Electrical resistance measurements under pressure on NbTe2 single crystal

Transition metal dichalcogenides, because of their layered structure, are well suited for extreme pressure lubrication. These materials being semiconducting and of layered structure may undergo structural and electronic transitions under pressure. Here we report the details of the preparation and characterization of single crystals of NbTe₂ and the results of electrical resistance measurements under pressure carried out on it to investigate this possibility. Single crystals were grown by the chemical vapor transport technique, using iodine as a transporting agent. The composition of the grown crystals was confirmed on the basis of Energy Dispersive Analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray Diffraction (XRD) studies. Electrical resistance was measured employing a Bridgman anvil set up to 10?GPa and diamond anvil cell (DAC) assembly up to 25?GPa. A technique slightly modified from that described in the literature for carrying out electrical resistivity measurements in the diamond anvil cell (DAC) under pressure has been standardized.     

Tellurium recrystallization under microgravity conditions and the resulting properties of samples

Three experiments on the tellurium recrystallization by a modified Bridgman method were performed under microgravity conditions on board the Mir orbital space laboratory using a ChSK-1 Kristallizator furnace. The physical properties of samples were studied, including the final crystal structure, the distribution of impurities and defects, and the charge carrier concentration and mobility. The results were compared to the analogous parameters of crystals remelted using the same method under the normal gravity conditions. It is established that the samples recrystallized in a close volume under the on-board microgravity conditions “break off” from the container walls and touch the walls only in a few points. This circumstance gives rise to special effects, such as the growth of crystals with a free surface and deep supercooling. Study of the distribution of electrically active impurities over the length of ingots shows evidence of the presence of thermocapillary convective flows in the melt under the microgravity conditions. The flows tend to increase upon separation of the melt from the container walls. The contributions due to impurities and electrically active structural defects to the charge carrier distribution are taken into account. The single-crystal sample obtained upon the partial recrystallization of tellurium in a close container volume under the on-board microgravity conditions exhibits the electrical characteristics comparable to those of a crystal grown by the Czochralski technique under the normal gravity conditions.     

New OSL detectors based on LiMgPO4 crystals grown by micro pulling down method. Dosimetric properties vs. growth parameters

Rod-shape crystals of the LiMgPO₄ compound were grown by micro pulling down technique under the different growth conditions. Influence of the different growth rates, thermal setups and gaseous atmospheres on the crystals dosimetric properties was investigated. Samples were irradiated with ⁹⁰Sr/⁹⁰Y β particles and optically stimulated luminescence spectra were measured with the automatic Risø TL/OSL-DA20 reader. The sensitivity level, repeatability, dose–response dependence and short-time fading were compared for all grown crystals. It was found that the crystal grown from the iridium crucible was about three times more sensitive to radiation as compared to the crystal grown from the graphite crucible. Also the radio-sensitivity measured for the crystals grown from the graphite crucible was higher in case of higher growth rates. It was also shown that the residual OSL signal measured one and two weeks after the irradiation was higher for the crystals grown with higher growth rates. There was also no correlation observed between the growth conditions and the level of dose–response nonlinearity. Over the studied dose range the response nonlinearity of the studied samples fluctuated around over a dozen percent, regardless of the applied growth parameters. The obtained results tend to suggest that LiMgPO₄ crystals may be considered as promising dosimeters in different fields of research.     

Metastable phases in vapour-quenched Al78Cu22 film

The structure and structural changes of an Al₇₈Cu₂₂ (at%) alloy film produced by coevaporation onto a cooled substrate have been investigated with transmission electron microscopy. A series of metastable phase transitions were observed during the crystallization from the as-deposited amorphous phase to the final equilibrium state. The deposit structure at room temperature consists of a mixture of fine grained α' crystallites (fcc, α = 4.81 Å) with an amorphous phase, and some welldefined crystals with an fcc lattice (a = 7.18 Å, namely α₁). The subsequent annealing led to the grown crystals transforming to an ordered α′ phase. It is argued that the development of such a transition is the result of a gradual atomic rearrangement – first topologically then a chemically predominant disorder-order transition.     

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals grown under various conditions of heat and mass transfer are studied by methods of X-ray topography, high-resolution X-ray diffractometry, and digital image processing. It is established that the inhomogeneity of crystals is determined by specific features of impurity microsegregation during growth under conditions of nonstationary convection in a melt and by peculiarities of the dislocation structure of crystals. The processes related to the initial stage of the decay of the Si supersaturated solid solution in GaSb contribute considerably to the inhomogeneity of crystals on the micro- and macrolevels.     

