Local order in fully deuterated liquid N-methylacetamide (C3D7NO) as studied by neutron diffraction and density-functional theory calculations

A structural investigation of fully deuterated liquid N-methylacetmide (NMAd7) is performed at 308 K and atmospheric pressure by using neutron diffraction together with density-functional theory (DFT). The analysis of experimental data yields the total structure factor SM(Q), the molecular form factor F1(Q), and the distinct pair correlation function gL(r). The DFT calculations are performed to study the relative stability of the two possible isomers (trans and cis) and to examine some possible clusters recently published that may describe the intermolecular arrangement in the liquid state. Neutron measurements can be interpreted in terms of trans linear trimer (T1) and cis cyclic trimer (T2) where the total number of hydrogen bonds is respectively equal to two and three. The theoretical structure factors obtained on the basis of intermolecular arrangements agree fairly well with the experimental one beyond Q = 2 A-1. All through the study, a comparison is made with complementary X-ray results.     

Crystal structure of a high‐pressure phase of magnesium chloride hexahydrate determined by in‐situ X‐ray and neutron diffraction methods

A high‐pressure phase of magnesium chloride hexahydrate (MgCl₂·6H₂O‐II) and its deuterated counterpart (MgCl₂·6D₂O‐II) have been identified for the first time by in‐situ single‐crystal X‐ray and powder neutron diffraction. The crystal structure was analyzed by the Rietveld method for the neutron diffraction pattern based on the initial structure determined by single‐crystal X‐ray diffraction. This high‐pressure phase has a similar framework to that in the known ambient‐pressure phase, but exhibits some structural changes with symmetry reduction caused by a subtle modification in the hydrogen‐bond network around the Mg(H₂O)₆ octahedra. These structural features reflect the strain in the high‐pressure phases of MgCl₂ hydrates.     

High density amorphous ices: disordered water towards close packing

The structure of amorphous ice under pressure has been studied by molecular dynamics at 160 K. The starting low-density phase undergoes significant changes as the density increases, and at rho=1.51 g/cm(3) our calculated g(OO)(r) is in excellent agreement with in situ neutron diffraction data obtained at 1.8 GPa and 100 K on very high density amorphous ice made at 150 K. As the system is further compressed, in the theoretical simulations, up to rho=1.90 g/cm(3), the structural modifications are continuous up to the highest density. The analysis of orientational distributions reveals that dense amorphous ice is characterized by major distortions of the tetrahedral geometry, and that the pressure structural changes, already observed experimentally at lower densities, can be interpreted as a trend towards a disordered closed-packed structure.     

Synthesis, structure, and properties of BaAl2Si2

Single crystals of BaAl2Si2 were grown from an Al molten flux and characterized using single-crystal X-ray diffraction at 10 and 90 K and neutron diffraction at room temperature. BaAl2Si2 crystallizes with the alpha-BaCu2S2 structure type (Pnma), is isostructural with alpha-BaAl2Ge2, and is an open 3D framework compound, where Al and Si form a covalent cagelike network with Ba2+ cations residing in the cages. BaAl2Si2 has a unit cell of a=10.070(3) A, b=4.234(1) A, and c=10.866(3) A, as determined by room-temperature single-crystal neutron diffraction (R1=0.0533, wR2=0.1034). The structure as determined by single-crystal neutron and X-ray diffraction (10 and 90 K) indicates that BaAl2Si2 (Pnma) is strictly isostructural to other (alpha)-BaCu2S2-type structures, requiring site specificity for Al and Si. Unlike BaAl2Ge2, no evidence for an alpha to beta (BaZn2P2-type, I4/mmm) phase transition was observed. This compound shows metallic electronic resistivity and Pauli paramagnetic behavior.     

Rigid films of an anionic porphyrin and a dialkyl chain surfactant

The 2D complex formed at the air-water interface between the dialkyl chain cationic surfactant, dihexadecyldimethylammonium bromide, and the anionic porphyrin, tetrakis-(4-sulfonatophenyl) porphine, was studied using surface pressure-area isotherms as well as X-ray and neutron reflection measurements. The surface structure of these films was determined by the use of simultaneously constrained analysis of the neutron and X-ray reflectometry data and BAM images. Isotopic contrast variation methods were employed to enhance the information content of the neutron reflection data. The rigid complex forms at the interface due to the electrostatic interaction between the cationic headgroups of the surfactant and the anionic functional groups at the meso position of the porphyrin. The surface pressure-area isotherms show three distinct regions on compression: an initial condensed phase that ends with a pressure peak at 36 mN m-1, a second plateau region of high compressibility, and a final condensed phase. BAM images show that at the beginning of the plateau region in the isotherm there is complete surface coverage by a monolayer. The constrained simultaneous fitting of neutron and X-ray data measured just prior to and after the pressure peak shows a structurally similar 2D complex at the interface. Modeling of X-ray reflectometry data also reveals that in the final high-pressure phase the film has folded to form a trilayer. The conclusion is that the plateau region of the isotherm is due to the formation of trilayer surface coverage through localized buckling or folding, and that after this is complete there is some condensation before final film collapse.     

