Conversion of non-stoichiometry of LiNbO3 to constitutional stoichiometry by impurity doping

Several chemical components exist in the oxide melt, including ionic species. During crystal growth these are partitioned into the solid phase with their own equilibrium partitioning coefficients. Therefore, they are usually segregated, even at the congruent composition. However, when a compound has a stoichiometric structure at the congruent composition, the equilibrium partitioning coefficients of all chemical components become unity and segregation does not occur. Segregation of ionic species was observed at the binary congruent composition of LiNbO₃ because of its non-stoichiometric structure. On the other hand, the segregation of ionic species did not occur at the ternary congruent composition of MgO-doped LiNbO₃ (Li₂O:Nb₂O₅:MgO=45.5:50:4.5). This demonstrates that non-stoichiometry of congruent LiNbO₃ was converted to the constitutional stoichiometry at the ternary congruent composition by MgO doping.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

Growth of large-sized Ce:Y3Al5O12 (Ce:YAG) scintillation crystal by the temperature gradient technique (TGT)

In this work, Ce:YAG crystal with the size of ?4 in was successfully grown by the TGT method. The optical and scintillation properties of as-grown Ce:YAG crystals were investigated. Three obvious absorption bands at 223, 340 and 460 nm and two weak color-center absorption bands at 296 and 370 nm are observed in as-grown Ce:YAG crystal. Fluorescence with an emission peak at 398 nm is observed due to the color centers, and absorption bands of the color centers can be eliminated by annealing in O₂ or H₂ atmosphere at 1673 K for 24 h. Yellow-green fluorescence centered at 530 nm is found when the crystal was excited at 460 nm and the 530 nm excitation spectrum shows two peaks at 340 and 460 nm. X-ray fluorescence spectrum of as-grown crystal shows three emission peaks at 300, 360 and 530 nm. An average light output of 1360 phe/MeV and a single exponential decay with the decay time constant of 62.97 ns are found in as-grown Ce:YAG crystal.     

Effect of BaO content on the sintering and physical properties of BaO-B2O3-SiO2 glasses

A BaO-B₂O₃-SiO₂ glass system was chosen as a candidate composition for the application to Pb-free low temperature sinterable glass. The effect of BaO content on the crystallization, sintering behavior, and properties of the glasses was examined. Both the glass transition temperature and crystallization temperature decreased as the BaO content increased. Crystallization easily occurred during sintering with a BaO content of more than 50 mol%, which effectively inhibited the over-firing phenomenon. The dielectric characteristics and thermal expansion coefficient of the glasses were examined and the results were explained on the basis of the crystallization and densification of the specimens.     

Floating zone growth and compositional inhomogeneity study of La0.75Y0.05Sr0.2MnO3 single crystals

Single crystals of La_0.75Y_0.05Sr_0.2 MnO₃ have been grown by the floating zone technique. The yttrium distribution in the radial and axial directions in the crystals was studied for different growth rates. The sample grown at high rate showed inhomogeneity in the yttrium content in the growth direction, while the sample grown at relatively low rate was homogeneous in the growth direction, but demonstrated compositional separation between the Y-poor central part and Y-enriched peripheral part in the perpendicular plane. The compositional separation led to magnetic inhomogeneity.     

Influence of pulsed laser deposition rate on the microstructure and thermoelectric properties of Ca3Co4O9 thin films

The effects of deposition rate on the microstructure and thermoelectric (TE) properties of Ca₃Co₄O₉ thin films fabricated by pulsed laser deposition (PLD) technique were investigated. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) revealed that a fast deposition rate resulted in not only low crystallinity but also the existence of the Ca_xCoO₂ secondary phase. Formation of Ca_xCoO₂ was inevitable during the thin film growth, and this was discussed from both structural and compositional point of view. With longer deposition interval or with sufficient oxygen at a lower deposition rate, the Ca_xCoO₂ phase was able to transit into the desired Ca₃Co₄O₉ phase during the coalescence process. The quality of the thin films was further analyzed by electrical properties measurements. The Ca₃Co₄O₉ thin film fabricated at a slower deposition rate was found to exhibit a low electrical resistivity of 9.4 mΩ cm and high Seebeck coefficient of 240 μV/K at about 700 °C, indicating a good quality film.     

