Diffraction of electrons in the CubicTi5O5 superstructure of titanium monoxide

Annealed titanium monoxide TiO_1.087 has been studied by the electron diffraction method. A cubic model of the Ti₅O₅ superstructure (Ti₅O₅ (Ti₉₀?₁₈O₉₀??₁₈)) of nonstoichiometric titanium monoxide Ti _x O _z has been proposed on the basis of experimental data and representations about the disorder-order transition channel. It has been shown that reflections observed on the electron diffraction pattern are identified in the space group $Pm\bar 3m$ . The period of the unit cell of the cubic Ti₅O₅ superstructure is larger than that for the B1 basic disordered structure of Ti _x O _z monoxide by a factor of 3. The disorder-order transition channel Ti _x O _z (space group $Fm\bar 3m$ )-Ti₅O₅ (space group $Pm\bar 3m$ ) includes 75 superstructure vectors of seven stars {k ₁₀}, {k ₇}, {k ₆₍₁₎}, {k ₆₍₂₎}, {k ₄₍₁₎}, {k ₄₍₂₎}, and {k ₁}. The distribution functions of Ti and O atoms over the sites of the cubic Ti₅O₅ (space group $Pm\bar 3m$ ) superstructure have been calculated.     

Simulation of the short-range order in disordered cubic titanium monoxide TiO1.0

A model of the atomic structure with the short-range order in the vacancy distribution for the disordered cubic phase of titanium monoxide TiO_1.0 has been proposed. The effect of the short-range order on the electronic structure and the stability of the compound has been studied by the supercell method within the DFT-GGA approximation with the use of pseudopotentials. It has been established that the appearance of the short-range order considerably decreases the total energy. The decrease in the energy is comparable with the energy gain during the ordering of the vacancies according to the type of monoclinic superstructure Ti₅O₅ to the long-range order parameter η = 0.7. It has been shown that the discrepancies between the theoretical and experimental electronic spectra of titanium monoxide can be explained by allowance for the short range order.     

Two new superstructures Ba4Ti5O10 and Ba4Ti4O11 in epitaxial barium titanate nanodomains determined by nanobeam electron diffraction and high-resolution transmission electron microscopy

Barium titanate thin films deposited on MgO (0?0?1) by RF magnetron sputtering are composed of two new superstructures Ba₄Ti₅O₁₀ and Ba₄Ti₄O₁₁ formed as epitaxial nanodomain structures. Due to the nanometre scale size of domains and the complexity of the structure in the film, the single-crystal X-ray diffraction technique does not have the capability for the crystallographic structure determination of the two new superstructures. We used nanobeam electron diffraction to reconstruct the three-dimensional diffraction space and hence the symmetry of the new superstructures. Both Ba₄Ti₅O₁₀ and Ba₄Ti₄O₁₁ are monoclinic structures with a space group of Cm (b-unique axis). Ba₄Ti₅O₁₀ has a lattice parameter a?=?16.49?Å, b?=?3.94?Å, c?=?8.94?Å and β?=?103°, while Ba₄Ti₄O₁₁ has a?=?17.88?Å, b?=?3.94?Å, c?=?7.21?Å and β?=?98°. Atomic structural models for the two new superstructures were established by reconstructing the high-resolution transmission electron microscopy (HRTEM) images taken from the three major axes and refined by matching the simulated HRTEM images and calculated electron diffraction patterns with the experimental results. The two superstructures are epitaxially grown on MgO with their b-axis parallel to the growth direction.     

Two-sublattice ordering in titanium monoxide

The disordered and ordered structures of nonstoichiometric titanium monoxide Ti_xO_z≡TiO_y(y=z/x) containing structural vacancies simultaneously in the nonmetallic and metallic sublattices were studied. In the stoichiometry range from TiO_0.9 to TiO_1.1, an ordered monoclinic phase [space group C2/m (A12m/1)] of the Ti₅O₅ type is formed in the TiO_y monoxide at temperatures below 1300 K. The disorder-order TiO_y?Ti₅O₅ phase-transition channel involves Lifshitz {k₁₀} and non-Lifshitz {k₄} and {k₁₁} star rays. The ordering proceeds as a first-order phase transition with a decrease in the volume of the basal cubic lattice. The titanium and oxygen distribution functions in the metallic and nonmetallic sublattices of titanium monoxide are calculated. The domain of allowed values is determined for the long-range order parameter.     

