Single crystal growth in PMN-PT and PMN-PZT

Single crystal growth of lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN-32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN-37PT-21PZ) ceramics via templated grain growth (TGG) was investigated. (001)- and (111)-oriented BaTiO₃ (BT) single crystals and (001)-oriented SrTiO₃ (ST) single crystals (of approximately 2.5 × 2.5 × 1 mm) were utilized as seeds for the growth experiments. The piezoelectric single crystals were produced in a process that involves at first hot pressing of single crystal in cold isostatically pressed ceramics followed by subsequent sintering of the samples. Growth of (001)-oriented single crystals with BT seeds was observed in both PMN-32PT and PMN-37PT-21PZ matrices. The measured growth lengths were up to 140 and 65 μm, respectively. The grown (001)-oriented single crystals grown were rectangular. The measured growth lengths of the pyramidal-shaped (111) BT single crystals were up to 1 mm, which is much larger than the growth lengths of the (001) single crystals. Experiments on (001) ST-seeded single crystals were not successful. No single crystal growth was observed due to the dissolution of the ST single crystals in the PMN-PZT matrix. The differences were explained by defect-chemical considerations.     

Single crystal growth of organic photoconductors: phthalocyanine

An effective method of growing single crystals of organic photoconductors such as phthalocyanine in the presence of doping impurity such as iodine by vacuum sublimation is discussed in this paper. This method is very useful especially when an organic material does not have a melting point but decomposes above a particular temperature. So far, doping has been done by exposing the previously grown organic single crystals to the dopant vapour, but growing the crystal in the presence of dopant vapour makes the doped-crystal more efficient and stable. The photosensitivity as well as the photoconductivity of the doped crystals by this method increases significantly.     

Single crystal growth and electrical properties of TlTe

Single-crystals of TlTe were prepared by Czochralski and horizontal zone melting methods. Measurements of the electrical resistivity and Hall effect between 2,66 K and 295 K were carried out on TlTe samples with different residual resistivity ratios. The results indicate that no discontinuities in the resistivity and Hall-coefficient data are present in this temperature range.     

Single crystal growth and structure of LaOBr and SmOBr

Single crystals of LaOBr and SmOBr were grown from a flux of the corresponding anhydrous rare earth bromide by slow cooling. The crystals were obtained as thin platelets with dimensions up to 5×5×0.1 mm. In the case of LaOBr the formation of needle-like crystals (1×0.1×0.1 mm) was also observed. The crystal structures of LaOBr and SmOBr were determined from single crystal X-ray diffraction data. They both crystallize in the PbFCl structure with z(La) = 0.16206(6)z(Br) = 0.6352(2) (LaOBr) and z(Sm) = 0.1464(4) z(Br) = 0.6550(7) (SmOBr).     

Single crystal growth,structure and properties of TlHgBr3

High-quality inclusion-free single crystals of ternary thallium mercury bromide, TlHgBr₃, were successfully grown by Bridgman–Stockbarger method. For the pristine surface of the TlHgBr₃ single crystal, X-ray photoelectron core-level and valence-band spectra were measured. The comparison on a common energy scale of the X-ray photoelectron valence-band spectrum of TlHgBr₃ and the X-ray emission Br Kβ₂ band, representing peculiarities of the energy distribution of the Br 4p states revealed that the main contribution of the valence Br p states, occurred in the upper portion of the valence band, with also their significant contributions in other valence band regions. It has been determined that TlHgBr₃ is a semiconductor with the bandgap energy value of E_g = 2.51 eV at 100 K. The E_g value decreased up to 2.44 eV when temperature increased to 300 K.     

Single crystal growth of the transition metal monoxides by skull melting

A modified Bridgman technique by crucibleless skull melting has been applied to the growth of equiaxed, cmsized Mn_1?xO, Co_1?xO and Fe_1?x single crystals in f_o2 buffered atmospheres. The use of COCO₂ gaseous mixtures, suitable for growth and subsolidus annealing, has been investigated in order to optimize homogeneity, as indicated by X-ray diffraction and polarized reflected light microscopic characterization.     

Single crystal growth of cadmium manganese selenides with controlled carrier concentration

Single crystal boules of Cd_1?xMn_xSe with x = 0.01, 0.05, and 0.10 have been grown by a modified Bridgeman method using 5% excess selenium to lower the melting point. A technique has been developed for the growth of high quality, single grain boules with demonstrated uniform manganese distribution. Free carriers have been introduced in a controlled fashion by adding gallium to the melt and annealing crystal slices in cadmium vapor. Reproducible carrier concentrations from 2 × 10¹⁷ to 2 × 10¹⁸cm^?3 have been achieved.     

Single crystal growth and characterization of LaLuO3

Wide shoulder and fast lateral growth are characteristics of LaLuO₃ during crystallization onto an iridium rod in Czochralski's configuration. Under careful temperature control, necking in and further low-speed pulling provided for single-crystal growth, but spontaneous separation of the growing crystal from the melt is a strongly limiting factor for production of large boules. Ce- and Pr-doped single crystals of up to 0.5 cm³ were obtained and used in measurements of the X-ray data, density, melting point and optical properties and in annealing studies.     

Single crystal growth of Mn2N using an In-Na flux

Hexagonal platelet and pyramidal single crystals of Mn₂N were prepared by using an In-Na flux at 700 °C and 5 MPa of N₂ pressure. The crystal structure (space group P6₃/mmc) with lattice parameters a=2.8185(6) Å and c=4.5371(12) Å was confirmed by X-ray single-crystal diffraction. Nitrogen atoms statistically occupy a half of the octahedral interstitial sites in a hexagonal closest packing arrangement of Mn atoms.     

