Interrelation of the macrostructure and microstructure with the nonlinear optical properties of iodate and titanate crystals

Noncentrosymmetric crystals of iodates and titanates have been systematized, and the principles of the design of new nonlinear optical materials by means of revealing empirical composition-structure-property relationships have been considered. It is demonstrated that, in the diagram of chemical bond lengths, noncentrosymmetric crystals of simple and binary iodates and titanates are located within the rosette formed by three partially intersecting ellipses. The empirical dependences of the nonlinear optical susceptibility χ⁽²⁾ of the crystal on the length of the shortest metal-oxygen chemical bond L are established for iodates and titanates. These dependences exhibit maxima at L = 177 and 180 pm for iodates and L = 177 and 188 pm for titanates.     

N掺杂钛酸盐的电子结构和磁性研究

基于密度泛函理论,从头计算了N掺杂立方结构钛酸盐的电子结构和磁学性质.结果表明,N掺杂钛酸盐在自旋极化状态下的总能量比自旋非极化状态下的总能量小,说明N掺杂钛酸盐的基态具有铁磁性.从态密度和自旋密度分析可知,其磁性源于掺杂的N 2p电子和价带顶O 2p电子间的p-p耦合作用. 关键词： 钛酸盐 电子结构 磁学性质     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.     

Experimental investigation and ab initio calculation of the properties of Sc-, in-doped bismuth titanates with the pyrochlore type structure

Using ab initio calculations, the data have been obtained on the structural, electronic, and optical properties of bismuth titanates with the pyrochlore type structure and compounds with the substitution of scandium or indium atoms for bismuth and titanium atoms. The results of the theoretical calculations agree with the experimentally obtained structural and optical characteristics of the synthesized compounds doped with scandium or indium. It has been shown that the substitution of scandium or indium atoms for bismuth atoms in the pyrochlore structure is energetically favorable. The energies corresponding to the direct and indirect electronic transitions in scandium- and indium-doped bismuth titanates have been determined based on the optical spectroscopy data obtained for the studied samples. These energies are in agreement with the theoretically calculated values.     

Mechanisms of corrosion in surface layers at solid-solution interfaces

This review is focussed on solids in the broad categories of oxides, both simple (e.g. binary) and complex (e.g. silicates, aluminosilicates, titanates), and sulfides as ceramics, minerals and surface coatings. Mechanisms of surface reaction, corrosion and leaching associated with protons, hydroxide, water and simple cations and anions are considered. A variety of mechanisms that have some generality in determining the kinetics and reaction products at the solid-solution interfaces is illustrated. The focus of the experimental studies is on the surface layers; their development, distribution of species, depth and control. Mechanisms discussed and illustrated include: diffusion; protonation and ion transfer to solution; lattice reaction; ion exchange (leaching); precipitation; surface oxidation; surface reconstruction; intergranular films and grain boundaries. Theoretical models for the first four mechanisms are presented.     

Structural and conductivity investigations of doped Li-titanates

Polycrystalline lithium lanthanum titanates, Li_xLa_yTiO₃ (0<x<0.5, 0.5<y<0.7) show high ionic conductivity (10⁻⁴ to 10⁻³ S/cm, depending on x and y) at room temperature. Doping the lithium lanthanum titanates by Co and Ni results in perovskite-like structures, which may be readily synthesized by solid state reaction. Structural and conductivity characterizations are reported. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 999.     

Chemical changes induced by sputtering in TiO2 and some selected titanates as observed by X-ray absorption spectroscopy

We studied the chemical changes induced by Ar⁺-bombardment in TiO₂ and some selected titanates by means of soft X-ray absorption spectroscopy. The O 1s and Ti 2p spectra provide unique information on hybridization, valency and crystal-field effects. In TiO₂, we found that some Ti ions are partly reduced but no metallization is observed. On the other hand, the effect of ion-bombardment in titanates depends on the counter-ions. In Al₂TiO₅, the central Ti ions are strongly reduced while Al₂O₃ remains unaffected. In NiTiO₃, by contrast, we found the opposite behaviour with strong reduction of the counter Ni ions. This contrasting behaviour seems to be related to differences in the relative heat of formation of the sub-species. In PbTiO₃, both central- and counter-ions are affected by sputtering. Finally, the electronic structure of BaTiO₃ presents very little changes upon ion-bombardment.     

