Point,linear and extended defect structures in nonstoichiometric rutile

TiO_2-x crystals (0 < x ≤ 0.0035) prepared at 1323 K and given controlled thermomechanica treatment were examined by high-resolution electron microscopy (HREM). Crystallographic shear planes (CSP) were not present in non-deformed specimens quenched from 1323 K but appeared, mostly as closely-spaced pairs, in slowly-cooled or deformed and reduced specimens. Lateral and longitudinal disorder in the fine structure of the CSP, which was strongly dependent upon cooling rate was observed. Platelet defects, approximately parallel to {100}, formed at about 400–600°C. Defects intermediate in size between point defects and CSP were also discovered. In situ observations revealed some details of the mechanisms of formation and dissolution of CSP and platelets.

New interstitial cation structural models were derived using the above observations and electrostatic energy arguments. Linear defects, consisting of two face-shared pairs of octahedra containing trivalent cations are proposed. These have very much lower formation and migration energies than the traditional model. Diffusion mechanisms were derived which allow the linear defects to aggregate and readily form pairs of CSP or platelets without the necessity to nucleate dislocation loops.     

Domain structures in barium and strontium hexaferrites

Powder figures have been used to study the domain structure of monocrystals of barium hexaferrite; the domain boundaries on a side face respond to a magnetic field. The domain structure of strontium hexaferrite is reported; the star structure on the end face has a definite order, and the pattern changes to wavy bands as the thickness of the crystal is reduced. The domain structure of both ferrites corresponds generally to that of cobalt monocrystals.     

Lattice deffects in rutile,TiO2

Abstract

Rutile, TiO₂ having relatively high melting point exhibits strong optical absorption after neutron irradiation (8 × 10¹⁶ n_r/cm²) at 15K. The band peak is located near 0. 96 μ having FWHM 0. 87 eV (at LNT). After inverse recovery at 120K, lattice defects due probably to F centers are annealed out at about 220K.     

Domain wall dynamics: Growth laws,localized structures and stable droplets

We discuss the relation between the dynamics of walls separating two equivalent domains and the existence of different kinds of localized structures in systems far from thermodynamic equilibrium. In particular we focus in systems displaying a modulational instability of a flat front where an amplitude equation for the dynamics of the curvature allows to characterize different growth regimes and to predict the existence of stable droplets, localized structures whose stability comes from nonlinear curvature effects.     

Domain and wall structures in films with helical magnetization profile

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.     

Co-, Fe-, Ni-doped and co-doped rutile GeO2: insights from ab-initio calculations

Rutile germanium oxide (rutile GeO₂), a semiconductor, can act as a half-metallic compound and is a promising material for spintronic and optoelectronic applications. Calculations were performed using the Korringa–Kohn–Rostoker (KKR) approach and the coherent potential approximation (CPA), which were further combined with two approximations, the local density approximation (LDA) and the self-interaction corrected LDA approximation (LDA-SIC), to study the electronic structure of bulk rutile GeO₂ doped and co-doped with three transition-metal impurities: Fe, Co, and Ni. The doping value was set to 10%, while the co-doping level was set to 5% for each impurity. The main findings of this work are: (1) a direct ultrawide bandgap of 4.80 eV is observed and the rutile GeO₂ exhibits an N-type semiconducting property. (2) Doped and co-doped GeO₂ acquire a magnetic behavior and exhibit half-metallicity. (3) The mechanism responsible for these properties is also studied. (4) The critical temperature can reach 334 K when GeO₂ is doped with Fe, while it rises to 398 K when it is co-doped with Fe and Co. (5) The spin polarization can be improved by co-doping. It can be inferred that rutile GeO₂ doped or co-doped with (Co, Fe) transition metals can be considered to be potential candidates for spintronic and optoelectronic applications.     

