Crystal Engineering最新文献

The materials properties of many ABO₃ perovskites result primarily from the B cations, but are tuned by the cation(s) at the A sites. Notable examples are ferroelectric titanates, magnetoresistive manganites, and layered A₂BO₄ type superconducting cuprates. The A cation control of these properties can be described through a simple ionic approach by considering the sizes and charges of the A cations. The size control is approximated well by the mean A cation radius, equivalent to the traditional perovskite tolerance factor, and the size variance which describes the mismatch in ionic radii. Studies in recent years have shown that the latter term is extremely significant; ferroelectric, ferromagnetic, superconducting and structural transition temperatures all vary linearly with the size variance. A variance vs. size plot defines the ‘chemical window’ which enables property variations to be displayed usefully. The variance control of phase separation phenomena in manganite perovskites has also been demonstrated. In contrast, the A cation charge variance is shown to be unimportant in manganites and cuprates.

The formation of calcium carbonate is not only a common ionic reaction that takes place in natural processes, but also creates a problem known as scaling, which is present in our every day life and in various industrial processes and technologies. In spite of the simplicity of the reaction there is considerable variability in the properties of the solid product, such as: crystal form, particle size distribution, electro-kinetics potential, etc. The influence of the magnetic field on calcium carbonate precipitation has been known for a long time but despite a lot of effort, which has been made to explain this effect, researchers still disagree on the mechanism(s) responsible for it. The focus of our research work was to follow systematically the influence of the magnetic field on the crystal form of calcium carbonate precipitated from low concentration water solutions. By changing the strength of the field and the flow rate of the water through the system the calcite/aragonite/vaterite ratio varied. The crystal form and the particle-size distribution of the precipitated calcium carbonate were determined by using X-ray analyses and TEM. The theoretical part of the work was to study the mechanism of the influence of the magnetic field on the nucleation and further crystallization of calcium carbonate. Starting from ab initio calculations the fundamental physics knowledge was used to propose a mechanism for a better understanding of the phenomena.

The energy position and the spacing of the levels of the 4f^n–15d electronic configuration of the trivalent rare earth ion dopands in wide band gap fluoride crystal, depend on the symmetry and the type of the host matrix. Crystal field splitting of the 4f²5d electronic configuration of the Nd³⁺ ions in SrF₂ crystals have been observed in the VUV region of the spectrum. The absorption bands were due to the interconfigurational 4f³→4f²5d dipole allowed transitions between the ground state with 4f³ electronic configuration of the Nd³⁺ ions and the Stark components of the 4f²5d electronic configuration. The VUV spectra can be interpreted by applying the crystal field model, and taking into consideration both that lanthanide contraction of the 4f^n–15d electronic configuration of the rare earth ions is taking place, and that the contribution of the positively charged ions in the total electric field, is effectively decreased with increasing numbers of the electrons in the 4fⁿ electronic configuration due to effective charge shielding.

In this work, we report on the interest of the rapid thermal annealing (RTA) for densification of sol-gel (SG) indium tin oxide (ITO) thin films. The crystalline structure of these ITO films was visualized by transmission electron microscopy (TEM) and corresponding electron diffraction pattern were compared with data from pure In₂O₃. The average grain size, measured from TEM micrographs, ranges from 5 to 50 nm. The film densification was followed by Rutherford backscattering spectrometry coupled with cross-section TEM observations. A comparison with classical annealing is discussed.

Silicon on insulator (SOI) structures (Si / SiO₂ layer / Si) were prepared by bonding the oxidised Si wafer with the hydrogen implanted one and a cleavage of the last wafer by the Smart Cut technique. Effect of high temperature and hydrostatic pressure (HT–HP) treatment at temperatures up to 1570 K and pressure up to 1.2 GPa, typically for 5 h, on the SOI structures was investigated by Transmission Electron Microscopy, X-Ray and photoluminescence measurements.

The point and extended defects are created at HT–HP, especially near the SOI surface. That effect depends on the SOI preparation method and treatment conditions and is related to the hydrogen and pressure assisted oxygen outdiffusion from SiO₂ to the SOI surface and bulk.

