Structural description of the macroscopic piezo- and ferroelectric properties of lead zirconate titanate.

An in situ structural description of the origin of the ferroelectric properties as a function of the applied electric field E was obtained by synchrotron x-ray diffraction. A setup was used to average the effects of the preferred orientation induced by the strong piezoelectric strain and solve in situ the crystal structure as a function of the applied electric field. Hence, we were able to describe the microscopic origin of the macroscopic ferro- and piezoelectric properties of the most widely used ferroelectric material, lead zirconate titanate.

The role of oxygen stoichiometry on phase stability, structure, and magnetic properties of Sr2CoIrO(6-δ).

The phase stability, crystal structure, and magnetic properties of perovskite-like nonstoichiometric Sr(2)CoIrO(6-δ) were studied. Oxygen deficiency can be well controlled and reversibly varied up to δ = 0.33. A single phase exists at least for partial oxygen pressures between 10(-5) and 1 bar at 1273 K, followed by phase decomposition at higher temperature with the elimination of metallic Ir and the formation of a new phase with approximately Sr(3)CoIrO(6) composition crystallizing in K(4)CdCl(6) structure type. The structural features of Sr(2)CoIrO(6-δ) are dependent on both temperature and oxygen content and were determined by synchrotron and neutron powder diffraction. Both the increasing amount of oxygen vacancies at constant temperature and increasing temperature at constant oxygen content result in the same higher crystal symmetry of Sr(2)CoIrO(6-δ): (1) The oxygen-stoichiometric phase Sr(2)CoIrO(6.00) is monoclinic (I2/m or P2(1)/n) at room temperature but cubic (Fm-3m) for Sr(2)CoIrO(5.67). (2) A sequence of phase transitions [Formula: see text] was observed for Sr(2)CoIrO(6.00) in air. All Sr(2)CoIrO(6-δ) compositions show weak ferromagnetism at low temperature with a canted but predominantly antiferromagnetic ground state. The magnetic ordering temperature decreases monotonously with increasing oxygen deficiency, while pronounced extrema are observed for the paramagnetic moment and the Curie-Weiss temperature at an oxygen deficiency δ ≈ 0.10, which corresponds to the P2(1)/n ↔ I2/m phase transformation.

Polymorphism of K(2)Co(2)Mo(3)O(12): variations in the packing schemes and changes in molybdenum coordination under high pressure.

Within systematic studies on the K-Co-Mo-O system so-called high-temperature and high-pressure modifications of K2Co2Mo3O12 were found. The Birch-Murnaghan fits for the ambient-conditions modification alpha (Z = 4) and the high pressure phase II (Z = 8) lead to V0 = 1193.09 (4) A3, K =30.8 (8) GPa, K'0= 5.4 (4) and V0 = 2170 (10) A3, K =51 (2) GPa with K'0 fixed at 4.0, respectively. The high-pressure phase transition is denoted as pseudoreconstructive [Wiesmann et al. (1997). J. Solid State Chem. 132, 88-97], because some [MoO4] tetrahedra turn into edge-sharing pairs of [MoO5] pyramids or face-sharing pairs of [MoO6] octahedra. The new phases are presented and compared with the alpha phase.

Anomalous thermal expansion in rare-earth gallium perovskites: a comprehensive powder diffraction study.

Crystal structures of rare-earth gallium perovskites LaGaO(3), PrGaO(3), NdGaO(3) and Pr(1-x)Nd(x)GaO(3) (x = 0.25, 0.50, 0.75) solid solutions were investigated in the temperature range 12-300 K by high-resolution powder diffraction using synchrotron or neutron radiation. The previously reported negative thermal expansion in the b direction of the PrGaO(3) lattice has been found to be persistent in Pr(1-x)Nd(x)GaO(3) solid solutions and its magnitude has been revealed as proportional to the amount of praseodymium. Evaluation of the obtained temperature evolution of cell dimensions indicated a weak anomalous behaviour of the b lattice parameter in NdGaO(3), and its origin is supposed to be the same as in PrGaO(3), i.e. a coupling of the crystal electric field levels with phonon excitations of about 23-25 meV energy. The performed bond length analysis revealed an anomalous behaviour of both LnO(12) (Ln-rare-earth) and GaO(6) coordination polyhedra, which can be a structural manifestation of anomalous thermal expansion in the considered compounds.

Magnetic structure of the kagome mixed compound (Co(0.5)Ni(0.5))(3)V(2)O(8).

We report the magnetic structure of (Co(0.5)Ni(0.5))(3)V(2)O(8) (CNVO) deduced by single crystal neutron diffraction. This compound exhibits features which differ from that of its parent compounds, which are absolutely collinear along the a axis for Co(3)V(2)O(8) (CVO) or exhibit magnetic moments predominantly in the a-b plane with small components along c in the case of Ni(3)V(2)O(8) (NVO). The averaged magnetic moments of the statistically distributed Ni(2+) and Co(2+) ions in CNVO are oriented in the a-c plane and form loops of quasiferromagnetically coupled spins. These loops are connected along the a axis and separated along the c axis by cross-tie spins forming a quasiferromagnetic wave with the upper part of the respective neighbouring loops. The magnetic moments are sinusoidally modulated by the propagation vector k = (0.49,0,0) with an average amplitude of 1.59(1) µ(B) for a magnetic ion on a cross-tie site and 1.60(1) µ(B) for the spine site. In addition to neutron diffraction, specific heat and magnetization data, which confirm that the only magnetic phase transition above 1.8 K is the onset of antiferromagnetic order at T(N) = 7.4(1) K, are presented.