Magnetic Properties of Bismuth-Doped Ytterbium and Holmium Pyrochlore Titanates

Physics of the Solid State - Bismuth-doped ytterbium and holmium titanates have been synthesized and their magnetic properties have been investigated. The magnetization curves and temperature...     

Magnetic Susceptibility of Nanocomposite Rare-Earth Titanates in Alternating Fields

Physics of the Solid State - The magnetic susceptibility of nanocomposite rare-earth titanates has been studied experimentally at low temperatures in the frequency range from 1 to 10 kHz. The...     

Mn-substituted titanates as efficient anodes for direct methane SOFCs

A new family of perovskite titanates with formula La₄Sr₈Ti_12−xMn_xO_38−δ has been investigated as fuel electrode materials for SOFCs. These phases present a rhombohedral (R− 3c) unit cell. Mn substitution does not have a large impact on the bulk conductivity of the phases studied, which remains close to the values observed in other related titanates, although the grain boundary contributions are largely improved. Relatively low polarisation resistances were observed under both hydrogen and methane conditions, e.g., 0.3 and 0.7 Ω cm² at 950 °C, respectively. Despite the polarisation resistance in methane being more than twice that in hydrogen, the performances are very similar, which might indicate certain methane activation at that temperature. Surprisingly, the anodic overpotential was fairly low compared to those reported in the literature for other materials and especially for titanate-based anodes, i.e., a value of 55 mV at 0.5A/cm², at 950 °C, under wet hydrogen was obtained. Additionally, a value 72 mV was obtained in the same conditions under methane. These values indicate that the use of Mn as dopant for perovskite-related titanates enhanced electrochemical performance of these anodes, especially at high temperatures.     

Bottom-up assembly of single-domain titania nanosheets on (1 x 2)-Pt(110)

A bottom-up route towards the synthesis of titania nanosheets is explored, alternative to the exfoliation of layered titanates. Nanosheets are assembled from the constituent elements and epitaxially matched to a suitable substrate: (1 x 2)-Pt(110). Their basic lepidocrocite structure is modulated at the nanoscale due to coincidence with the substrate. Density functional calculations reveal the structure details of the nanosheet, which is also shown to be in close relationship with a (001)-oriented anatase bilayer.     

Influence of acceptor doping on ionic conductivity in alkali earth titanate perovskites

The electrical conductivity of the SrTi_1−xFe_xO_3−δ, BaTi_1−xFe_xO_3−δ and SrTi_1−xMn_xO_3−δ systems has been studied in a range of oxygen partial pressures between 10⁻¹⁶ and 0.21 atm at 900 and 1000 °C. The materials exhibit predominantly ionic conductivity in a wide range of intermediate oxygen partial pressures. It has been found that in Fe doped strontium and barium titanates, the dependencies of the ionic conductivity on the acceptor concentration show a local maximum near x=0.2. Taking into account that in the CaTi_1−xFe_xO_3−δ system (x=0−0.5), the concentration dependence of the ionic conductivity also has a maximum near x=0.2, it can be concluded that this is a common phenomenon for Fe doped alkali earth titanates. An assumption has been made that a scheme of defect formation devised earlier for Fe doped calcium titanate is applicable for other alkali earth titanates.     

Ab initio calculations of phonon spectra in ATiO3 perovskite crystals (A = Ca,Sr, Ba,Ra, Cd,Zn, Mg,Ge, Sn,Pb)

First-principles calculations of phonon spectra based on the density functional theory are carried out for calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with a perovskite structure. By analyzing unstable modes in the phonon spectrum, the possible types of lattice distortion are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn concerning the nature of ferroelectric phenomena in the crystals studied. It is shown that the main factors determining the possible appearance of off-center atoms in the A position are the geometric size and electronic configuration of these atoms.     

