into the La-site.

Short-range ordered phase of the double-exchange model in infinite dimensions

Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T=0 phase diagram of the double-exchange model on a Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, we also study a class of disordered phases with magnetic short-range order (SRO). In the weak-coupling limit, a SRO phase has a higher transition temperature than the AF phase for all fillings p below 1 and can even have a higher transition temperature than the FM phase. At T=0 and for small Hund's coupling J_H, a SRO state has lower energy than either the FM or AF phases for 0.26\le p < 1. Phase separation is absent in the J_H --> 0 limit but appears for any non-zero value of J_H.

COMPARISON OF THE MAGNETIZATION BEHAVIORS IN PEROVSKITE COMPOUNDS La0.7-xLnxPb0.3MnO3 (Ln=Pr AND Sm)

The magnetization behaviors of two manganite oxide systems, La 0.7-x Ln x Pb 0.3 MnO 3 ( Ln = Pr and Sm ), have been synthesized. The replacement of La ions by Pr or Sm results in a considerable decrease in the ferromagnetic ordering temperature T C and clearly irreversible behavior in the zero-field-cooling-field-cooling curve at a low applied field, showing a short-range spin order phase. These facts are in agreement with the smaller ionic radii of Pr (0.130 nm) and Sm (0.124 nm) ions in contrast to La ion (0.136 nm), and the corresponding larger distortion of perovskite structures. The saturation magnetization M S decreases as Sm content increases relative to the increase of M S as Pr content increases. This can be interpreted in terms of the competition between suppression of ferromagnetism due to structure tuning induced by the small ionic radius of the interpolated cations into the La -site and the increase of ferromagnetically interacting spins due to the introduction of magnetic Pr or Sm ions with f-shell electrons.

Observation of critical behaviors in squaric acid by 1H nuclear magnetic resonance

1H nuclear magnetic resonance (NMR) was employed in order to investigate the phase transitions in a two-dimensional antiferroelectric system, squaric acid (H 2C 4O 4). As a result, in addition to the critical behaviors around the long range order phase transition, similar behaviors were observed at higher temperatures where a short range order phase transition presumably takes place.