Crystallization of polarons through charge and spin ordering transitions in 1T-TaS2.

The interaction of electrons with the lattice in metals can lead to reduction of their kinetic energy to the point where they may form heavy, dressed quasiparticles-polarons. Unfortunately, polaronic lattice distortions are difficult to distinguish from more conventional charge- and spin-ordering phenomena at low temperatures. Here we present a study of local symmetry breaking of the lattice structure on the picosecond timescale in the prototype layered dichalcogenide Mott insulator 1T-TaS2 using X-ray pair-distribution function measurements. We clearly identify symmetry-breaking polaronic lattice distortions at temperatures well above the ordered phases, and record the evolution of broken symmetry states from 915 K to 15 K. The data imply that charge ordering is driven by polaron crystallization into a Wigner crystal-like state, rather than Fermi surface nesting or conventional electron-phonon coupling. At intermediate temperatures the local lattice distortions are found to be consistent with a quantum spin liquid state.

What Is the Actual Local Crystalline Structure of Uranium Dioxide, UO2? A New Perspective for the Most Used Nuclear Fuel.

Up to now, uranium dioxide, the most used nuclear fuel, was said to have a Fm3̅m crystalline structure from 30 to 3000 K, and its behavior was modeled under this assumption. However, recently X-ray diffraction experiments provided atomic pair-distribution functions of UO2, in which UO distance was shorter than the expected value for the Fm3̅m space group. Here we show neutron diffraction results that confirm this shorter UO bond, and we also modeled the corresponding pair-distribution function showing that UO2 has a local Pa3̅ symmetry. The existence of a local lower symmetry in UO2 could explain some unexpected properties of UO2 that would justify UO2 modeling to be reassessed. It also deserves more study from an academic point of view because of its good thermoelectric properties that may originate from its particular crystalline structure.

Structural Changes in the Local Environment of Uranium Atoms in the Three Phases of U4O9.

The crystal structure of U4O9 remains an enigma because of its differences with U(4+) and U(5+) coordination polyhedral mixtures, as shown in the XANES experimental results. To better understand this crystal structure, its diffraction pattern was measured at seven different temperatures using neutron diffraction before being independently refined by Rietveld's method and pair distribution function analysis. The O cuboctahedron-a structural element consisting of 13 oxygen atoms-is a specific feature of the U4O9 crystal structure. The volume of the cuboctahedron decreases when the temperature increases, whereas the overall volume of the crystal cell increases. This feature can be correlated with the two U4O9 phase transitions that induce sharp changes in the cuboctahedron geometry, suggesting that this structural element has internal dynamics. In particular, these structural modifications in the γ phase suggest that the high-temperature phase can be described as a mixture of U(4+) and U(5+) coordination polyhedra, the latter having U-O distances shorter than 2.2 Å, that are absent in the former. These changes in uranium polyhedra as a function of temperature are tentatively interpreted using steric arguments. They also raise the question of charge localization on the different U ion sites in the low-temperature phases of U4O9.

Density functional theory calculations of UO2 oxidation: evolution of UO(2+x), U4O(9-y), U3O7, and U3O8.

Formation of hyperstoichiometric uranium dioxide, UO2+x, derived from the fluorite structure was investigated by means of density functional theory (DFT) calculations. Oxidation was modeled by adding oxygen atoms to UO2 fluorite supercells. For each compound ab initio molecular dynamics simulations were performed to allow the ions to optimize their local geometry. A similar approach was used for studying the reduction of U3O8. In agreement with the experimental phase diagram we identify stable line compounds at the U4O9-y and U3O7 stoichiometries. Although the transition from fluorite to the layered U3O8 structure occurs at U3O7 (UO2.333) or U3O7.333 (UO2.444), our calculated low temperature phase diagram indicates that the fluorite derived compounds are favored up to UO2.5, that is, as long as the charge-compensation for adding oxygen atoms occurs via formation of U(5+) ions, after which the U3O8-y phase becomes more stable. The most stable fluorite UO2+x phases at low temperature (0 K) are based on ordering of split quad-interstitial oxygen clusters. Most existing crystallographic models of U4O9 and U3O7, however, apply the cuboctahedral cluster. To better understand these discrepancies, the new structural models are analyzed in terms of existing neutron diffraction data. DFT calculations were also performed on the experimental cuboctahedral based U4O9-y structure, which enable comparisons between the properties of this phase with the quad-interstitial ones in detail.

