Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to 7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

Electromechanical coupling properties of [001], [011] and [111] poled Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystals

The electric field induced “butterfly” curves and polarization loops, and the stress induced strain and polarization responses of [001], [011] and [111] oriented Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32 PT) relaxor ferroelectric single crystals have been systematically investigated by experiment study. The focus is on the effect of constant compressive bias stress on the electromechanical coupling behavior along three crystallographic directions of PMN-0.32 PT single crystals. Dependence of the coercive field, remnant polarization, dielectric constant, and piezoelectric coefficient on the bias stress has been quantified for PMN-0.32 PT single crystals oriented in three different directions. Obtained results show that the large piezoelectric responses under zero compressive stress in [001] and [011] orientation are dominated by intrinsic crystal lattice while the engineered domain structure has a relatively minor effect. It is found that observed responses under stress cycle for [001] oriented crystals are due to polarization rotation and phase transformations. However, those for [011] and [111] oriented crystals are due to domain switching. The “butterfly” curves and polarization loops driven by electric field under different bias compression are described by two non-180° domain switching.     

Czochralski single crystal growth of Sr.61Ba.39Nb2O6 for surface acoustic wave applications

Ferroelectric Sr_.61Ba_.39Nb₂O₆ (SBN) single crystals approximately 2 cm. in diameter and 5–6 cm. long have successfully been grown by the Czochralski technique. All the crystals were grown in the [001] direction in argon or oxygen, and it was found that the degree of difficulty of growth increased as the diameter of the crystals increased. Temperature dependence measurements on the poled Z-cut SBN crystals showed that this composition has temperature compensated orientations which make this material useful for surface acoustic wave devices.     

Hydrothermal recrystallization of Bi2WO6

The feasibility of growing single crystal Bi₂WO₆ by the hydrothermal method is reported. Single domain crystals (～1mm) were readily grown at 400°C, well below the disruptive high temperature phase transformation, from 2N aqueous KF solutions. Data is given for X-ray diffraction and hot-stage microscopy observations. The significance of preparing strain-free, single domain, highly acentric polar crystals for new device applications is discussed.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Crystal Growth and Electrical Properties of β-CdP<Subscript>2</Subscript> Single Crystals

Optically homogeneous single crystals of tetragonal cadmium diphosphide (β-CdP₂) are prepared by vapor-phase growth, and their electrical conductivity, dielectric permittivity, and loss tangent are measured during slow heating from 78 to 400 K in the [001] and [100] directions at frequencies of 10², 10³, 10⁴, and 10⁶ Hz. The permittivity of the crystals increases with temperature. The electrical properties of β-CdP₂ are shown to be anisotropic.     

Influence of orientation,temperature, and strain-rate on the yield strength of Fe2B single crystals

The deformation behaviour of Fe₂B single crystals with different orientations has been studied by the compression test over the temperature range 800 to 1100° C. Three slip systems (1 1 0) [0 0 1], (1 1 0) [1¯1 1], and (hkl) [1¯11] have been found to be active in the present work. The operation of (h k l) [1 ¯1 1] slip system appears to require about ten times higher stress than those of (1 1 0) [0 0 1] and (1 1 0) [1 ¯1 1] slip systems. The temperature and strain-rate dependence of the yield stress suggests that the Peierls mechanism controls the plastic deformation of Fe₂B crystals. The activation energy for the thermally activated deformation of Fe₂B varies between 6 and 10 eV and the activation volume between 10b ³ and 160b ³. The remarkably high activation energies have been discussed in connection with the large Burgers vectors of Fe₂B.     

Vapor growth of hexagonal GeO2 needle crystals

Needle crystals of haxagonal GeO₂ were grown at 750–900°C by the oxidation of GeO vapor which was generated by the reduction of GeO₂ with graphite. The largest crystals were grown at higher temperature, and had a maximum size of 10 μm diameter and 6 mm length after growth at 900°C for 5 hours. The growth direction of the crystals was parallel to the a-axis.     