Ordered orthorhombic phases of titanium monoxide

Symmetry analysis is carried out for the ordered phases of cubic monoxide TiO_y with relative oxygen contents y<1 and y>1. It is established that a partially ordered orthorhombic phase (space group Immm)—a derivative of the orthorhombic M₃X₂□ superstructure (at y<1.0) or the inverse superstructure M₂■X₃ (at y>1)—may arise in TiO_y. The distribution of Ti and O atoms, oxygen vacancies □, and titanium vacancies ■ in unit cells of the orthorhombic ordered phases is determined. The phases are formed through the order-disorder transition channel along two rays of a non-Lifshitz star {k ₄}, and the ordering proceeds as a first-order phase transition. The distribution functions of Ti atoms over the sites of metallic and O atoms over the sites of nonmetallic sublattices are calculated for the orthorhombic superstructures of cubic titanium monoxide TiO_y.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Electrical conductivity and magnetic susceptibility of titanium monoxide

Conductivity and magnetic susceptibility of disordered cubic titanium monoxide TiO_y(0.920≤y≤1.262) are studied. Temperature dependences of the conductivity of TiO_y monoxides with y≤1.069 are described by the Bloch-Grüneisen function with Debye temperature 400–480 K, and temperature dependences of the susceptibility include Pauli paramagnetism of conduction electrons. The behavior of conductivity and susceptibility of TiO_y with y≥1.087 is typical of semiconductors with nondegenerate charge carriers obeying Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y(y≥1.087) is 0.06–0.17 eV, and effective mass of charge carriers is equal to 7–14 electron masses.     

Short-range order and twins in ordered titanium monoxide

The structure of annealed titanium monoxide TiO_1.087 containing monoclinic ordered phase Ti₅O₅ was studied by electron diffraction. Along with the set of structural, superstructural, and extra reflections, the diffraction pattern of titanium monoxide shows a set of plane diffuse fringes in the (112)_B1* section of the reciprocal lattice of the basis cubic structure B1. It is shown that some of the extra reflections are due to the twinning of the monoclinic superstructure along the (?1?11) plane of the reciprocal lattice of the basis cubic structure. The diffuse contours enclose plane areas of the reciprocal space with the fixed values of wave vectors K₁₀₀ ～ ±(h+0.07)k₁₀₀, K₀₁₀ ～ ±(k+0.07)k₀₁₀, and K₀₀₁ ～± (l+0.07)k₀₀₁ of the B1 structure. Their appearance is associated with the short-range displacement order.     

Twinning and short-range order in ordered titanium monoxide

Electron diffraction was used to study the annealed titanium monoxide TiO_1.087 containing the monoclinic ordered phase Ti₅O₅. The diffraction pattern of titanium monoxide in the (112)* _B1 plane of the reciprocal lattice of the parent B1 cubic structure contains not only structural, superstructural, and additional reflections but also a system of planar diffuse strips. It has been established that part of the additional reflections are twins of the superstructure reflections of the monoclinic ordered phase; the twinning plane is the ( $\overline 1 \overline 1 1$ ) plane of the reciprocal lattice of the parent cubic phase. The diffuse scattering contours cover finite plane areas in the reciprocal space characterized by the wave vectors K ₁₀₀ ～ ± (h + 0.07)k ₁₀₀, K ₀₁₀ ～ ±(k + 0.07)k ₀₁₀, and K ₀₀₁ ～ ±(l + 0.07)k ₀₀₁ in the B1 structure. The diffuse scattering is caused by short-range displacement order. Short-range substitution order and the corresponding diffuse scattering are absent.     

外电场作用下TiO光激发特性研究

利用密度泛函BLYP方法优化得到了TiO分子的稳定构型,并计算了TiO分子基态在外场作用下前线轨道变化情况,然后利用杂化组态相互作用CIS-DFT方法,比较了TiO分子在外电场下的激发特性.结果表明,在一定的电场范围内,随着电场的增大,α轨道的最高占据轨道与最低空轨道能隙逐渐变小,β轨道能隙逐渐变大,同时可跃迁的低激发态跃迁波长随电场的增大而变长,高激发态波长变化相对复杂,且基态跃迁至激发态的耦合强度随外电场的增大而加强.     

