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Abstract: The high pressure stability of α-Cu3V2O8 has been investigated via complementary high pressure synchrotron X-ray diffraction experiments and theoretical density functional theory calculations. The results of both experiment and theory are in close agreement. The main result of this work is that α-Cu3V2O8 undergoes a pressure-induced chemical decomposition into CuO and V2O5 at a modest pressure of ∼1.35 GPa according to the experimental observations, and at ∼2.45 GPa according to the calculations. The decomposition is investigated with enthalpy calculations and one of the main driving factors is the stability of the octhedral oxygen-coordination of the metal atoms in the decompositon products. Additionally, we determine the bulk modulus of α-Cu3V2O8 to be the lowest of all known vanadates, at 52(2) GPa, due to its crystal structure, and determine the corresponding isothermal compressibility tensor.

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Abstract: The tensile fractured surfaces of ZrTi-based bulk metallic glass and composite samples were studied using synchrotron X-ray total scattering. The scanned areas contain different shear bands densities. The shear bands create localized atomic strains, which in turn cause more ordered atomic structures. Such structural changes were reflected in the scattering structure factor, i.e. the higher the density of the shear bands, the higher the scattering structure factor. Similar phenomenon was also found in the metallic glass composite.

Open Access

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Abstract: We report an in-situ synchrotron X-ray diffraction study of K0.5Bi0.5TiO3-BiFeO3-PbTiO3 ceramics, which exhibit a Tc of around 450 °C. The electromechanical actuation mechanisms comprise contributions from coexisting tetragonal and rhombohedral phases. The tetragonal {200} grain family exhibited the highest effective lattice strain, up to 8.2 ×10-3 at 5 kV/mm. Strong strain anisotropy in the tetragonal phase and field-induced intergranular stresses facilitate a partial transformation from tetragonal (high strain anisotropy) to rhombohedral (low strain anisotropy) at high electric field levels, with an average linear transformation strain of -1.54×10-3. The domain switching behavior was effectively enhanced in both tetragonal and rhombohedral phases after the phase transformation, due to the release of intergranular stress. This observed self-adapting mechanism in tuning intergranular stress through partial phase switching in the morphotropic KBT-BF-PT composition with large lattice distortion could also be exploited in other perovskite systems in order to achieve high performance high temperature piezoelectric ceramics.

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Abstract: In this work we present the study of rare earth molybdates RE=(Eu, Tb and Ho) with formula RE2(MoO4)3. Under ambient conditions, Eu- and Tb-containing compounds can be found in the phases α and β0 , while holmium-containing can be found in the phase γ and β0 under these conditions. In the study of the β0 -Tb2(MoO4)3 compression by the CCDD group, the hypothesis of a transition to a new phase, called the phase δ, was considered. Subsequently, in the study of Y2(MoO4)3 synthesised unconventional conditions, non-stoichiometric oxide and molybdate phases with dif fractograms very similar to those of the phase δ were obtained. This opened the door to another possible hypothesis about the β0 → δ transition, that it was a decomposition induced by high pressure. For its verification, the synt hesis of the named compounds was carried out, modifying the solid state synthesis, applying a pressure of 0.66 GPa for compacting powder samples and increasing or decreasing the synthesis temperature, which was different for each case. We carry out a study of the crystalline structures of the most relevant phases involved: chelite-α, β-β0 and γ. Additionally, symmetry relationships provide clarity in understanding the phase transitions. A routine diffractogram was performed for each compound synthesised by using the X-ray diffractometer available at SIDIX (X-ray Facilitiy of the La Laguna Univer sity). Diffraction data collected under pressure were provided by the CCDD (group with which the supervisors of this work are researching). Note that, in addition to the β0 -Tb2(MoO4)3 data, β0 -phase of Ho and Eu molybdates data were also available. Using the ICSD database we simulated the different phases that would be expec ted to be found, and with which a visual identification of the phases was performed. Applying the Le Bail refinement method, the intensities of the full profile were refined as a verification of the existence of the expected phases. The synthesis of europium molybdate was carried out at 500oC, 550oC and 600oC. After the analysis and refinements, different mixtures of phases with struc tural types Sm2O3, MoO3, Eu4Mo7O27, Eu2Mo4O15 and the phase α-Eu2(MoO4)3 were detected. Furthermore, by analysing the pure β0 -Eu2(MoO4)3 phase under pres sure, it was observed that around 2.23 GPa a phase transition occurs, interpreted as a decomposition into the β0 , Eu2O3 and Eu2Mo4O15 phases, while at 5 GPa an amorphisation undergoes. Holmium oxide, β0 -Ho2(MoO4)3, nonstoichiometric Y2Mo4O15 phases were iden tified for the holmium molybdated synthetised at 600oC. Under pressure, as in the case of the europium molybdate, a phase transition occurs around 2.3 GPa in which the β0 -phase, the europium oxide and the Y2Mo4O15 phase are involved and the non-reversible amorphous phase starts at around 5 GPa. In these cases as well as the one studied by the CCDD group on Tb molybdate, when the decompression is carried out, the initial β0 -RE2(MoO4)3 phase is not com pletely recovered, so the phase transition is not reversible, which leads us to think that it is a decomposition induced by pressure. In addition to the experimental work and the analysis and discussion of the results, we would like to highlight the literature review carried out, which is a very important part of this dissertation. On the one hand, the research was contextualised, its interest was explained and an exhaustive description of the crystal structures of the materials studied was made. It was also necessary to: 1) review and introduce some crystallographic terms that were later used in the description of these structures. 2) Describe the experimental techniques used, reviewing their physical foundations. 3) Explain the tools used in the analysis of the data.

Open Access

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Abstract: Localized fluctuation in residual stress distribution for direct energy deposited 316L stainless steel thin plate was revealed through high-energy synchrotron X-ray diffraction. The macroscopic residual stress levels are changed steadily across the article according to the characteristic cooling rates, as implied by the measured dendrite arm lengths. Localized fluctuations of residual stress in the length scale, between 500 to 600 µm, coincides well with the morphological variations of the solidification microstructure that formed during direct energy deposition. Computations indicate a greater propensity for related mesoscopic thermal gradient fluctuations along the solidification front of the moving melt pool. Novel perspectives on residual stress across multiple length-scales and its relevance to the formation of solidification microstructure in metal additive manufacturing is analysed by considering the experimental results.