Spatial inhomogeneity of ferroelectric domain structure in Pb(Mg1/3Nb2/3)O3–30%PbTiO3 single crystals

Piezoresponse force microscopy studies of domain structures have been performed on (001)-oriented Pb(Mg_1/3Nb_2/3)O₃–30%PbTiO₃ single crystals with a rhombohedral ferroelectric state. Regular (narrow strip-like) and irregular (fingerprint-like) domain patterns were clearly observed on the different surfaces of the unpoled samples. The results reveal that the random internal field due to nanoscale structure irregularity plays an important role in the spatial inhomogeneity of ferroelectric domain structures in PMN-PT single crystals.     

Effect of orientation on the high-temperature superelasticity in Co49Ni21Ga30 single crystals

The temperature interval ΔT _SE of superelasticity in [001]-, [$ \bar 1 $ \bar 1 23]-, and [$ \bar 1 $ \bar 1 24]-oriented Co₄₀Ni₂₁Ga₃₀ (at. %) single crystals strained at compression has been studied. It is established that ΔT _SE in the [001]-oriented single crystal amounts to 441 K and the reversible B2-L1₀ martensite transformations in loaded samples take place at T ₂ = 698 K. In [$ \bar 1 $ \bar 1 23]- and [$ \bar 1 $ \bar 1 24]-oriented samples, ΔT _SE decreases to 233 K and the superelasticity is observed up to T ₂ = 523 K.     

Textured mullite at muscovite–kaolinite interface

Mullite crystallization was carried out by the inter-reaction of alternate layers of muscovite and kaolinite minerals. The nucleation and growth of mullite anisotropic crystals take place along the muscovite plane and specific structural relationships are observed, which confirm a topotactic effect with the high temperature form of muscovite. The [001]_mull axis is oriented parallel to [010]_musc, [310]_musc and axes. The mullite orientation is fully completed in a temperature range between the ternary eutectic at 985 °C and the ternary transition point at 1140 °C, of the SiO₂–Al₂O₃–K₂O system, which strongly suggests an influence of a small quantity of liquid phase at the interface. Along the kaolinite–muscovite interface, the realisation of highly textured ceramics can be achieved.     

Growth and characterizations of Ba2Ti2Fe2As4O single crystals

Abstract

Single crystals of a new iron-based superconductor Ba₂Ti₂Fe₂As₄O have been grown successfully via a Ba₂As₃-flux method in a sealed evacuated quartz tube. Bulk superconductivity with T_c ～ 21.5 K was demonstrated in resistivity and magnetic susceptibility measurements after the as-grown crystals were annealed at 500 °C in vacuum for a week. X-ray diffraction patterns confirm that the annealed and the as-grown crystals possess the identical crystallographic structure of Ba₂Ti₂Fe₂As₄O. Energy-dispersive x-ray spectra indicate that partial Ti/Fe substitution exists in the [Fe₂As₂] layers and the annealing process redistributes the Ti within the Fe-plane. The ordered Fe-plane stabilized by annealing exhibits superconductivity with magnetic vortex pinned by Ti.     

Elaboration et caracterisation de monocristaux de V2O3 par transport en phase vapeur

Single crystals of V₂O₃ are obtained by HCl or TeCl₄ chemical transport method in sealed quartz tubes with temperature gradients of 1050–930°C and 990–900°C respectively. The hexagonal prims are 5 mm long and 1 or 2 mm wide. Both kinds of crystals are characterized by sharp insulator-metal transition at 156 and 137 K respectively, they present a large hysteresis. The sharp decrease in resistivity with increasing temperature is of the order of 10⁸ at the transition.     

Processing and ferroelectric behavior of textured KSr2Nb5O15 ceramics

<001>-textured KSr₂Nb₅O₁₅ (KSN) ceramics were fabricated in a matrix of SrNb₂O₆ and KNbO₃ powders via reactive templated grain growth (TGG), using <001>-oriented acicular KSN particles as the template for grain growth. KSN phase formation started at 1100 °C and was complete at 1300 °C. Template particles grew at the expense of matrix grains with increasing sintering time and temperature, resulting in improved texture fraction, f. A maximum fraction of 0.98 was obtained in 1.5 wt% Nb₂O₅ and 10 wt% template containing samples sintered at 1450 °C for 6 h. A template content of 10 wt% resulted in a high quality of texture with a narrow distribution of elongated grains in [001]. The highly textured ceramics (f ≥ 0.9) had a remanent polarization of 0.19 C/m² in the polar c direction, [001], as compared to 0.03 C/m² in the non-polar a or b direction and 0.04 C/m² as for the random sample. The estimated saturation polarization (P _sat) was 0.25 C/m² which is comparable to reported KSN single crystal value of 0.25 C/m². This indicates that TGG is a viable method to fabricate textured ceramics having ferroelectric properties close to the single crystals.     