Single-crystal X-ray diffraction analysis of pyrene II at 93 K

The crystal and molecular structure of the low temperature phase of the polycyclic aromatic hydrocarbon pyrene has been determined for the first time by single-crystal X-ray diffraction methods. The crystal structure at 93 K is shown to be in excellent agreement with that obtained by the refinement of powder neutron diffraction data for pyrene II obtained at 4.2 K.     

From molecular clusters to liquid structure in AlCl3 and FeCl3

Melting of aluminum and iron trichloride is accompanied by a structural transition from sixfold to fourfold coordination of the trivalent metal ions, and a widely accepted interpretation of the structure of their melts near freezing is that they mainly consist of strongly correlated dimers formed from two edge-sharing tetrahedra. We carry out classical molecular dynamics simulations to examine how a polarizable-ion force law, determined on isolated molecular monomers and dimers in the gaseous phase of these compounds, fares in accounting for the pair structure of their liquid phase and for mean square displacements and diffusion coefficients of the two species in each melt. The model reproduces the main features of the neutron diffraction structure factor, showing peaks due to intermediate range order and to charge and density short-range order, and accounts for the experimental data at a good semi-quantitative level. We find agreement with the neutron and X-ray diffraction data on metal–halogen and Cl–Cl bond lengths in the melt, and demonstrate the high sensitivity of the results for the width of the first-neighbor shell to truncation in obtaining it by Fourier transform of the neutron-weighted structure factor in momentum space. We also report comparisons with a recent first-principles study of the structure of the AlCl₃ melt by the Car–Parrinello method. Finally, we demonstrate break-up of dimers into monomers upon raising the liquid temperature in the case of AlCl₃.     

Down to the quantum limit at the neutral-to-ionic phase transition of (BEDT-TTF)-(ClMe-TCNQ): symmetry analysis and phase diagram

We report on the neutral-to-ionic (N-I) phase transition in the one-dimensional organic complex (BEDT-TTF)-(ClMeTCNQ). The X-ray studies at room temperature show that the neutral phase of (BEDT-TTF)-(ClMeTCNQ) is already characterized by a polar long-range ordering, at variance with other charge-transfer compounds comprising noncentrosymmetric molecules. From a detailed neutron diffraction study of this complex under high pressure, we present the phase diagram of the N-I transition down to the quantum limit. We discuss the symmetry breaking associated with the transition and the evolution of its first-order character under pressure.     

Magnetism and structural chemistry of the n=2 Ruddlesden-Popper phase La3LiMnO7

Polycrystalline samples of the n=2 Ruddlesden-Popper phase La₃LiMnO₇ have been prepared and characterized. X-ray and neutron diffraction suggest that the structure is tetragonal with a disordered distribution of Li and Mn cations over the octahedral sites, but ⁶Li MAS NMR shows that the Li and Mn are 1:1 ordered locally. Electron microscopy shows that the stacking of the cation-ordered, perovskite-like bilayers along the crystallographic z-axis is disordered on the distance scale sampled by X-ray and neutron diffraction. Magnetometry data and neutron diffraction data collected at 2 K together suggest that the Mn cations within each structural domain order antiferromagnetically at 14 K, but that the disorder along z prevents the establishment of long-range magnetic order.     

Structural chemistry of A3CuBO6 (A = Ca, Sr; B = Mn, Ru, Ir) as a function of temperature

Variable temperature X-ray and neutron powder diffraction techniques have been used to identify structural phase transitions in Cu-rich A(3)A'BO(6) phases. A transition from monoclinic to rhombohedral symmetry was observed by X-ray diffraction between 700 and 500 K in Sr(3)Cu(1-x)M(x)IrO(6) (M = Ni, Zn; 0 < or = x < or = 0.5). The temperature of the phase change decreased in a linear manner with Cu-content and was essentially independent of the nature of M. Ca(3.1)Cu(0.9)MnO(6) was shown to pass from a rhombohedral phase to a triclinic phase on cooling below 290 K; the structure of the triclinic phase was refined against neutron diffraction data collected at 2 K. Ca(3.1)Cu(0.9)RuO(6) undergoes a transition between a disordered rhombohedral phase and an ordered monoclinic phase when cooled below 623 K. Neutron diffraction has been used to determine the structure as a function of temperature in the range 523 < or =T/K < or = 723 and hence to determine an order parameter for the low temperature phase; the second-order transition is shown to be incomplete 100 K below the critical temperature.     

