Crystal Engineering最新文献

An electron conductive BaTiO₃ crystal without shallow traps is desirable for photorefractive applications. For this report, a nominally undoped BaTi0₃ crystal was grown by the top-seeded solution method, then cut into several pieces, and these pieces were systematically reduced. By investigating the absorption spectra, the light-induced absorption changes, the two-beam coupling gain and the dark-conductivity, we found that the sample reduced at 10⁻¹² atm oxygen partial pressures to be electron conductive and the shallow traps to be inactive.

The RNiO₃ perovskites (R=rare earth) are well known for their metal to insulator transition, which temperature can be modulated by changing the nature of the rare earth. It makes this family of oxides very attractive for several applications such as thermochromic coating. In this work we have studied NdNiO₃ thin films, grown on (110) NdGaO₃, single crystal substrate, with different thicknesses. The temperature of the metal to insulator transition varies in the range 150 –200 K. Residual stress in films was analyzed using the sin²ψ method. Each film exhibits in plane tensile stress, their magnitude depending on the synthesis conditions. A possible relationship between the metal-insulator transition temperature and residual stress is discussed.

Next generation microelectronic circuits will have minimum dimensions below 100 nm. It is envisioned that 157 nm laser lithography will be the next step of optical lithography, A.C. Cefalas, E. Sarantopoulou, Microelectronic Engineering, V53, (2000) 465, followed by lithographies at shorter wavelengths e.g. 13 nm. At 157 nn vacuum ultraviolet (VUV) illumination of the mask target lithographic features with dimensions less than 100 nm on the photoresist could be achieved. However, there are problems related with the design of the optical projection system. This is mainly because most of the optical materials in one hand have high absorption coefficient and their optical properties degrade constantly with time under VUV irradiation. Taking into consideration the imaging requirements for this type of application, the refractive index variation over the illuminated volume of the optical material should be better than 10^-6, and hence optical elements should be prepared from ultra high purity materials. Crystals have been examined with the Jeol 2010 F microscope equipped by the energy dispersive X-ray spectroscopy (EDXS), and it has been proved to be an efficient quality control technique for identifying defects and impurities in crystal samples. A non-uniform distribution of concentration of various elements in wide band gap dielectric crystals in confined space regions from 2 to 50 nm was found, and this result sets the limitations in the optical quality of the crystals.

La_1−xMnO_3+δ (x=0–0.3) nanopowders were prepared at low temperature (825 °C) from a sol–gel method. As results of chemical analyses and X-ray diffraction of products, two ranges of La/Mn ratios can be distinguished: for La/Mn<0.9, a phase separation into La_0.9MnO₃ and Mn₃O₄ occurs while single phases with vacancies on both La and Mn sites are obtained for La/Mn>0.9. La and Mn vacancy concentrations were determined from combined Rietveld refinements of X-ray and neutron diffraction data. Correlations between these structural characteristics and magnetic properties were made. Curie temperatures remain constant at a very high value of 295 K for La/Mn<0.9, while they decrease for increasing ratios of La/Mn>0.9. The decrease of Curie temperatures can be related to an increasing amount of Mn vacancies up to a limit corresponding to a La_0.93Mn_0.93O₃ compound with La/Mn=1. For this compound a cluster–glass behaviour is observed. Finally, neutron diffraction was performed as a function of temperature in order to determine the nature of the magnetic interactions.

Titanium aluminides based on γ-TiAl are potential materials for high temperature aerospace application. The mechanical properties of these alloys are very sensitive to microstructure. Influence of ion irradiation on phase transformation behavior of quenched Ti-48Al-2Nb alloy during aging at 650 and 800 °C has been studied by X-ray diffraction. The quenched samples were irradiated by 60 keV Ti⁺ ions. Next, the recovery of microstructure during aging at 650 and 800 °C has been studied. It is revealed that the preliminary irradiation of quenched samples greatly affects the dynamics of phase transformations during aging. It is established that the speed of the phase transformations in preliminary irradiated and non irradiated samples is different.

