On the microscopic mechanism behind the purely orientational disorder-disorder transition in the plastic phase of 1-chloroadamantane.

Globular molecules of 1-chloroadamantane form a plastic phase in which the molecules rotate in a restrained way, but with their centers of mass forming a crystalline ordered lattice. Plastic phases can be regarded as test cases for the study of disordered phases since, contrary to what happens in the liquid phase, there is a lack of stochastic translational degrees of freedom. When the temperature is increased, a hump in the specific heat curve is observed indicating a change in the energetic footprint of the dynamics of the molecules. This change takes place without a change in the symmetry of the crystalline lattice, i.e. no first-order transition is observed between temperatures below and above the calorimetric hump. This implies that subtle changes in the dynamics of the disordered plastic phase concerning purely orientational degrees of freedom should appear at the thermodynamic anomaly. Accordingly, we describe, for the first time, the microscopic mechanisms behind a disorder-disorder transition through the analysis of neutron diffraction and QENS experiments. The results evince a change in the molecular rotational dynamics accompanied by a continuous change in density.

Structural study of SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskite oxides by symmetry-adapted mode analysis.

Powder SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskites were synthesized by the solid-state reaction method, and their crystal structure was investigated by Rietveld analysis (using the symmetry-mode procedure) by x-ray, synchrotron and neutron powder diffraction data. SrPrMRuO6 materials are monoclinic perovskites at room temperature, adopting the space group P21/n, [Formula: see text], c ≈ 2ap, Z = 2. The unit cell parameters increase through the series as the B-cation size increases. The tilting of the octahedra is associated with irreducible representations GM(+)4 and X(+)3. In the studied series, Sr(2+)/Pr(3+) are in eight-fold coordination and are displaced from the center of the Sr/PrO8 polyhedron along [010] by the X(+)5(A10) mode. The size of the first coordination sphere of Sr/Pr increases and the second coordination sphere reduces with temperature, suggesting an actual change in coordination. While changing the interatomic distances, the distortion of the structure diminishes, as observed in distortion mode amplitudes and octahedral tilt angles. The temperature driven [Formula: see text] phase transition was observed for Ni and Mg materials at ≈1025 K and 950 K, respectively. Tilting of the octahedra in the trigonal phase is associated with GM(+)4, which is the unique active mode associated with the not experimentally observed [Formula: see text] phase transition.

Differences in first neighbor orientation behind the anomalies in the low and high density trans-1,2-dichloroethene liquid.

Trans-1,2-dichloroethene (HClC=CClH) has several structural and dynamic anomalies between its low- and high-density liquid, previously found through neutron scattering experiments. To explain the microscopic origin of the differences found in those experiments, a series of molecular dynamics simulations were performed. The analysis of molecular short-range order shows that the number of molecules in the first neighbor shell is 12 for the high-density liquid and 11 for the low-density one. It also shows that the angular position of the center of mass of the first neighbor is roughly the same although the molecular orientation is not. In both liquids the first neighbor and its reference molecule arrange mainly in two configurations, each being the most probable in one of the liquids. First neighbors in the configuration that predominates in the high-density liquid tend to locate themselves closer to the reference molecule, an evidence that they are more strongly bonded. This arrangement facilitates a better packing of the rest of molecules in the first neighbor shell so that on average an additional molecule can be included, and is proposed to be the key in the explanation of all the observed anomalies in the characteristics of both liquids.

Thermal expansion of layered cobaltites Y(Ba1 - xSrx)Co2O5 + δ with x = 0, 0.05 and 0.10.

The crystal structure and thermal expansion of Sr-doped layered cobaltites Y(Ba(1 - x)Sr(x))Co(2)O(5 + δ) (x = 0, 0.05 and 0.10) were studied by means of in situ neutron thermodiffraction in the temperature range 20 K ≤ T ≤ 570 K. The evolution with temperature of lattice parameters for the phases which crystallize in this system is presented, as well as their dependence on the oxygen non-stoichiometry δ. Each phase's volume has been fitted using available models based on the Grüneisen approximation to the zero-pressure equation of state and using the Ruffa model based on the Morse potential, both using a Debye model for the internal energy. The coefficient of volumetric thermal expansion, Debye temperature and other thermodynamic parameters are presented and compared with other perovskite compounds.

Neutron scattering and ab initio molecular dynamics study of cross-linking in biomedical phosphate glasses.

Details of the microscopic structure of phosphate glasses destined for biomedical applications, which include sodium, magnesium and calcium cations, have been obtained from the static structure factor measured by means of neutron scattering. A complementary, molecular dynamics study has been performed on a range of phosphate glasses using density functional theory methods, which allow structural fluctuations, including bond breaking, in the liquid phase before quenching to the glass phase. Good agreement between experiment and simulation allows the molecular dynamics trajectories to be analysed in detail. In particular, attention is focused on the cross-linking of divalent cations in contrast with the structural aspects associated with monovalent cations. Magnesium cations are found equidistant and bridging between the phosphorus atoms of different phosphate chains, leading to a shorter phosphorus-phosphorus second neighbour distance (that is, a more compact packing of neighbouring phosphate chains) compared to the effect of sodium cations. Calcium cations show behaviour intermediate between those of magnesium and sodium. Molecular dynamics simulations give access to the cation mobility, which is lowest for magnesium, reflecting its structural, cross-linking role.