Neutron diffraction study of unusual phase separation in the antiperovskite nitride Mn3ZnN

The antiperovskite Mn(3)ZnN is studied by neutron diffraction at temperatures between 50 and 295 K. Mn(3)ZnN crystallizes to form a cubic structure at room temperature (C1 phase). Upon cooling, another cubic structure (C2 phase) appears at around 177 K. Interestingly, the C2 phase disappears below 140 K. The maximum mass concentration of the C2 phase is approximately 85% (at 160 K). The coexistence of C1 and C2 phase in the temperature interval of 140-177 K implies that phase separation occurs. Although the C1 and C2 phases share their composition and lattice symmetry, the C2 phase has a slightly larger lattice parameter (Δa ≈ 0.53%) and a different magnetic structure. The C2 phase is further investigated by neutron diffraction under high-pressure conditions (up to 270 MPa). The results show that the unusual appearance and disappearance of the C2 phase is accompanied by magnetic ordering. Mn(3)ZnN is thus a valuable subject for study of the magneto-lattice effect and phase separation behavior because this is rarely observed in nonoxide materials.     

Neutron powder diffraction study of A2BeF4 (A=K, Rb, Cs): Structure refinement and analysis of background

The crystal structure of potassium, rubidium and caesium fluoroberyllates have been re-examined by neutron powder diffraction at room temperature and at 1.5 K. Previously, their structures, obtained from X-ray data, were described in the Pn2₁a space group. However, the results obtained from Rietveld refinements, using powder neutron diffraction, at both temperatures, indicated that all structures are orthorhombic with space group Pnma. The known phase transition at high temperature is probably related to the appearance of a hexagonal pseudo-symmetry instead of the elimination of the mirror plane between the above mentioned orthorhombic space groups. A possible phase transition, at very low temperature, was discarded considering the stereochemical criteria concerning the structural stability of A₂BX₄ compounds. This was confirmed by thermal analysis. On the other hand, a modulated background has been detected in all samples during the refinements. This is compatible with the presence of an amorphous phase, coexisting with the crystalline phase, or with a disordered component within the main crystalline phase. Instead of using a polynomial function, the background was modelled by Fourier filtering improving the fit for all patterns. The radial distribution function (RDF) was obtained from the analysis of the calculated background and compared with the RDF from the average crystal structure. The advantages of neutron with respect to X-ray diffraction were evidenced for this type of compound with β-K₂SO₄-type structure.     

Topotactic oxidation pathway of ScTiO3 and high-temperature structure evolution of ScTiO3.5 and Sc4Ti3O12-type phases

The novel oxide defect fluorite phase ScTiO(3.5) is formed during the topotactic oxidation of ScTiO(3) bixbyite. We report the oxidation pathway of ScTiO(3) and structure evolution of ScTiO(3.5), Sc(4)Ti(3)O(12), and related scandium-deficient phases as well as high-temperature phase transitions between room temperature and 1300 °Cusing in-situ X-ray diffraction. We provide the first detailed powder neutron diffraction study for ScTiO(3). ScTiO(3) crystallizes in the cubic bixbyite structure in space group Ia3 (206) with a = 9.7099(4) ?. The topotactic oxidation product ScTiO(3.5) crystallizes in an oxide defect fluorite structure in space group Fm3m (225) with a = 4.89199(5) ?. Thermogravimetric and differential thermal analysis experiments combined with in-situ X-ray powder diffraction studies illustrate a complex sequence of a topotactic oxidation pathway, phase segregation, and ion ordering at high temperatures. The optimized bulk synthesis for phase pure ScTiO(3.5) is presented. In contrast to the vanadium-based defect fluorite phases AVO(3.5+x) (A = Sc, In) the novel titanium analogue ScTiO(3.5) is stable over a wide temperature range. Above 950 °C ScTiO(3.5) undergoes decomposition with the final products being Sc(4)Ti(3)O(12) and TiO(2). Simultaneous Rietveld refinements against powder X-ray and neutron diffraction data showed that Sc(4)Ti(3)O(12) also exists in the defect fluorite structure in space group Fm3m (225) with a = 4.90077(4) ?. Sc(4)Ti(3)O(12) undergoes partial reduction in CO/Ar atmosphere to form Sc(4)Ti(3)O(11.69(2)).     

