Bi2Te3与SnBi2Te4的电子结构与热电性能研究

用离散变分密度泛函分子轨道方法(DFT-DVM)和线性扩展平面波能带方法(LAPW)计算了Bi2Te3与SnBi2Te4,讨论了电子结构与热电性能之间的关系.Te(Ⅱ)-Bi离子键强度和Te(Ⅰ)-Bi差别不大,而Te(Ⅱ)-Bi共价键比Te(Ⅰ)-Bi强.Te(Ⅰ)-Te(Ⅰ)原子层之间的主要相互作用是范得华力而最弱.Bi2Te3掺Sn后Te-Bi离子键增强而共价键减弱,且费米能级处带隙变小.Sn主要影响导带结构.     

Catalyst-free growth of Sb2Te3 nanowires

In this study, a catalyst-free growth method was discovered to prepare the high-quality single crystal Sb₂Te₃ nanowires from the Al:Ge:Sb:Te thin films. The diameters of Sb₂Te₃ nanowires were found to be ~ 100 nm and their lengths were as great as tens of micrometers. The Al content and the annealing temperature play an important role in the growth of Sb₂Te₃ nanowires. When the Al content (> 12.4 at.%) was sufficiently contained in Al:Ge:Sb:Te film, Sb₂Te₃ nanowires were extruded spontaneously on the surface of thin film with increase in annealing temperatures. Compared with the vapor-liquid-solid method, our method has advantages of low temperature (~ 300 °C) and no impurities, such as a metal catalyst.     

Crystal structure of GeTe and Ge2Sb2Te5 meta-stable phase

Direct X-ray diffraction measurement of the erased state of the Ge–Sb–Te recording layer in a four-layered phase change optical disk, which was produced by an optical disk drive, was performed. It was identified as an fcc crystal structure. In order to carry out the detailed crystal structure analysis by the powder X-ray diffraction method with Rietveld refinements, somewhat larger amount of the fcc crystal powder was prepared from deposited 10 μm thick films. It revealed that Ge₂Sb₂Te₅ belongs to the NaCl type structure (Fm m) with the 4a site including 20% vacancies. The conclusion was supported by the results of the density measurements with Grazing Incidence of X-ray Reflectivity.     

Dielectric and photoconducting properties of Ga2Te3 and In2Te3 crystals

Single crystals of defect III–VI semiconductors Ga₂Te₃ and In₂Te₃ have been grown by the Bridgman method. Capacitance vs frequency measurements have been carried out from which the low frequency dielectric constants ?₅ have been determined to be 10.95 ± 0.26 and 12.3 ± 0.13 respectively. These values are compared with the high-frequency dielectric constants ?₆₀ calculated from the Phillips' model. Dark conductivity and photoconductivity have been studied as a function of annealing upto 210°C, maxinum photosensitivity being obtained for both crystals for T_anneal = 80°C. This behaviour has been related to lattice ordering through x-ray diffraction studies. Measurements of photo conductive gain indicate carrier life-times of 2 × 10^?4s and 5 × 10^?4s respectively at room-temperature.     

Crystal growth and structure determination of K2TiO3: A five coordinate titanate

Long, needle-shaped single crystals of K₂TiO₃ were grown out of a reactive high temperature hydroxide melt. The structure was determined by single crystal X-ray diffraction and found to crystallize in the orthorhombic Cmcm space group with a = 10.0283(2) Å, b = 6.9346(2) Å, c = 5.4534(1) Å, V = 379.242(15) Å³, and Z = 4. The structure is related to the K₂SnO₃ and K₂ZrO₃ structures, which crystallize in the Pnma space group, and that of K₂PbO₃, which crystallizes in the Cmc2₁ space group. The structure consists of chains of trans edge-sharing TiO₅ rectangular pyramids with alternating apical directions. The chains are separated by KO₇ polyhedra.     

Resistivity studies on Ga2Te3 and In2Te3 under high pressures

The resistivities of III–VI semiconductors Ga₂Te₃ and In₂Te₃ were found to decrease abruptly under hydrostatic pressures between 1 and 7 GPa. The transition was reversible for Ga₂Te₃ but irreversible in the case of In₂Te₃ due to decomposition. These results are compared with the behaviour of II–VI and III–V semiconductors and transition pressures correlated with lattice constants.     

