Fddd Structure Research Articles

First-principles study of the crystal and magnetic structures of multiferroic Cu2OCl2

Recently, the discovery of multiferroicity in pyrochlore-like compound Cu2OCl2 has generated significant interest, and several studies have been performed in this area. This transition metal oxychloride is unique because the divalent copper atoms create an correlated insulator and the pyrochlore lattice tends to frustrate spins. From neutron powder diffraction measurements, an incommensurate magnetic order of the ordering vector emerges below the Néel temperature of 70 K. At this temperature or slightly above, ferroelectricity (FE) or antiferroelectricity, accompanying a lattice distortion, has been observed. Experimentally, some discrepancies remain. In this paper, we report our first-principles simulation results by evaluating the possible lattice and spin spiral states. We found that the Fddd structure is not more stable than , which is supported by our reexamination of the x-ray diffraction data. In addition, we find that after we include magnetism in the calculation, it predicts that the lattice with a helical (proper screw) spin structure is energetically more stable than other spin configurations. Our results indicate charge-order-driven FE that subsequently induces magnetism.

FdddStructure in Polystyrene-block-polyisoprene Diblock Copolymer/Polystyrene Homopolymer Blends

We have induced Fddd structure in polystyrene-block-polyisoprene diblock copolymer (SI) which does not have Fddd phase by adding polystyrene (hPS) homopolymer. We prepared the SI/hPS blend samples in the range of 0.627 < ϕPI < 0.646 and investigated the phase boundary of Fddd phase by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), where ϕPI is volume fraction of polyisoprene (PI) in SI/hPS. We found the Fddd phase as an equilibrium phase in the range of 0.631 ≤ ϕPI ≤ 0.641 and 26.6 < χN < 30.7, where χ and N are respectively the Flory–Huggins interaction parameter between PS and PI and polymerization index of the SI. The Fddd phase boundary in the SI/hPS is similar to that in the neat SI block copolymer since the effect of the addition of the hPS with the low molecular weight on the phase diagram corresponds to that of the same change of PS volume fraction in the neat block copolymer.

Lattice stability and high-pressure melting mechanism of dense hydrogen up to 1.5 TPa

Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reach absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

Computer simulations of three-dimensional Turing patterns in the Lengyel-Epstein model.

We investigate numerically Turing patterns in the Lengyel-Epstein model in three dimensions. In a bulk homogeneous system under periodic boundary conditions, we obtain not only lamellar, cylindrical, and spherical structures but also several interconnected periodic structures including the Schwartz P-surface structure. In order to examine Turing patterns in the conditions accessible experimentally, we consider inhomogeneous systems where a parameter in the reaction-diffusion equations depends on the space coordinate with either Dirichlet or Neumann boundary conditions. In this situation, we find that a perforated-lamellar structure and an Fddd structure, both of which have a uniaxial symmetry, appear depending on the boundary conditions.

Review on distorted face-centered cubic phase in yttrium via genetic algorithm

We present two distorted face-centered cubic (dfcc) structures of yttrium under high pressure, which have been found by a first-principles genetic algorithm technique. The structures are a tetragonal P43 (dfcc-I) and a triclinic P1¯ (dfcc-II), formed by slight distortions from a trigonal R3¯m structure reported as the dfcc phase earlier. The enthalpy difference between the two dfcc structures is less than 0.2 mRy/atom, and dfcc-I is marginally more stable than dfcc-II in lower pressure region. The enthalpy comparison among candidate structures indicates the structural phase transitions into dfcc-I at 41 GPa, into dfcc-II at 81 GPa, and into an orthorhombic Fddd structure at 106 GPa.

Structure of intermediate phase II of LiNH2 under high pressure.

A new intermediate phase (phase II) was found between phases I and III in LiNH2 in the pressure range of 10 to 13 GPa through the analysis of infrared and powder X-ray diffraction measurements to 25 GPa at room temperature. This result agreed with the prediction of a stable phase between phases I and III through theoretical calculations. Powder X-ray diffraction measurement and DFT calculation showed that this phase has a monoclinic structure with space group C2/c (Z = 8), which is the same structure as that of a slightly tilted crystal lattice of the Fddd structural model. The enthalpy of the C2/c structure was also found to be almost the same as that of the Fddd structure.