C掺杂TiO薄膜的制备及其第一性原理研究

TiO在微电子结构器件中有重要的应用前景. 本文以CO₂作反应气体, 采用直流反应磁控溅射方法成功制备出C掺杂TiO薄膜. 采用XRD, XPS和四探针电阻计对薄膜结构、成分和电阻率进行表征. 在实验结果的基础之上建立起TiO和C掺杂TiO的计算模型并采用第一性原理 方法计算其能带结构和电子态密度. 实验结果表明薄膜相结构为面心立方的岩盐结构, C取代O的阴离子掺杂为主要掺杂方式, 薄膜电阻率为52.2 μΩ·cm. 第一性原理计算结果表明, 费米能级穿过TiO的导带, TiO具有金属性导电的能带结构特征; C掺杂TiO 后, 其金属性导电的能带结构没有改变, 只是在费米面附近出现C 2p态提供的杂质能级, 杂质能级扩展了TiO的导带宽度并提高了费米面附近的电子能态密度, 从而导至TiO电导增加, 电阻率降低. 第一性原理计算结果与实验结果一致. 关键词： C掺杂TiO 直流反应磁控溅射 第一性原理 电子结构     

Electronic structure of titanium monoxide with randomly distributed vacancies

The electronic structure of titanium monoxide TiO _y (0.810 ≤ y ≤ 1.262) in the high-temperature cubic phase with vacancies randomly distributed over the titanium and oxygen sublattices is calculated in the coherent potential approximation. The changes in the electronic spectra with the concentration of vacancies are retraced. The calculated spectra are compared to the available experimental data.     

Electrostatic stabilization of an ordered phase in titanium monoxide

The influence of the ordering of titanium and oxygen atoms on the Madelung energy has been studied using the computer simulation. The dependence of the Madelung constant on the crystal size for the titanium monoxide with ordered and disordered distributions of atoms and vacancies has been obtained for the first time. It has been demonstrated that the Madelung energy in the ordered titanium monoxide is considerably (by 6%) lower than that in the disordered monoxide. The electrostatic interaction between titanium and oxygen ions stabilizes the ordered phase in titanium monoxide up to high temperatures and should substantially affect the disordering processes that occur in this compound at high temperatures.     

Investigation of structural vacancies in titanium monoxide by electron-positron annihilation

Local atomic environment of vacancies in nonstoichiometric titanium monoxide ranging in composition from TiO_0.74 to TiO_1.26 was studied by electron-positron annihilation. Analysis of the Doppler broadening spectra of the annihilation gamma line for titanium and liquid oxygen showed that positrons in titanium monoxide are trapped by titanium vacancies. Experiments revealed that the lifetime of positrons in ordered and disordered titanium monoxide TiO _y increases with increasing oxygen content y and varies from 184 to 210 ps. Data on the valence electron density permitted the prediction that the lifetime of free positrons in stoichiometric titanium monoxide is about 140 ps and the lifetime of positrons localized in an oxygen vacancy is about 170 ps. The method used to analyze the gamma-line Doppler broadening spectra makes it possible to determine the type and number of atoms around a vacancy and to investigate order-disorder phase transformations in nonstoichiometric compounds.     

Observation of structural vacancies in titanium monoxide using transmission electron microscopy

Structural vacancies were directly observed in nonstoichiometric ordered titanium monoxide using high-resolution transmission electron microscopy under a magnification of 4×10⁶. The observation of structural vacancies became possible due to their ordering and the formation of continuous vacancy channels in certain crystallographic directions. Microdiffraction was employed to orient the sample in the direction permitting the observation of vacancy channels. Transmission electron microscopy providing a magnification of tens of thousands of times revealed that titanium monoxide grains do not contain cracks and macropores and confirmed that the free volume detected picnometrically in the titanium monoxide is concentrated in structural vacancies on the titanium and oxygen sublattices.     