Lattice vibrations in an α- and ß-AgCuS superionic conductor: experimental time-of-flight inelastic neutron scattering studies.

Inelastic neutron scattering (INS) measurements of a powder specimen of cation conducting AgCuS have been performed in its ß-phase at 298 and 348 K and in its α-phase at 398 K by neutron time-of-flight (TOF) scattering. The neutron-weighted phonon density of states, G(ε), of the superionic and non-superionic phases of AgCuS has been obtained. G(ε) reveals a non-Debye behaviour at low energy that is caused by the presence of low-energy modes at 2.6 meV observed in the dynamic structure factor. The origin of this mode is not clear. The [Formula: see text] phase transition at 366 K is accompanied by considerable softening of several modes and simultaneous broadening of all peaks that result in a large increase of quasi-elastic intensity. Thermodynamic variables have been derived on the basis of G(ε).

Correction: Composition-dependent charge transfer and phase separation in the V1-xRexO2 solid solution.

Correction for 'Composition-dependent charge transfer and phase separation in the V1-xRexO2 solid solution' by D. Mikhailova, et al., Dalton Trans., 2017, 46, 1606-1617.

Composition-dependent charge transfer and phase separation in the V1-xRexO2 solid solution.

The substitution of vanadium in vanadium dioxide VO2 influences the critical temperatures of structural and metal-to-insulator transitions in different ways depending on the valence of the dopant. Rhenium adopts valence states between +4 and +7 in an octahedral oxygen surrounding and is particularly interesting in this context. Structural investigation of V1-xRexO2 solid solutions (0.01 ≤ x ≤ 0.30) between 80 and 1200 K using synchrotron X-ray powder diffraction revealed only two polymorphs that resemble VO2: the low-temperature monoclinic MoO2-type form (space group P21/c), and the tetragonal rutile-like form (space group P42/mnm). However, for compositions with 0.03 < x ≤ 0.15 a phase separation in the solid solution was observed below 1000 K upon cooling down from 1200 K, giving rise to two isostructural phases with slightly different lattice parameters. This is reflected in the appearance of two metal-to-insulator transition temperatures detected by magnetization and specific heat measurements. Comprehensive X-ray photoelectron spectroscopy studies showed that an increased amount of Re leads to a change in the Re valence state from solely Re6+ at a low doping level (≤3 at% Re) via mixed-valence states Re4+/Re6+ for at least 0.03 < x ≤ 0.10, up to nearly pure Re4+ in V0.70Re0.30O2. Thus, compositions V1-xRexO2 with only one valence state of Re in the material (Re6+ or Re4+) can be obtained as a single phase, while intermediate compositions are subjected to a phase separation, presumably due to different valence states of Re.

The crystal structure of Tm5Re2O12.

The crystal structure of Tm(5)Re(2)O(12), pentathulium dirhenium dodecaoxide, was determined by synchrotron diffraction on a reticular merohedral twin, revealing space group C2/m with a = 12.3717 (7), b = 5.6744 (3), c = 7.4805 (4) Å, beta = 107.816 (2) degrees and Z = 2. Distorted ReO(6) octahedra form chains with alternating rhenium-rhenium distances of 2.455 (1) and 3.219 (1) Å. Early reports on Ln(2)ReO(5) compounds are critically reviewed in the light of our results for Tm(5)Re(2)O(12).

Experimental charge density of hematite in its magnetic low temperature and high temperature phases.

Structural parameters of hematite (α-Fe(2)O(3)), including the valence electron distribution, were investigated using convergent beam electron diffraction (CBED) in the canted antiferromagnetic phase at room temperature and in the collinear antiferromagnetic phase at 90K. The refined charge density maps are interpreted as a direct result of electron-electron interaction in a correlated system. A negative deformation density was observed as a consequence of closed shell interaction. Positive deformation densities are interpreted as a shift of electron density to antibinding molecular orbitals. Following this interpretation, the collinear antiferromagnetic phase shows the characteristic of a Mott-Hubbard type insulator whereas the high temperature canted antiferromagnetic phase shows the characteristic of a charge transfer insulator. The break of the threefold symmetry in the canted antiferromagnetic phase was correlated to the presence of oxygen-oxygen bonding, which is caused by a shift of spin polarized charge density from iron 3d-orbitals to the oxygen ions. We propose a triangular magnetic coupling in the oxygen planes causing a frustrated triangular spin arrangement with all spins lying in the oxygen planes. This frustrated arrangement polarizes the super-exchange between iron ions and causes the spins located at the iron ions to orient in the same plane, perpendicular to the threefold axis.

Tunable Nanostructures and Crystal Structures in Titanium Oxide Films.

Controllable nanostructures in spin coated titanium oxide (TiO(2)) films have been achieved by a very simple means, through change of post deposition annealing temperature. Electron beam imaging and reciprocal space analysis revealed as-deposited TiO(2) films to be characterized by a dominant anatase phase which converts to the rutile form at 600 degrees C and reverts to the anatase modification at 1,200 degrees C. The phase changes are also accompanied by changes in the film microstructure: from regular nanoparticles (as-deposited) to nanowires (600 degrees C) and finally to dendrite like shapes at 1,200 degrees C. Photoluminescence studies, Raman spectral results, and X-ray diffraction data also furnish evidence in support of the observed solid state phase transformations in TiO(2).