Cu₃(OH)₄SO₄ and Cu₄(OH)₆SO₄, synthetic equivalent of antlerite and brochantite minerals, have been obtained by hydrothermal synthesis from copper sulfate, soda and H₂O or D₂O. Their structures have been refined from single crystal X-ray and/or powder neutron diffraction data. They consists of edge-shared copper octahedron infinite chains connected to each other to define either triple chains in the case of antlerite or corrugated planes in the case of brochantite. Sulfate groups connect the former copper entities. From magnetic susceptibility measurements, both samples reveal a transition towards a 3D antiferromagnetic long range order at low temperature. In the paramagnetic domain, AF interactions are predominant for brochantite whereas ferromagnetic ones are evidenced for antlerite. The magnetic structures have been determined from powder neutron diffraction data and reveal the presence of ferromagnetic chains AF coupled. The magnetic results have been related to the Cu-O-Cu bridge angle values.

The magnitudes of off-center Li displacements and the relaxation energies related to reorientation of Li are calculated in ABO₃ perovskites using the semi-empirical Hartree-Fock-based method of the Intermediate Neglect of Differential Overlap (INDO). The spatial extent of a lattice relaxation around Li impurities and contributions from different neighbours to the relaxation energy are discussed. We have applied the INDO method for the study of interaction between Li impurities in KTaO₃. The Li-Li interaction energy as a function of the Li-Li distance and orientation are calculated, including the lattice relaxation around Li-Li impurities. According to our calculations, the Li-Li interaction energies are smaller and less long-range than those estimated from earlier shell model calculations. Lastly, we discuss theoretical modelling of KTaO₃ perovskites with a small amount of Li and simultaneously diluted by Nb (K_1–xLi_xTa_yNb_1–yO₃, KLTN).

Stoichiometric LaMnO₃ is an insulator whose A-type antiferromagnetic structure at low temperature is mainly connected to the Jahn-Teller distortion of the MnO₆ octahedra. The substitution of non magnetic d¹⁰ Ga³⁺ ions on the manganese site disrupts the orbital and spin orderings of the manganese ions and has a strong influence on the physical properties. The antiferromagnetic order is progressively destroyed while the ferromagnetic component increases. At the same time, the distortion of the orthorhombic cell decreases and the high temperature structural Jahn-Teller transition disappears. Such studies on manganese site substitutions give insight into the physics of the Mn-O three-dimensional network, which is supposed to be the key feature in the CMR manganites.

The efficiency of the laser crystals doped with trivalent rare-earth (RE) ions depends on the degree of the homogeneous distribution of the concentration of the RE ion in the crystal volume. In this communication, and as a first contribution to the subject, the concentration of the Pr³⁺, Er³⁺ and Tm³⁺ ions in BaF_2, SrF₂, CaF₂ and KY₃F₁₀ wide band-gap dielectric crystal hosts was determined by measuring the magnetic moment of the ions, using the vibrating sample magnetometer (VSM) method, for the first time to our knowledge. With this experimental method concentration of the RE ions could be determined within 0.1% accuracy in a non-destructive way. In addition, by applying laser induced fluorescence spectroscopy in the vacuum ultraviolet region of the spectrum (100–200 nm), the magnetic properties of the 4fⁿ⁻¹5d electronic configurations can be predicted from the optical spectra for electronic states lying 8 eV above the ground electronic state of the 4fⁿ electronic configuration.

Metal-transition oxides are potential materials for thermoelectric devices operating at high temperatures (T≫300 K). In particular, compounds exhibiting small resistivity and large Seebeck coefficient are required. Two kinds of oxides have been retained: the ‘misfit’ cobaltites which are p-doped metallic phases and the electron doped perovskite manganites. For the former, it is found that the Ca for Sr substitution enhances the thermopower values in the Pb/Sr/Co/O misfit cobaltite whereas it has only a moderate effect in the case of the Tl/Sr/Co/O phase. From this study, it appears that the oxidation state of Co in the conducting layers plays a crucial role on the physical properties. Moreover, for the manganites, we show that electron-doped SrMnO₃ can be prepared by a two-steps method. The doping, created via substitution at either the A-site (Pr³⁺ for Sr²⁺) or B-site (Mo⁶⁺ for Mn⁴⁺), leads to metallic compounds with large thermopower values.