First-principles study of electronic structure and optical properties of barium strontium titanates (BaxSr1−xTiO3)

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigate the energy band structures and total density of states (TDOS) of barium strontium titanates (BSTs). Direct band gaps of 1.89 and 1.87 eV at the Γ point in the Brillouin zone are calculated for Ba_xSr_1−xTiO₃ (x = 2/3 and 1/3), respectively. The optical properties of the above perovskites in the core-level spectra are investigated by the first-principles under scissor approximation. The real and imaginary parts of the complex dielectric function and derive optical constants, viz., the refractive index, extinction coefficient, reflectivity, and the electron energy-loss spectrum are calculated.     

Unusual symmetries in the Kugel-Khomskii Hamiltonian

The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d(1) ions having threefold degenerate t(2g) orbitals. Since orbitals do not couple along "inactive" axes, perpendicular to the orbital planes, the total number of electrons in |alpha> orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.     

Physical properties of titanates,semiconductors and nickelates derived from ionization energy theory

Ionization energy theory is exploited to predict the changes to atomic polarizability for both anions and cations, the polarization in doped titanates and the energy gap in III–V and II–VI semiconductors. We then extend the above analysis to discuss the physics of metallic and superconducting phases in the recently discovered superconducting nickelates. In doing so, we are able to prove the existence of Ni²⁺ cations and oxygen vacancy in the metallic normal state of nickelates. We find that the normal state resistivity of the nickelates follows exactly as predicted by the ionization energy theory. Quantitative estimates are also given for the concentrations of Ni²⁺ and oxygen vacancy in superconducting nickelates.     

Hyperfine interactions in titanates: Study of orbital ordering and local magnetic properties

Hyperfine magnetic fields induced on the nuclei of nonmagnetic ions ¹³⁹La and ⁸⁹Y in LaTiO₃ and YTiO₃, respectively, have been microscopically calculated. The dependence of the hyperfine fields on the orbital and magnetic structures of the compounds under study has been analyzed. The comparative analysis of the calculated and known experimental data confirms the existence of the static orbital structure in lanthanum and yttrium titanates.     

The electronic structure of RbTiOPO(4) and the effects of the A-site cation substitution in KTiOPO(4)-family crystals

The electronic structure of RbTiOPO(4) has been investigated with x-ray photoemission spectroscopy. Detailed photoemission spectra of the element core levels have been recorded under excitation by nonmonochromatic Al Kα radiation (1486.6?eV). The chemical bonding parameters are compared to those reported for complex titanates and phosphates. The band structures of KTiOPO(4), RbTiOPO(4), K(0.535)R(0.465)TiOPO(4) and TlTiOPO(4) have been calculated by ab?initio methods and compared to available experimental results. It is found that the band structure of KTP-type phosphate crystals is weakly dependent on the nature of the A-site (A=K, Rb, Tl) element.     

Sol-gel preparation of selected high temperature proton conductors

The sol-gel preparation of the compounds BaCe_0.8Y_0.2O_2.9 (BCY20), BaCe_0.8Gd_0.2O_2.9 (BCG20), BaTiO₃, BaTi_0.9Y_0.1O_2.95 and BaTi_0.9Er_0.1O_2.95 using the acrylamide route is described. The advantages lie in the short reaction time leading to the gels and the large batch sizes possible. In the case of the titanates there was the additional advantage that one of the starting compounds was metallic Ti powder which was dissolved in H₂O₂ and NH₃. Basically, all 5 compounds could be synthesized phase pure. The cerates were well described by PDF 35-1318 and absorb large quantities of water vapor in TG experiments. The doped titanates were cubic and were well characterized by PDF 31-0174. They only absorb relatively small amounts of water vapor but still may be termed proton conductors.