Synthesis of GaN crystal by the reaction of Ga with Li3N in NH3 atmosphere

A novel method to synthesize GaN crystals was studied by the reaction of Ga with Li₃N under NH₃ atmosphere. We have already reported the synthesis technique of GaN by the reaction of Ga₂O₃ with Li₃N. However, the size of GaN crystals obtained by this method was limited to be smaller than several micrometers because of the solid phase reaction. In order to increase the size of GaN crystals, the method using liquid Ga as gallium source was studied for solid–liquid phase reaction. We found that the GaN crystals with the size of more than 100 μm were synthesized at 750 °C for 24 h under NH₃ atmosphere. We propose the possible reaction mechanism as follows. Lithium amide (LiNH₂) is synthesized by the reaction of Li₃N with NH₃ gas and then the crystal growth of GaN occurs by the reaction of Ga with LiNH₂. We found that LiNH₂ is a useful nitrogen source for the GaN synthesis method.     

Fabrication and properties of novel low-melting glasses in the ternary system ZnO-Sb2O3-P2O5

Glasses in the ternary system ZnO-Sb₂O₃-P₂O₅ were investigated as potential alternatives to lead based glasses for low temperature applications. The glass-forming region of ZnO-Sb₂O₃-P₂O₅ system has been determined. Structure and properties of the glasses with the composition (60 − x)ZnO-xSb₂O₃-40P₂O₅ were characterized by infrared spectra (IR), differential thermal analysis (DTA) and X-ray diffraction (XRD). The results of IR indicated the role of Sb³⁺ as participant in glass network structure, which was supported by the monotonic and remarkable increase of density (ρ) and molar volume (V_M) with increasing Sb₂O₃ content. Glass transition temperature (T_g) and thermal stability decreased, and coefficient of thermal expansion (α) increased with the substitution of Sb₂O₃ for ZnO in the range of 0-50 mol%. XRD pattern of the heat treated glass containing 30 mol% Sb₂O₃ indicated that the structure of antimony-phosphate becomes dominant. The improved water durability of these glasses is consistent with the replacement of easily hydrated phosphate chains by corrosion resistant P-O-Sb bonds. The glasses containing ?30 mol% Sb₂O₃ possess lower T_g (<400 °C) and better water durability, which could be alternatives to lead based glasses for practical applications with further composition improvement.     

From Te nanotubes to CoTe2 nanotubes: A general strategy for the formation of 1D metal telluride nanostructures

The transformation of Te nanotubes to CoTe₂ nanotubes have been achieved through a solvothermal process in a mixed solvent of ethylene glycol (EG) and oleic acid (OA) at 200 °C for 24 h. Single crystalline Te nanotubes generated in situ could serve as the Te source and template, and then transformed as self-assembly CoTe₂ nanotubes. In the unique reaction system, CoTe₂ nanotubes were obtained when the volume ratio of EG and OA (V_EG/V_OA) was 5/3. The facile approach was extended to prepare other 1D telluride nanostructures, including CdTe, PbTe, Sb₂Te₃ and Bi₂Te₃.     

Synthesis and characterization of ilmenite NiTiO3 and CoTiO3 prepared by a modified Pechini method

Powders of ilmenite structure NiTiO₃ and CoTiO₃ were prepared by a simple method based on the modified Pechini process. The raw compounds and citric acid (CA) were mixed in ethanol (EA) with the molar ratio Ni(Co)/Ti/CA/EA = 1/1/1/7.5. The DTA curve shows exothermic peaks only around 300-350 °C and 600 °C, which correspond to the decomposition of the organic compound and direct crystallization of the ilmenite phase. X-ray diffraction patterns indicated that the ilmenite phase was successfully synthesized as the Ni(Co)-Ti precursor calcined above 600-900 °C for 3 h, and the activation energies of NiTiO₃ and CoTiO₃ were calculated to be about 8.84 and 13.23 kJ/mol. TEM bright field images showed that the grain sizes of powders of NiTiO₃ and CoTiO₃ at 600-900 °C were estimated to be about 10-250 and 20-200 nm depending on the nature of the aggregate. The samples of NiTiO₃ calcined at 600-800 °C have the larger specific surface area of 31.51, 18.78, and 6.01 m²/g, respectively. The UV-Vis diffuse reflectance spectra show the optical band gaps of NiTiO₃ and CoTiO₃ as 3.02 and 2.43 eV.     

Physical properties of self-flux and WO3-containing solutions useful for growing type III KGd(PO3)4 single crystals

Density, surface tension and dynamic viscosity of self-flux and WO₃-containing solutions useful for growing type III KGd(PO₃)₄ single crystals have been measured at temperature near the saturation temperature. The thermal behavior of these physical properties has also been studied. Solutions containing WO₃ show a higher density than self-flux solutions and the density decreases linearly when the temperature increases, in the two cases. Near the saturation temperature, self-flux solutions present a surface tension slightly lower than that of WO₃-containing solutions. The dynamic viscosity of WO₃-containing solutions is slightly lower than that of self-flux solutions when this property was measured at the same temperature. We observed that, in WO₃-containing solutions, the saturation temperature is lower than in self-flux solutions. Thus, at the growth temperatures, the two solutions present dynamic viscosities only slightly different, so we expect that the introduction of WO₃ up to 10 mol% in the growth solution does not represent any important improvement in its hydrodynamics and this small change does not compensate for the possibility of introduction of tungsten impurities in the crystal structure affecting the physical properties expected for these crystals. Taking into account the values measured for these physical properties, we choose the initial conditions for growing type III KGd(PO₃)₄ single crystals from self-flux solutions.     