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.     

High-temperature superelasticity during B2-L1<Subscript>0</Subscript> martensite transformations in Co<Subscript>40</Subscript>Ni<Subscript>33</Subscript>Al<Subscript>27</Subscript> crystals

The temperature interval ΔT _SE of superelasticity in [001]-oriented Co₄₀Ni₃₃Al₂₇ (at. %) single crystals strained by tension and compression has been studied. It is established that ΔT _SE in tension amounts to 220 K, and the reversible B2-L1₀ martensite transformations in loaded samples take place at 590 K. In the samples strained by compression, ΔT _SE decreases to 105 K, and the superelasticity is observed up to 420 K.     

Microdurete de monocristaux de V2O5

Microhardness measurements of V₂O₅ single crystals were undertaken in order to study the anisotropy of their mechanical properties. Results are presented for both Vickers and Knoop hardness values on (100), (010) and (001) planes of orthorhombic V₂O₅ single crystals.A strong anisotropy is observed in each plane. As expected, hardness appears to be particularly small when the diamond indenter is parallel to the clivage directions in the (100) and (001) planes. A remarkably small value is observed along the [100] direction in the (001) plane, which could be related to the model proposed by Gillis in order to explain the structural charge into V₆O₁₃ occuring in non-stoichiometric V₂O₅.     

Dislocation structures of low-angle boundaries in Nb-doped SrTiO3 bicrystals

Dislocation core structures in low-angle boundaries of Nb-doped SrTiO₃ bicrystals were investigated by high-resolution electron microscopy. Bicrystals with tilt angles of 2°, 4°, 6° and 8° with respect to the [001] zone axis were prepared by joining two single crystals at 1873 K. All of the boundaries consisted of a regular array of dislocations whose spacing gradually decreased with an increase in tilt angle. Except for the 2° tilt-angle boundary, the dislocation cores exhibited a dissociation from a[010] into two partials of a/2[010] on (100). Furthermore, two kinds of dislocation core structures were observed; Sr–Sr atomic columns and Ti–O atomic columns inside the cores. In addition, it was found that the positioning of adjacent cores along the boundary tended to change from a linear form to a zig-zagg shape as the tilt angle was increased from 4° to 8°. In the case of the linear array, dislocation core structures including Sr–Sr columns or Ti–O columns alternately appear. In contrast, only one core structure was observed in the zig-zagged array. On the other hand, the dislocation cores in the 2°-tilt-angle boundary had another type of dissociation with a/2[110] or a/2[111] partials, which included the twist component at a tilt axis of [001].     

Epitaxial growth of ferromagnetic CrO2 films in air

Epitaxial growth of CrO₂ films under atmospheric pressure has been investigated. Single crystal films of CrO₂ were obtained by the thermal decomposition of gaseous CrO₃ onto the substrates of rutile single crystals in air. The optimum temperature of the substrate for the pure CrO₂ epitaxial films was found to be 390°C. At the substrate temperature of 380°C, the obtained film included Cr₂O₅ as impurities, and Cr₂O₃ appeared at 400°C. Magnetic domain patterns of these films were observed by longitudinal Kerr effect. The growth patterns of domain were obtained with applied field.     

Electrical conductivity in Ta2O5

The defect structure of undoped polycrystalline Ta₂O₅ was investigated by determining the temperature [850–1050°C] and oxygen partial pressure [10⁰–10^?19 atm.] dependence of the electrical conductivity. The data were found to be proportional to the $\text{～?}\text{1}\text{4}\text{th}$ power of the oxygen partial pressure for the oxygen pressure range <10^?8 atm. and independent of the oxygen partial pressure for P_O2 > 10^?6 atm. The enthalpy of formation of doubly ionized oxygen vacancies plus two electrons is estimated to be 118.31 Kcal/mole [5.13 eV]. The observed conductivity data are explained on the basis of the presence of unknown acceptor impurities in the undoped samples.