The disorder-order transition in cubic vanadium monoxide with vacancies in the metal sublattice

The disorder-order transition in cubic vanadium monoxide VO_y (y = 1.29, 1.30) possessing a B1 type structure and containing vacancies only in the metal sublattice has been studied by X-ray diffraction and symmetry analyses. It is established that the formation of a tetragonal (space group I4₁/amd) ordered V₅₂O₆₄ phase in cubic vanadium monoxide VO_y proceeds in the form of the first-order phase transition via a channel involving 22 nonequivalent superstructural vectors of four stars ({k ₁₀}, {k ₄}, {k ₃}, and {k ₂}). The distribution function of V atoms in the tetragonal V₅₂O₆₄ superstructure is calculated, and it is found that the real ordered V_51.6O₆₄ phase exhibits significant atomic displacements. The boundaries of the domain of existence of the V₅₂O₆₄ phase at 54–60% O are determined in the phase diagram of the V-O system.     

A single-crystal study of the electrochemically Li-ion intercalated spinel-type Li4Ti5O12

Single crystals of Li_4 + xTi₅O₁₂ were prepared by means of electrochemical Li-ion intercalation technique using parent Li₄Ti₅O₁₂ single crystals. The obtained Li_4 + xTi₅O₁₂ (x = 1.35) crystallizes in the cubic spinel-related type structure, space group Fd3?m, and lattice parameters of a = 8.346(2) Å and V = 581.3(5) Å³ and Z = 8. The Li-ion intercalated sites were successfully determined to be both the 8a and 16c sites by using the difference Fourier synthesis map. The structure was determined by single-crystal X-ray structure analysis and refined to the conventional value of R = 3.7% for 132 independent observed reflections. The chemical composition has been determined to be Li_5.35Ti₅O₁₂ from the result of site-population refinements. In addition, theoretical electron density distributions and total energy were calculated for three postulated compounds of “Li_4.5Ti_4.5O₁₂” and “Li_4.5 + xTi_4.5O₁₂” with x = 1.5 and 3.0.     

Effects of Li/Ti ratios on the electrochemical properties of Li4Ti5O12 examined by time-resolved X-ray diffraction

Li₄Ti₅O₁₂ for anodic active material of lithium ion batteries is synthesized using different Li/Ti ratios of 3.5/5.0, 4.0/5.0 and 4.5/5.0 by a solid-state reaction between Li₂CO₃ and anatase TiO₂ at 850?°C. All samples contain a small amount of transformed rutile TiO₂ in the final Li₄Ti₅O₁₂, where the amount of rutile TiO₂ decreases with the increase in Li/Ti ratio. A stoichiometric Li₄Ti₅O₁₂ with Li/Ti = 4.0/5.0 shows a slightly larger particle size and higher charge capacity than those of Li-deficient and Li-excessive particles, while the discharging rate capability is shown to mainly depend on particle size regardless of Li/Ti ratio. According to the time-resolved X-ray diffraction patterns using a synchrotron source, however, no significant difference is found in spite of the difference in Li/Ti ratio, indicating the structural stability of Li₄Ti₅O₁₂ during the Li insertion and extraction process.     

Improvement electrochemical performance of Li1.5Ni0.25Mn0.75O2.5 with Li4Ti5O12 coating

Layered cathode Li_1.5Ni_0.25Mn_0.75O_2.5 has been synthesized and coated by Li₄Ti₅O₁₂. The pristine and coated Li_1.5Ni_0.25Mn_0.75O_2.5 powders are characterized by X-ray diffraction (XRD), indicating the materials remained the layered structure before and after coating. The coated Li₄Ti₅O₁₂ has been detected by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (DEX). The electrochemical performance, especially rate performance of Li_1.5Ni_0.25Mn_0.75O_2.5 electrode, is improved effectively after Li₄Ti₅O₁₂ coating. The first discharge capacity, coulombic efficiency, and capacity retention of Li₄Ti₅O₁₂-coated Li_1.5Ni_0.25Mn_0.75O_2.5 electrode are 244 mA h g^?1, 81.5 %, and 98.3 % after 50 cycles, respectively. The Li₄Ti₅O₁₂-coated Li_1.5Ni_0.25Mn_0.75O_2.5 electrode exhibits 108 mA h g^?1 at 10 °C rate. Electrochemical impedance spectroscopy (EIS) results show that the charge transfer resistance (R _ct) of Li_1.5Ni_0.25Mn_0.75O_2.5 electrode decreases after coating, which is due to the existence of Li₄Ti₅O₁₂ with high lithium ion diffusion coefficient and suppression of the solid electrolyte interfacial (SEI) layer development and is responsible for the excellent rate capability and cyclic performance.