Crystal growth and structure refinement of monoclinic Li2TiO3

Colorless platelet crystals of monoclinic Li₂TiO₃ with a maximum size of 5.0 mm × 5.0 mm × 0.5 mm were successfully grown by a flux method at 1373 K using a LiBO₂-Li₂O system flux. The stoichiometric chemical composition of Li₂TiO₃ was determined by the SEM-EDX, ICP-AES and density measurement using the single crystal samples. The thermal conductivity of the Li₂TiO₃ single crystals was evaluated using hot-disk method. A single-crystal X-ray diffraction study confirmed the monoclinic Li₂SnO₃-type structure, space group C2/c and the lattice parameters of a = 5.0623(5) Å, b = 8.7876(9) Å, c = 9.7533(15) Å, β = 100.212(11)°, and V = 427.01(9) Å³. The crystal structure was refined to the conventional values of R = 2.4% and wR=3.3% for 2187 independent observed reflections. The cationic arrangement of (LiTi₂) layers in Li₂TiO₃ was precisely revealed by the structure analysis.     

Crystal growth and X-ray structural investigatton of two forms of HgGa2Te4

We have synthesized the HgGa₂ Te₄ system in two different forms showing cubic and tetragonal symmetry, respectively.Single crystals of α-HgGa₂Te₄ suitable for X-ray investigations were obtained by crystallization of the polycrystalline compound using KBr as flux. The X-ray analysis showed a cubic blende-type structure with cations and vacancies disordered on the metallic sites, in agreement with the model proposed by Hahn.Single crystals of β-HgGa₂Te₄ were prepared by chemical vapour deposition. The X-ray structural investigation indicated a partially ordered defect chalcopyrite structure.     

Strain-sensitive ferromagnetic two-dimensional Cr2Te3

Searching for room temperature magnetic two-dimensional(2D)materials is a charming goal,but the number of satisfied materials is tiny.Strain can introduce considerable deformation into the lattice structure of 2D materials,and thus significantly modulate their intrinsic properties.In this work,we demonstrated a remarkable strain-modulated magnetic properties in the chemical vapor deposited Cr₂Te₃ nanoflakes grown on mica substrate.We found the Curie temperature of Cr₂Te₃ nanoflakes can be positively and negatively modulated under tensile and compressive strain respectively,with a maximum varied value of -40 and-90 K,dependent on the thickness of samples.Besides,the coercive field of Cr₂Te₃ nanoflakes also showed a significant decrease under the applied strain,suggesting the decrease of exchange interaction or the change of the magnetization direction.This work suggests a promise to employ interfacial strain to accelerate the practical application of room temperature 2D magnetics.     

Optimization of Bi2Te3 and Sb2Te3 thin films deposited by co-evaporation on polyimide for thermoelectric applications

The optimization of the deposition process of n-type Bismuth Telluride and p-type Antimony Telluride thin films for thermoelectric applications is reported. The films were deposited on a 25 μm-thick flexible polyimide (kapton) substrate by co-evaporation of Bi and Te, for the n-type element, and Sb and Te, for the p-type element. The evaporation rate of each material was monitorized by an oscillating crystal sensor and the power supplied to each evaporation boat was controlled with a PID algorithm in order to achieve a precise user-defined constant evaporation rate.The influence of substrate temperature (in the range 240-300 °C) and evaporation rates of Bi, Te and Sb on the electronic properties of the films was studied and optimized to obtain the highest Seebeck coefficient. The best n-type Bi₂Te₃ films were deposited at 300 °C with a polycrystalline structure, a composition close to stoichiometry, electrical resistivity ∼20 μΩ m and Seebeck coefficient −195 μV/°C. The best p-type Sb₂Te₃ films were deposited at 240 °C, are slightly Te-rich, have electrical resistivity ∼20 μΩ m and Seebeck coefficient +153 μV/°C. These high Seebeck coefficients and low electrical resistivities make these materials suitable for fabrication of Peltier coolers and thermopile devices.     

Electron-microscopic study of glassy As2Te3

Electron-microscopic investigations of glassy As₂Te₃ reveal that this material is a homogeneous glass and does not contain any crystalline phase. In order to verify the detection limit of the employed method with respect to crystalline phase the presence of the latter was provoked by heating the glass in DTA. The crystallization procceeds in two stages: (1) in T_g region the growth of crystallization centres originating probably from the initial melt starts; (2) closely below the recrystallization temperature T_r spontaneous crystallization in the whole glassy bulk is initiated.     

Phase relations in the InBr3 - In2Te3 system and the crystal structure of InTeBr

The quasibinary system InBr₃-In₂Te₃ contains the intermediate phase InTeBr, which melts peritectically at 477 °C. Crystals of InTeBr suitable for crystal structure determination were grown by Bridgman technique. InTeBr: monoclinic, $\text{P}\text{2}{}_1\text{c}$; a = 7.350, b = 7.577, c = 8.343 Å B = 117.61 °; d_x = 5.2 g cm^?3; Z = 4. Intensities were measured on an automatic diffractometer, and the structure was refined with anisotropic temperature factors to R = 9.9 %. The structure consists of InTe₃Br tetrahedra which are connected to layers parallel to the b/c-plane; the layers are translationally equivalent along the a - axis direction. Within a layer the tetrahedra form dimeric units (Br atoms in trans-terminal positions) by sharing common Te vertices to give a $\text{2}\text{∞}$ framework. Te atoms are surrounded by three In atoms in a distorted trigonal pyramidal arrangement. A band gap of about 2.45 eV for InTeBr has been determined by optical transmission measurements.     