High-temperature behavior of Sr-doped layered cobaltites Y(Ba(1-x)Sr(x))Co2O5.5: phase stability and structural properties.

In this paper we present a neutron diffraction in situ study of the thermal evolution and high-temperature structure of layered cobaltites Y (Ba, Sr)Co(2)O(5 + δ). Neutron thermodiffractograms and magnetic susceptibility measurements are reported in the temperature range 20 K ≤ T ≤ 570 K, as well as high-resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with δ≈0.5, we show that the room temperature phases remain stable up to 550 K, where they start losing oxygen and transform to a vacancy-disordered structure with tetragonal symmetry. Our results also show how the so-called '122' structure can be stabilized at high temperature (around 450 K) in a sample in which the addition of Sr at the Ba site had suppressed its formation. In addition, we present the structural and magnetic properties of the resulting samples with a new oxygen content δ≈0.25 in the temperature range 20 K ≤ T ≤ 300 K.

Structure of liquid n-hexane.

The structure of deuterated liquid n-hexane has been investigated at room temperature by neutron diffraction and molecular dynamics simulations. By carrying out a careful analysis of the measurements, experimental data were obtained in very good agreement with the simulated data. This allowed a thorough analysis of the simulation results aiming at the evaluation of the partial, intra-, and intermolecular components of the n-hexane structure. We finally compare the intramolecular differential cross sections calculated from the most probable n-hexane molecular configurations with the measured and simulated data.

Enhancing spin state phase separation by strontium addition in the Y (Ba(1-x)Sr(x))Co(2)O(5.5) layered cobaltite.

In this paper we present a neutron diffraction study of the effects of replacing Ba with smaller Sr cations in the layered cobaltite YBaCo(2)O(5+δ) for δ≈0.5. Neutron thermodiffractograms are reported in the temperature range 20 K≤T≤300 K, as well as high-resolution neutron diffraction experiments at selected temperatures. The systematic refinement of the series of samples with Sr substituting at the Ba site permits the understanding of the complex magnetic behaviour of this system, and reinforces a two-phase scenario for the parent compound. We have found that Sr addition destabilizes the ferrimagnetic spin state ordered phase, as well as the '122' order characteristic of cobaltites with δ = 0.5, favouring instead a tetragonal and less ordered structure.

On the atomic structure of liquid Ni-Si alloys: a neutron diffraction study.

The atomic structure of the liquid NiSi and NiSi(2) alloys is investigated by means of neutron diffraction experiments with isotopic substitution. From experimental data-sets obtained using four Ni isotopes, partial structure factors and pair correlation functions are obtained by applying a reverse Monte Carlo modelling approach. Both alloys were found to exhibit a strong tendency to hetero-coordination within the first coordination shell. In particular, covalent Si-Si bonds with somewhat greater distances seem to influence the structure of the liquid NiSi alloy.

Structure of molten yttrium aluminates: a neutron diffraction study.

We used the aerodynamic levitation technique combined with CO2 laser heating to study the structure of liquid yttrium aluminates above their melting point with neutron diffraction. For various yttria contents, we determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. In particular, we derived Al-O and Y-O bond distances and coordination numbers. Experimental data are compared with ab initio molecular dynamics, carried out using the VASP code where the interatomic forces are obtained from density functional theory. In particular, partial pair correlation functions have been calculated and are in relatively good agreement with the experimental observations.

Direct experimental assessment of the strength of orientational correlations in polar liquids.

The strength of molecular orientational correlations in polar liquids is assessed by means of comparison of the diffuse scattering patterns of a liquid composed by molecules devoid of permanent electric dipole but having a weak quadrupole moment and those for a liquid composed by permanent molecular dipoles. The extent of orientational correlations within the liquid phases is in both cases assessed by comparison of the liquid radial distributions to those present in the rotator-phase (plastic) crystal phases of both compounds. For such disordered-crystal phases, information concerning orientational correlations is directly derived from the experimental scattering patterns by means of analysis of the diffuse scattering background present beneath the Bragg peaks. The results show that rather than long-ranged, orientational correlations in polar or polarizable liquids are confined within distances comprising the second coordination sphere.

Chemical isomerism as a key to explore free-energy landscapes in disordered matter.

The effects of a minor chemical modification on the microscopic structure of a material in its glass and crystal phases are investigated by the concurrent use of neutron diffraction and computer simulation. Significant changes in short-, intermediate-, and long-range order are found, resulting from the change in molecular structure. These differences are explainable by a shift in the balance between directional and excluded-volume interactions.