Remarkably Rich Variety of Nanostructures and Order–Order Transitions in a Rod–Coil Diblock Copolymer

A remarkably rich variety of nanophase-separated structures and various order–order transitions are observed in a series of low-molecular weight (MW) rod–coil block copolymers (BCPs) with the rod blocks of different lengths (LRod’s). The rod–coil diblock copolymer studied herein is poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS), in which PMPCS is a rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior. When the polymerization degree of the PMPCS rod block (NRod) is less than 32 (LRod < 8 nm), the PMPCS block is always amorphous in the entire temperature range. And the corresponding PDMS-b-PMPCS BCPs with NRod from 11 to 29 and the volume fraction of the PMPCS rod (fRod) from 43% to 67% self-assemble into various equilibrium nanostructures after annealed at temperatures above the glass transition temperatures of the PMPCS blocks. When NRod = 11 and fRod = 43%, the BCP forms a lamellar structure (LAM); when NRod = 15 and fRod = 51%, the BCP forms a double gyriod structure (GYR) ; when NRod = 20 and fRod = 57%, the BCP forms a GYR structure after annealed below 180 °C and transforms to the Fddd structure after annealed above 180 °C; when NRod = 29 and fRod = 67%, the nanostructure of the BCP after annealed below 180 °C is hexagonally packed cylinders (HEX) and changes to a body centered cubic structure (BCC) after annealed above 180 °C. When NRod > 32 (LRod > 8 nm), the PMPCS rod block is amorphous at low temperatures and transforms to a stable columnar LC phase after annealed at high temperatures. Correspondingly, the PDMS-b-PMPCS BCP with NRod = 44 and fRod = 75% forms a HEX structure after annealed at lower temperatures at which the PMPCS block is amorphous, and the nanostructure transforms to LAM after the sample is annealed at higher temperatures at which the PMPCS block enters into the LC phase. Therefore, only by a small change of the rod length in the low-MW PDMS-b-PMPCS rod–coil BCPs, various nanostructures including LAM, GYR, Fddd, HEX, and BCC are obtained. In addition, by increasing annealing temperatures, GYR-to-Fddd and HEX-to-BCC transitions are observed in the BCPs with the amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC PMPCS block undergoes an isotropic-to-LC phase transformation.

Structure determination of ultra dense magnesium borohydride: A first-principles study

Magnesium borohydride (Mg(BH4)2) is one of the potential hydrogen storage materials. Recently, two experiments [Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov, and H. Hagemann, Angew. Chem., Int. Ed. 50, 11162 (2011); L. George, V. Drozd, and S. K. Saxena, J. Phys. Chem. C 113, 486 (2009)] found that α-Mg(BH4)2 can irreversibly be transformed to an ultra dense δ-Mg(BH4)2 under high pressure. Its volumetric hydrogen content at ambient pressure (147 g/cm(3)) exceeds twice of DOE's (U.S. Department of Energy) target (70 g/cm(3)) and that of α-Mg(BH4)2 (117 g/cm(3)) by 20%. In this study, the experimentally proposed P4(2)nm structure of δ-phase has been found to be dynamically unstable. A new Fddd structure has been reported as a good candidate of δ-phase instead. Its enthalpy from 0 to 12 GPa is much lower than P4(2)nm structure and the simulated X-ray diffraction spectrum is in satisfied agreement with previous experiments. In addition, the previously proposed P-3m1 structure, which is denser than Fddd, is found to be a candidate of ε-phase due to the agreement of Raman shifts.

Determination of theFdddPhase Boundary in Polystyrene-block-polyisoprene Diblock Copolymer Melts

We determined the phase boundary of the Fddd phase in the phase diagram of polystyrene-block-polyisoprene (SI) diblock copolymer by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). We prepared eight SI diblock copolymers having different volume fractions of polyisoprene, fPI, in the range of 0.627 ≤ fPI ≤ 0.653 by living anionic polymerization. At the lowest fPI, 0.627, SI diblock copolymer showed only the disorder−lamellar (L) transition without any order−order transitions. SI diblock copolymer having the highest fPI, 0.653, exhibited disorder−gyroid (G)−L transitions with decreasing temperature but did not have the Fddd structure. Except for two samples described above, SI diblock copolymers exhibited disorder−G−Fddd−L transitions with decreasing temperature. It was found that the location of the Fddd region agreed with those obtained by using self-consistent-field theory calculation and Ginzburg−Landau theory.