Crystal growth and structure of BiVO4

Large crystals of BiVO₄ were grown. Extensive twinning can be related to the ferroelastic transition at 528K. The structure of BiVO₄ was refined at 4.5, 295 and 566K from powder neutron diffraction data and at 295K from powder X-ray diffraction data by the Rietveld profile technique. Space groups of $\text{I}\text{4}{}_1\text{a}$ above and $\text{I}\text{2}\text{b}$ below 528K were confirmed. It is suggested that the transition in BiVO₄ is driven by the lone-pair cation Bi³⁺. The Bi-O polyhedron is regular above the transition but becomes significantly distorted below the transition. This lone-pair distortion increases on cooling from 295 to 4.5K. The V-O tetrahedron remains regular at all temperatures.     

Crystallization behavior and resistance change in eutectic Si15Te85 amorphous films

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si₁₅Te₈₅ amorphous thin films. The Si₁₅Te₈₅ amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si₁₅Te₈₅ amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si₂Te₃ crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250-295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.     

Crystal structure of LuRuB2

Ternary metal borides of the formula MTB₂ have been synthesized in a new structure for M=Sc, Y, Tb, Dy, Ho, Er, Tm, Lu, and T=Ru, Os. Single crystal x-ray diffraction data on LuRuB₂ were refined on an orthorhombic unit cell, space group Pnma, Z=4, and yielded an R factor equal to 0.085. All metal atoms are coplanar with short bond distances of 3.10Å between rare earth atoms which are arranged in zig-zag chains.     

Hydrothermal synthesis of single-crystalline Bi2Te3 nanoplates

Novel Bi₂Te₃ nanoplates with about 0.2-1 μm in diagonal and 100 nm in thickness have been facilely synthesized via hydrothermal routes in the presence of polyvinylpyrrolidone (PVP). Various techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform infrared spectrometry (FT-IR) have been used to characterize the obtained products. The results show that the existence of PVP is vital to the formation of the plate-like morphology. Other factors, such as the reaction temperature and the different surfactants also have influence on the morphology of the final products to some extent.     

Crystal structure of Li2 CaUF8

Compound Li₂CaUF₈ is tetragonal. The unit cell, with $\text{a}\text{=5.2290(12)}\text{A}\text{?}$,$\text{c}\text{=11.0130(18)}\text{A}\text{?}$, contains two formula units and the space group is I$\text{bar}\text{?}$m2. The crystal structure has been solved from single crystal diffractometer data by Patterson and Fourier synthesis and refined by a least-squares method. The final value of R is 0.062 for 602 reflections. The cationic distribution is the same as in the scheelite structure, with an additional cationic ordering. Moreover, we observe that the fluorine atoms are randomly distributed at two half occupied sites.     

Zur kenntnis von Na2Cu4S3 und KCu3Te2

The crystal structures of the new compounds Na₂Cu₄S₃ and KCu₃Te₂ have been solved. Na₂Cu₄S₃ crystallizes in the K₂Ag₄S₃ structure (space group: C2/m, a = 1563(3) pm, b = 386(2) pm, c = 1033(2) pm, β = 107.6^o, N = 4), KCu₃Te₂ in the CsAg₃S₂ structure (space group: C2/m, a = 1645.3(9) pm, b = 429.4(4) pm, c = 866.1(6) pm, β = 111.86^o, N = 4).     

The preparation and characterization of Ba3Te2O9; a new oxide structure

The structure of Ba₃Te₂O₉ has been determined from x-ray powder diffraction data and by profile refinement of neutron diffraction data. We find tellurium octahedrally coordinated as expected and the same face-shared [B₂O₉]^6? unit as observed in Ba₃W₂O₉. The phase Ba₃Te₂O₉ has, however, a Cs₃Fe₂F₉ type structure ($\text{P}\text{6}{}_3\text{mmc}$, $\text{a}\text{=}\text{5}\text{.8603(1)}\text{A}\text{?}$, $\text{c}\text{=14.3037(6)}\text{A}\text{?}$) rather than the Cs₃Tl₂Cl₉ structure (R3c) found for the tungsten analogue. The two oxide structures have the same BaO₃ layer sequence but differ only in the spatial arrangement of the [B₂O₉] groups in the lattice. Infrared and Raman spectra confirm the different site symmetries associated with the different packing of the tungstate and tellurate anions in their respective structures.