Yttrium oxide thin films were in-situ deposited by ion beam sputtering on Si, MgO and SrTiO₃ substrates. These Y₂O₃ thin films were investigated mainly by means of x-ray diffraction. The strained state of the oxide layers was studied by the sin²ψ method as a function of the deposition parameters as well as the post annealing treatments. An in situ study of the kinetics of the internal strain relaxation process was performed as a function of temperature. The Arhenius plot of relaxation rate gives the activation energy of this strain relaxation process, which is 1.3 eV. The results obtained in this work were interpreted in terms of crystal chemistry and the stoichiometry-microstructure relationship.

The hexagonal polymorph of BaTiO₃ (P6₃/mmc) has been stablished at room temperature by partial replacement of Ti by Ga, where BaTi_1-yGa_yO_3-y/2 and 0.06≤y≤0.125 for samples prepared at 1300°C. The unit cell expands with increasing y and Rietveld Refinement of Neutron diffraction data shows that oxygen vacancies occur only in the hexagonal close packed layers between the face-sharing Ti₂O₉ dimers. Exaggerated grain growth (>100 μm) occurs for ceramics processed at ≥1400°C and/or for sintering periods ≥2 hours. Electrical measurements show the materials to be electrically insulating with room temperature permittivity values of ~70–80. Dense ceramics (94–97% of the theoretical X-ray density) resonate at microwave frequencies with Q.f values of ~4000–8000 at ~5.5 GHz.

Titanium aluminides based on TiAl and Ti₃Al are potential materials for high temperature aerospace application. Their low density, high temperature creep resistance and strength, high oxidation resistance, make them excellent potential engine materials. It is reasonable to develop processing strategies for protective and high temperature coatings based on them. On magnetron sputtering the Ti–48% Al alloy films, the composition of which corresponds to the target’s one, have been obtained. When annealed, the aluminium atoms have been discovered to diffuse into the substrate. The redistribution and the leveling of the films’ compositions in the volume take place. After annealing the film contains a higher titanium concentration. On sputtering the formation of the Ti₃Al-based metastable phase in the films has been observed. This phase is the main one in the films and has a different morphology on different substrates. It is stable and doesn’t decompose at annealing. During annealing the new intermediate metastable phases, which are stable only within a definite temperature and concentration range, have developed. The structure, kinetics of phase transformation and evolution of the film microstructures depend on the structure and morphology of the substrate.

Chemical self-diffusion coefficients D(Δ) as a function of temperature T and metal (Zn or Cd) vapor pressure p_Zn or p_Cd has been studied in undoped ZnS and in undoped CdSe single crystals at high temperature. In the temperature range from 750 to 850 °C, D(Δ) in undoped ZnS can be described as D(Δ)=4.5×10⁻³ exp(−0.69 eV/kT) and in the temperature range from 1050 to 1150 °C, D(Δ) can be described as D(Δ)=1.2×10⁻⁴ exp(−0.43 eV/kT). The ZnS phase transition region is characterized by confused values of D(Δ) because of change due to mixture of phases. D(Δ) in ZnS and in CdSe is about three orders of magnitude faster than self-diffusion in these crystals in the same conditions. It was shown that the doubly ionized interstitial metal atoms are the dominanting diffusible defects in ZnS and in CdSe at high metal vapor pressure. D(Δ) is found to be almost independent on metal vapor pressure for undoped ZnS and for undoped CdSe at high p_Zn or p_Cd. In the temperature range from 565 to 700 °C, D(Δ) for CdSe can be expressed as D(Δ)=1.6×10⁻² exp(−0.41 eV/kT).

The Ln_1–xCa_xCoO_3–δ (x = 0, 0.3, 0.4 and 0.5) (Ln = La, Er) phase formation process from chemical methods (amorphous and crystalline precursor decomposition) and a physical method (Pulsed Laser Deposition (PLD)) was monitored with high temperature X-ray powder diffraction (HT-XRD), thermal analysis, and high resolution transmission electron microscopy (HRTEM). The morphology and crystallographic structure of the products with identical composition obtained from different synthesis routes were systematically compared and evaluated for the possible application as electrocatalysts in bifunctional air electrodes.