过渡金属(Fe,Co,Ni,Zn)掺杂金红石TiO2的电子结构和光学性质

采用基于密度泛函理论的第一性原理方法对未掺杂以及不同浓度过渡金属Fe,Co,Ni,Zn掺杂金红石TiO₂的超晶胞体系进行了几何优化,并讨论了其晶格常数,电子能带结构和光学性质.研究结果表明:掺杂前后的晶格参数与实验值偏差在3.6%以下;适量的过渡金属掺杂不但影响体系能带结构,拓宽光吸收范围,而且扮演着俘获电子的重要角色,有利于光生电子-空穴对的有效分离以及增强光吸收能力;Fe,Co,Ni,Zn最佳理论掺杂体系分别为Ti_0.75Fe_0.25O₂,Ti_0.75Co_0.25O₂,Ti_0.75Ni_0.25O₂,Ti_0.83Zn_0.17O₂;Fe,Co,Ni3d态分裂为t_2g和e_g态,分别贡献于价带高能级和导带低能级部分,促进了电子-空穴对的生成,从而可提高TiO₂的光催化性能;Zn3d态电子成对填满轨道,不易被激发,故光催化活性无明显提高.     

Correlation between defect structures and dielectric constant in gallia-doped rutile (TiO2)

Dielectric constants and a.c. resistivities of rutile (TiO₂) doped with gallia (Ga₂O₃) have been measured as a function of frequency in the range 30–60 kHz at 20°C. The specimens were prepared by heating rutile single crystals on Ga₂O₃ pellets, at 1300°C, in air for various times. The dielectric constants depend on the frequency of measurement. In the c axis, the dielectric constant at low frequencies (30–100 Hz) increases as the sample heating time is extended to 1 week, but it decreases rapidly when the heating time is prolonged over 1 week. However, in the a axis, the dielectric constant does not change from the value of the starting material, being independent of the heating time. Besides dielectric measurements, the defect structures were determined using transmission electron microscopy, which showed that low concentrations of planar faults on {120} planes occur. The changes in dielectric constant are interpreted as being due to space change effects associated with Ga³⁺ ions located in the tunnels which lie parallel to the rutile c axis. Calculations are also included which show why other defects in the crystals have no effect on the value of the dielectric constant compared to pure rutile.     

Domain structures in orthorhombic KNbO3 and characterisation of single domain crystals

Domain configurations in large KNbO₃ single crystals grown by a top seeded flux growth technique are studied by optical methods and etching techniques. The orientation of observed domain walls is consistent with the principles of mechanical compatibility and electrical neutrality. Experimental evidence for the existence of a new type of ferroelectric domain wall (S-wall) is presented. Miller indices of such walls are irrational and depend on spontaneous strain tensor coefficients. The preparation of large single domain crystals (up to 1/3 cm³) is described. Assessment of crystal perfection by x-ray topographic and interference methods reveals a very small mosaic spread (one minute of arc) and a good optical homogeneity.     

Domain wall pinning in narrow ferromagnetic ring structures probed by magnetoresistance measurements

We present a magnetoresistance study of magnetization reversal and domain wall pinning effects in a mesoscopic narrow ferromagnetic Permalloy ring structure containing notches. The size and strength of the attractive pinning potential created by a notch is measured and the resistance minimum at remanence is found to occur when a single transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. When a field is applied in the direction corresponding to a potential well edge, a novel magnetic state with a very wide domain wall is stabilized, giving rise to a characteristic signature in the magnetoresistance at such angles.     

Cathodoluminescence spectra of diamonds in UHP rocks from the Kokchetav Massif, Kazakhstan

We propose to investigate the diamonds from the Kokchetav Massif, northern Kazakhstan, which is the best example of diamond-eclogite facies metamorphism, using cathodoluminescence (CL) techniques. CL spectra measurements of diamonds from garnet-pyroxene-quartz rocks and dolomitic marbles, made at 80 K, revealed peaks at 2.156, 2.463, and 3.188 eV and a broad band at 2.72-2.80 eV. Panchromatic and monochromatic diamond images analysed by CL reveal concentric zones of variable luminescence. This indicates that the diamond crystals grow during several metamorphic stages under ultra high pressure (UHP) metamorphism conditions. The inhomogeneous broadening and lower 2.156 eV ZPL peak suggests the presence of a higher concentration of defects and stresses in the rim compared to the core.     