Role of antisite disorder on preamorphization swelling in titanate pyrochlores.

Ion irradiation experiments and atomistic simulations were used to demonstrate that irradiation-induced lattice swelling in a complex oxide, Lu2Ti2O7, is due initially to the formation of cation antisite defects. X-ray diffraction revealed that cation antisite formation correlates directly with lattice swelling and indicates that the volume per antisite pair is approximately 12 Å3. First principles calculations revealed that lattice swelling is best explained by cation antisite defects. Temperature accelerated dynamics simulations indicate that cation Frenkel defects are metastable and decay to form antisite defects.

Refinement of the α-U4O9 crystalline structure: new insight into the U4O9 → U3O8 transformation.

The oxidation reaction of UO(2) into U(3)O(8) is studied as a function of the crystalline distortion of interstitial oxygen clusters, named cuboctahedra, which appear in U(4)O(9) and U(3)O(7) intermediate phases. For that purpose, the refinement of α-U(4)O(9) was performed because this phase undergoes a trigonal distortion from cubic ß-U(4)O(9) when the temperature is decreased. In α-U(4)O(9), the cuboctahedra can be described as crumpled sheets taken from a fragment of U(3)O(8). The manner by which the accumulation of crumpled sheets can lead to the formation of U(3)O(8) is discussed.

Influence of Ca content and oxygen partial pressure on microstructural evolution of (Co,Ca)O at elevated temperatures.

Ca-doped (1, 1.7, 5 and 10 mol% CaO) cobalt oxide single-crystal samples, with an [001] orientation, were annealed at elevated temperatures of 1000-1200 degrees C for different times and at different oxygen partial pressures. The microstructure was examined by means of transmission light and electron microscopy. High-temperature X-ray diffractometry was used, with the aim of determining the temperature of the CoO <--> Co(3)O(4) transition in these materials. Extensive precipitation of Ca-free Co(3)O(4) spinel crystals was observed with increasing Ca content and oxygen activity. It is suggested that the electrical conductivity changes in this material may be related to this precipitation, because it changes the electronic state of cobalt cations.

Neutron diffraction study of the size-induced tetragonal to monoclinic phase transition in zirconia nanocrystals.

Accurate neutron powder diffraction experiments at several temperatures allow one to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. The structure of the tetragonal phase observed in the nanocrystals is identical to that observed in micrometric zirconia above 1400 K. A uniaxial strain depending on grain size is observed. The phase transition occurs above a threshold crystal size. These results are analyzed within the Landau theory and can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.

Single-crystal study of the m = 2 tubular cobalt oxide, Bi(4)Sr(12)Co(8)O(30-delta).

The structure of the m = 2 tubular compound Bi(4)Sr(12)Co(8)O(30-delta), bismuth strontium cobalt oxide, was determined by single-crystal X-ray diffraction. This phase of orthorhombic symmetry exhibits a very strong tetragonal pseudosymmetry. The structure consists of 90 degrees -oriented Bi(2)Sr(2)CoO(6+delta) slices, four Co atoms wide, forming [Sr(4)Co(4)O(13)](infinity) pillars at their intersection. The Co atoms in these pillars form four corner-sharing CoO(5) bipyramids. In the resulting [Co(4)O(13)] cluster, an anionic disorder is evidenced and discussed. Then, an accurate description of the particular structure of the pillars is given. Finally, a comparison with the Mn tubular compound Bi(3.6)Sr(12.4)Mn(8)O(30-delta) is carried out.

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6.

The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic 12/m(alpha0gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) A, beta = 90.047 (5)degrees, q(inc) = 0.9000 (9)a* + 0.1735 (6)c*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.