SYNTHESIS AND CHARACTERIZATION OF NOVEL CHIRAL SMECTIC C(Sc*) PHASE SHISH-KEBAB TYPE LIQUID CRYSTALLINE BLOCK COPOLYMERS*

A new series of chiral shish-kebab type liquid crystal block copolymers that form the smecticC(Sc~*) phase was synthesized by solution polycondensation. The copolymers were characterized by GPC.DSC. TG, POM. X-ray diffraction and polarimeter. The copolymers 7 entered into liquid crystal phase whenthey were heated to their melting temperatures (T_m) and the copolymers 8 were in liquid crystal phase at roomtemperature with low viscosities. The smectic sanded texture or focal-conic texture were observed on POM.All the chiral block copolymers showed high optical activity. No racemization has happened. Temperature-variable X-ray diffraction study together with POM and polarimetric analysis realized that they are chiralsmectic C(Sc~*) phase. Thus we offer in this report the first example of shish-kebab type liquid crystal blockcopolymers that form a chiral smectic C(Sc~*) phase. The variation of melting and isotropization temperatureswith molecular structure was also discussed.     

Synthesis, structural transformation, thermal stability, valence state, and magnetic and electronic properties of PbNiO3 with perovskite- and LiNbO3-type structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.     

Pressure-driven phase transitions in NaBH4: theory and experiments

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.     

Endohedral clusterization of ten water molecules into a "molecular ice"within the hydrophobic pocket of a self-assembled cage

An adamantanoid (H2O)10 cluster is formed within the hydrophobic cavity of a self-assembled coordination cage. This cluster is termed "molecular ice" because it is the smallest unit of naturally occurring Ic-type ice. X-ray structural analysis, coupled with neutron diffraction study, reveals that the molecular ice is formed not by a simple space-filling effect but by efficient molecular recognition within the cage via H2O:...pi interaction.     

X-ray diffraction from the uniaxial and biaxial nematic phases of bent-core mesogens

X-ray diffraction patterns for the uniaxial and biaxial nematic phases exhibited by rigid bent-core mesogens were calculated using a simple model for the molecular form factor and a modified Lorentzian structure factor. The X-ray diffraction patterns depend strongly on the extent of the alignment of the molecular axes as well as the orientation of molecular planes. The X-ray diffraction can be unequivocally used to identify the biaxial nematic phase, study the uniaxial-biaxial phase transition, and estimate the order parameters of the nematic phase.     

Polymorphism of dense, hot oxygen

The phase diagram and polymorphism of oxygen at high pressures and temperatures are of great interest to condensed matter and earth science. X-ray diffraction and Raman spectroscopy of oxygen using laser and resistively heated diamond anvil cells reveal that the molecular high-pressure phase ε-O(2), which consists of (O(2))(4) clusters, reversibly transforms in the pressure range of 44 to 90 GPa and temperatures near 1000 K to a new phase with higher symmetry. The data suggest that this new phase (η') is isostructural to a phase η reported previously at lower pressures and temperatures, but differs from it in the P-T range of stability and type of intermolecular association. The melting curve increases monotonically up to the maximum pressures studied (～60 GPa). The structure factor of the fluid measured as a function of pressure to 58 GPa shows continuous changes toward molecular dissociation.     

Notes of the recent structural studies on lead zirconate titanate

Atomic scale structure has a central importance for the understanding of functional properties of ferroelectrics. The X-ray and neutron diffraction studies used for the average symmetry determination of lead zirconate titanate [Pb(ZrxTi(1- x))O3, PZT] ceramics and powders are reviewed. The results obtained through two frequently used local probes, transmission electron microscopy (TEM) combined with electron diffraction (ED) and Raman scattering measurements, are summarized. On the basis of these studies, structural trends as a function of composition x and temperature are outlined. There are two distinguished intrinsic structural features, (i) lead-ion shifts and (ii) local structural distortions related to different B cations and the spatial composition variation of x, which have a pronounced effect on the functional properties of PZT. Particular attention is paid to the morphotropic phase boundary (MPB) compositions for which a large number of different structural models have been proposed. Earlier symmetry considerations show that the monoclinic phase cannot serve as a continuous bridge between tetragonal and rhombohedral phases. This suggests that the two-phase coexistence has an important role for the piezoelectric properties. Near the MPB, the extrinsic contribution to piezoelectricity includes pressure (or electric-field)-induced changes in phase fractions and domain wall motion. It was recently shown that the domain contribution is crucial for the electromechanical properties of PZT in the vicinity of the MPB. The dependence of domain widths on crystal size and shape should also be properly accounted for when TEM/ED measurements complement X-ray and/or neutron diffraction experiments. The structure-piezoelectric property relations are summarized.