Single crystal growth and characterization of nearly stoichiometric LiVO2

LiVO₂ undergoes an imperfectly understood orbital ordering transition near 500 K resulting in a loss of magnetic moment below the transition. Studies of the transition have been hampered by a lack of high-quality stoichiometric single crystals. Here we report the growth and basic characterization of large, nearly stoichiometric LiVO₂ single crystals. The crystals were characterized by magnetic susceptibility, electrical resistivity, differential scanning calorimetry, and specific heat measurements over a temperature range from 2 to 650 K. A first-order phase transition with large hysteresis near T_t≈500 K was observed in all measurements. An anisotropy of the order of 100 was observed in the in-plane versus out-of-plane resistivity, and the inferred semiconducting energy gap was 0.18 eV for T<T_t and 0.14 eV for T<T_t. Electron diffraction experiments were performed on LiVO₂ single crystals at temperatures below and above T_t. Superlattice reflections were observed below T_t and disappeared upon heating above the phase transition temperature. Upon cooling below T_t, the supperlattice spots reappeared. Bright field electron micrographs indicate that the crystals develop a roughly hexagonal network of cracks.     

Single crystal GaN nanowires

Single crystal gallium nitride nanowires have been obtained by heating gallium acetylacetonate in the presence of carbon nanotubes or activated carbon in NH3 vapor at 910 degrees C. GaN nanowires also were obtained when the reaction of gallium acetylacetonate with NH3 was carried out over catalytic Fe/Ni particles dispersed over silica. The former procedure with carbon nanotubes is preferable because it avoids the presence of metal particles in the nanowire bundles.     

Single crystal growth of α-Bi2O3

Single crystals of α-Bi₂O₃ were grown by the pulling method from melts containing 6.25 mol % of Li₂O. The addition of Li₂O to Bi₂O₃ melt lowers the melting point below the cubicmonoclinic phase transition point and makes it possible to obtain crack-free α-Bi₂O₃ single crystals. The purity of starting materials has great influence on the quality of crystals. The crystal is optically positive and the refractive indices are n_x=2.574, n_y=2.659 and n_z=2.786 at the wavelength of 0.589 μm.     

Single crystal cleavage of brittle materials

Cleavage of brittle single crystals is reviewed and the historical criteria for the phenomenon are critically examined. Previously proposed criteria, including those based on crystal structure (crystal growth planes, the planes bounding the unit cell, and planar atomic packing) and crystal properties (ionic charge of possible cleavage planes, bond density, elastic modulus, and surface free energy), are found to be applicable only to particular crystals or to isostructural groups, but each lacks universal application. It is concluded that the fracture toughness (K _Ic) of the crystallographic planes is the most appropriate criterion. Measurements reveal that the cleavage toughnesses of brittle single crystals are usually about 1 MPa m^1/2 or less.Measurements of the fracture toughnesses of brittle polycrystalline aggregates are then compared to the single crystal cleavage values in those instances where reliable results are available for the same crystal structures. Polycrystalline toughnesses are consistently higher, in part because of the lack of continuity of cleavage cracks through the polycrystalline aggregates. However, the increment of toughness increase is only 1–2 MPa m^1/2. The role of grain texture or preferred crystal orientation is also addressed. It is concluded that polycrystalline aggregate toughnesses are often highly anisotropic and that the values for intensely oriented microstructures may approach those for single crystal cleavage.     

Single crystal elastic constants of tungsten monocarbide

The elastic constants of single crystal tungsten monocarbide were determined using highfrequency (20 to 50 mHz) ultrasonic pulse-echo measurements. The measured values from this study are a full order of magnitude lower than the previously reported values estimated from X-ray elastic constants. The elastic modulus of polycrystalline tungsten carbide was also measured to verify the accuracy of the ultrasonic method used.     

Single crystal diamond probes for atomic-force microscopy

Results obtained in the development and testing of high-strength, chemically inert, and sharply pointed single crystal diamond probes for atomic-force microscopy are presented. The probes were fabricated on the basis of pyramidal diamond single crystals produced by selective oxidation of polycrystalline films grown by chemical vapor deposition. A procedure was developed for attachment of single needles to cantilevers of silicon probes. A transmission electron microscope was used to find that the apical angle of the pyramidal diamond crystallites is about 10° and the radius of curvature of the apex of the diamond crystallite is 2–10 nm. It is shown for the example of two test samples (graphite surface and DNA molecules) that the diamond probes can be effectively used in atomic-force microscopy and make it possible to improve the image quality compared with standard silicon probes.     

Floating-zone crystal growth of WC

Single crystals of tungsten carbide were grown by the floating-zone technique by adding boron in the molten zone. It is confirmed that WC decomposes into cubic WC_1–x and carbon at an elevated temperature and cannot coexist with the liquid which consists of only tungsten and carbon. It was theoretically expected that addition of boron would stabilize the WC structure so that the WC phase could coexist with the liquid. This was proved by the float zoning through the WC rod which contained boron. The WC phase was deposited from the molten zone which consists of tungsten, carbon and boron. The growth of single crystals of WC was carried out under pressurized helium gas atmosphere of 1 M Pa. The molten zone, whose temperature was about 2900°C, was inductively heated and driven downwards. The compositions of the molten zone and the feed rod were [W]/[C]/[B] = 1/0.65/0.06 and 1/1.08/0.007 (atomic ratio), respectively. Single crystals with dimensions 9 mm diameter and 5 mm long can be cut out from the zone-passed rod. The boron content in the crystal could be lowered to the order of 100 p.p.m.