Synthesis and characterization of visible light responsive N-TiO2 mixed crystal by a modified hydrothermal process

N doped TiO₂ with anatase and rutile mixed crystal were prepared by using tetrabutyl titanate as the precursor via a modified hydrothermal process and calcination at 320 °C. The microstructure and morphology of samples were characterized by XRD, UV-vis-DRS, FTIR and XPS. The results showed that N-TiO₂ particles were crystallized to anatase and rutile mixed crystal structure; they were presented narrow particle size distribution, and the average particle size was ca. 13.5 nm calculated from XRD results. It was found that the N-doped TiO₂ particles showed strong visible-light absorption and high photocatalytic activity for the mineralization of Rhodamine B under irradiation by visible light (400-500 nm). The high visible-light photocatalytic activity of the obtained N-doped TiO₂ might result from the synergetic effect of nitrogen doping and the mixed lattice structure of N-TiO₂. Possible mechanism of N-TiO₂ mixed crystal formed under hydrothermal conditions was discussed.     

Bulk crystal growth of congruent MgO-doped LiNbO3 crystal with stoichiometric structure and its second-harmonic-generation properties

A new LiNbO₃ bulk crystal has been grown by doping with MgO (cs-MgO:LN; Li₂O:Nb₂O₅:MgO=45.30:50.00:4.70, (Li_0.906Mg_0.047V^Li_0.047)NbO₃), which successfully has the congruent point coinciding with the stoichiometric point. Its second-harmonic-generation (SHG) properties were evaluated. It was found that cs-MgO:LN has a much more homogeneous composition leading to uniform in-plane distribution of the non-critical phase-matching wavelength than the conventional LiNbO₃ crystals such as congruent LiNbO₃ (c-LN), stoichiometric LiNbO₃ (s-LN), and MgO-doped congruent LiNbO₃ (5MgO:LN). This homogeneity arose from the observation that none of the solute components including ionic species were segregated at the interface during growth. The SHG conversion efficiency of cs-MgO:LN is comparable to those of s-LN and 5MgO:LN.     

Computational study on transamination of alkylamides with NH3 during metalorganic chemical vapor deposition of tantalum nitride

DFT calculations were employed to investigate transamination during metalorganic chemical vapor deposition (MOCVD) of transition metal nitrides films, such as titanium nitride (TiN) and tantalum nitride (TaN). The calculated energetics and rate constants for the ligand exchange of tert-butylimidotris(dimethylamido) tantalum (TBTDMT) with NH₃ demonstrated that NH₃ addition to form the ammonia adduct, TBTDMT·NH₃, proton transfer and dissociation of dimethylamine to afford net transamination of the dimethylamido ligand are facile even at low temperature (∼300 °C). The transamination of the tert-butylimido ligand, however, was relatively slow at those temperatures but became facile at temperatures appropriate for CVD growth (∼600 °C). Rapid transamination is consistent with lower temperature for growth of TaN by MOCVD in the presence of NH₃, efficient removal of carbon-containing ligands, and incorporation of higher levels of nitrogen in the resulting films.     

Layer-by-layer growth and growth-mode transition of SrRuO3 thin films on atomically flat single-terminated SrTiO3 (1 1 1) surfaces

We report on growth-mode transitions in the growth of SrRuO₃ thin films on atomically flat Ti⁴⁺ single-terminated SrTiO₃ (1 1 1) substrates, investigated by reflection high-energy electron diffraction and atomic force microscopy. Over the first ~9 unit cells, the dominant growth mode changes from island to layer-by-layer for the growth rate of 0.074 unit cells/s and the growth temperature of 700 °C. Moreover, in the course of growing SrRuO₃ films, the governing growth mode of interest can be manipulated by changing the growth temperature and the growth rate, which change allows for the selection of the desired layer-by-layer mode. The present study thus paves the way for integrations of SrRuO₃ thin layers into (1 1 1)-orientated oxide heterostructures, and hence multi-functional devices, requiring control of the sharp atomic-level interfaces and the layer-by-layer growth mode.