Stability of the Fddd Phase in Diblock Copolymer Melts

The stability of the Fddd phase as an equilibrium phase in diblock copolymer melts was examined by using small-angle X-ray scattering and transmission electron microscopy. After 2 days of annealing at 150 °C where Fddd was found in a previous study (Takenaka et al. Macromolecules 2007, 40, 4399) for the poly(styrene-block-isoprene) (S−I), the Fddd structure still survived. The thermoreversibility in the order−order transitions (OOTs) between lamella (L) and Fddd and between gyroid (G) and Fddd was investigated. The long-time annealing at 150 °C induced the transformation from L and G to Fddd, indicating that Fddd is more stable than L and G at 150 °C. Fddd transformed into L and G, respectively, by annealing at 130 and 170 °C. These results supported that the OOTs between L and Fddd and between G and Fddd are thermoreversible. The stability of the Fddd structure after 2 days of annealing and the confirmation of the thermoreversibility in OOTs clarified that the Fddd phase exists as an equilibrium phase in S−I diblock copolymer melts.

Fluctuation effects and the stability of the Fddd network phase in diblock copolymer melts

We examine the effect of composition fluctuations on the stability of the orthorhombic Fddd network phase in the diblock copolymer melt phase diagram within the self-consistent Hartree approximation to the Landau-Brazovskii theory. For weak fluctuations, the Fddd structure is an equilibrium phase; however, stronger fluctuations render this phase metastable. These results suggest a reinterpretation of a recent experiment beyond mean-field theory. Fluctuations may also explain why an equilibrium Fddd phase is not generally observed in analogous self-assembling systems.

Landau theory of the orthorhombicFdddphase

Numerical self-consistent-field theory calculations by Tyler and Morse [Phys. Rev. Lett. 94, 208302 (2005)] predict a stable orthorhombic network phase with space group in very weakly segregated diblock copolymer melts. Here, we examine the predicted stability of this phase within a simple Landau theory of weakly ordered crystals, and within a straightforward extension of Leibler's theory of weakly segregated diblock copolymer melts. An Fddd structure with a ratio of unit cell parameters (a:b:c)=(1:2:2 square root 3) is found to compete very closely with the hexagonal (H) and lamellar (L) phases along the predicted H-L phase boundary, and to be stable within a very narrow range of parameters around this metastable boundary.

Structural modulation of spinel ZnCr2Se4 in the vicinity of antiferromagnetic phase transition induced by an applied magnetic field

AbstractMagnetic and structural phase transitions of normal‐type spinel ZnCr2Se4 have been studied as functions of applied magnetic field and temperature by means of neutron diffraction. It is found that the spiral spin order of Cr3+ ions is largely affected by the magnetic field, and the satellite‐like magnetic reflections are largely decreased in the magnetic field of 0.6 T &lt; H &lt; 1.2 T and disappear at H ≥ 1.2 T, when the magnetic field was applied vertically to an orthorhombic axis [001]. This means that the spiral configuration of the ferromagnetic CrSe4 chains is perfectly disordered around two‐fold screw axes in an orthorhombic symmetry phase (Fddd structure) by the magnetic field H ≥ 1.2 T, though the structural phase transition (TC) between cubic and orthorhombic symmetry phases simultaneously occurs at the antiferromagnetic phase transition temperature (TN ≈︂ 21 K) in no magnetic field. It is also found that the main reflections show a magnetic field dependence similar to that of the magnetic reflections. Pseudo‐tetragonal displacements of Se ions below TN are modulated by the applied magnetic field. The 3 T magnetic field induces a metastable structural transition at about 15 K, after initially cooling down to about 2 K in no applied magnetic field. The transition is related to the modulation from pseudo‐tetragonal to pseudo‐cubic displacements of Se ions.