On the heavy elements content of sediments and rocks from two semiclosed ecosystems: Proglacial Lake Balea (Fagara Mountains) and Crater Lake St. Ana (Harghita Mountains)

The content of seven heavy elements (natural Sc and Cr, Co, As, Sb, Br and Se as possible pollutants) was determined by the Neutron Activation Analysis (NAA) in 29 samples of sediments and surrounding rocks collected from proglacial Lake Balea (Fagara Mountains) and crater St. Ana Lake (Harghita Mountains) to estimate the degree of anthropogenic pollution, as well as the sediments provenance. Consequently, we have found that, within experimental uncertainties, the content of the considered elements in sediments was close to those of surrounding rocks, and, at the same time, comparable with the normal environmental content as defined by the Romanian Regulations, which enabled us to consider them as normal, nonpolluting constituents of lake sediments. A further R-mode, Principal Component Analysis (PCA), performed for each lake, showed that Sc, Cr, and Co, on the one hand, and As, Sb, Br and Sb, on the other, form two distinct clusters, regardless of the lake, while the Q-mode analysis pointed to a different lake geochemistry with respect to sediments as well as rocks origin.     

Investigations of the defect structures and the EPR parameters for the substitutional Mo5+ centers in rutile type crystals

The defect structures and the electron paramagnetic resonance parameters for the substitutional Mo⁵⁺ centers in rutile type SnO₂, TiO₂ and GeO₂ crystals are theoretically investigated from the perturbation formulas of these parameters for a 4d¹ ion in rhombically compressed octahedra. The [MoO₆]^7? clusters suffer the Jahn–Teller effect and transform the ligand octahedra from original elongation on host tetravalent sites to compression in the impurity centers, with additional smaller rhombic (perpendicular) distortions when compared with those in the hosts. The defect structures and the importance of the ligand contributions are discussed.     

RP-HPLC测定大别山区不同产地天麻药材中天麻素含量研究

采用RP-HPLC外标曲线法测定了天麻原药材中天麻素含量,色谱柱为Phenomenex-C18(4.6mm×250mm,5μm);流动相乙腈-水(5∶95);检测波长225nm。流速0.5mL·min-1:柱温27℃。结果表明天麻素在0.1000—0.6000μg呈良好线性关系,r=0.9997,平均加样回收率为99.53%,RSD为1.7%。方法灵敏、快速、准确,可用于天麻中天麻素含量的测定。     

Electronic structures and optical properties of rutile TiO2 with different point defects from DFT + U calculations

The electronic states and formation energies of four types of lattice point defects in rutile TiO₂ are studied using the first-principles calculations. The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band, while the Ti interstitial forms two local states. It is predicted that oxygen vacancy prefers to combine with Ti-interstitial to form V_O–Ti_i dimer by a partial 3d electron transfer from the Ti_i to its neighboring V_O. The charge distribution between a Ti interstitial and its neighboring Ti ions partially shields the Coulomb interactions. Lastly, optical properties of these defective lattices are discussed.     

Domain structures of sodium tungsten bronzes,NaxWO3 (0.4 < x < 1)

Optical-microscope observations with polarized light have shown that the birefringent, twin-domain structure of sodium tungsten bronzes is exhibited by Na-deficient, epitaxial surface films and hence is not a bulk property as had been suggested elsewhere. The film is translucent, 10^-2-10^-3 mm thick or less, and often laminates to a multi-film layer The domain boundaries are sensitive to lateral stress and, apparently, to minute changes in the substrate structure. These and related properties of the film and the substrate are presented.