Study of ferromagnetism in Co-doped rutile powders and float-zone grown single crystals

Co-doped rutile samples in the form of both powders and bulk single crystals have been studied with particular emphasis on the dependence of their magnetic, compositional and structural properties upon the type of atmosphere used during their preparation. Both powders and single crystals were characterized using X-ray diffractometry and vibrating sample magnetometry, while the crystals were also studied using the X-ray Laue technique, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that an oxygen deficient environment during the preparation of Co-doped TiO₂ powders is crucial for the observation of room temperature ferromagnetism, while preparation in oxygen rich conditions destroys the ferromagnetism due to the formation of the paramagnetic second phase CoTiO₃. Floating zone growth of crystals under oxygen also led to the formation of material containing second phase CoTiO₃ that was paramagnetic at room temperature, while crystals grown under argon were ferromagnetic and contained Co-rich inclusions.     

Dependence of superconducting properties of underdoped Bi-2201 single crystals on the charge composition and growth conditions in gas-filled cavities

High-quality Bi_{2 + x} Sr_{2 ? y} CuO_{6 + δ} (Bi-2201) single crystals with the ratio Bi/Sr = 1.4–2.0 were grown by the free growth method in gas-filled cavities in KCl solution-melt in a range of doping levels, which provides variation in superconducting properties from insulators to optimally doped crystals. The charge composition Bi : Sr : Cu = 1.7 : 2.3 : 2.5 with excess Sr and Cu and synthesis conditions provided growth cavity formation in the KCl solution at the crystal growth stage. Lamellar single crystals and whiskers were grown under quasi-equilibrium conditions of lowered growth temperatures and partial oxygen pressure, achieved in closed gas-filled cavities.     

Investigation of the structure,physical properties,and phase transition in SrAlF<Subscript>5</Subscript>

Crystals of SrAlF₅ have been grown by the Bridgman method from the melt and by sintering of the components. Optical polarization studies and measurements of the thermal expansion and birefringence coefficients have been carried out over a wide temperature range. The electromechanical coefficient d ₃₃ has been measured, and the optical second harmonic, dielectric hysteresis loop, and optical quality of the crystal have been assessed. X-ray diffraction investigations have been performed to identify the revealed compounds. It has been demonstrated that the SrAlF₅ crystals obtained under the growth conditions chosen have I4₁/a symmetry and do not undergo structural phase transitions in the temperature range 100–800 K. Crystalline inclusions of the AlOF oxyfluoride have been revealed in crystals grown with an AlF₃ excess. The birefringence of the AlOF crystal is an order of magnitude higher than that of SrAlF₅, does not depend on temperature, and has no anomalies up to 800 K.     

Deep level structure and compensation mechanism in In-doped CdTe crystals

Deep Level Transient Spectroscopy (DLTS) and Optical Deep Level Transient Spectroscopy (ODLTS) experiments have been conducted on a series of In-doped CdTe crystals grown by the Bridgman or the travelling heater (THM) methods using Te as the solvent. The THM samples are n-type but strongly compensated. Annealing at 700°C under high Cd vapour pressure leads to a decompensation of the crystals. The electron concentration is then a measure of the donor (In) concentration, which was in the range 3 × 10¹⁶−1.5 × 10¹⁸cm⁻³. Six and eight electron traps are, respectively detected in the non-annealed and annealed samples at a concentration level 10²–10³ times below the net electron concentration. They cover the energy range 0.2–0.8 eV. Similar traps are found in both types of crystals, the concentration of which increases with In content and after annealing. The presence of In interstitial-type defects is suggested. A main hole trap at 0.12eV is detected by ODLTS in compensated samples with a concentration close to the donor concentration. Low temperature electrical measurements show that this trap is ionized under equilibrium conditions. It appears to be the main compensating acceptor centre. A plausible microscopic structure is In_cdV_cd. This study shows that In-doped CdTe grown by THM is electrically compensated and that In-containing neutral associates or precipitates play a minor role in the compensation mechanism.     

Lamellar changes during heat treatment in isotactic polystyrene crystals

In the present investigation, the melting and thickening processes in lamellar crystals of isotactic polystyrene have been studied by transmission electron microscopy. It is shown that under properly chosen experimental conditions for the polymer, one can continuously follow the physical changes involved during the thickening as well as melting of lamellar crystals on heat treatment. The study of crystals grown at different temperatures reveals that melting of a single lamella starts at various areas. A commonly observed feature is the preferential melting of elastically bent parts of a lamella. It is indicated that the occurrence of melting in the various parts is due to a structural variation along the surface of lamellae resulting in a hindrance of the lamellar thickening process. At particular temperatures, melting of lamellar crystals is followed by recrystallization. The occurrence of a solid-stage thickening process is the major process so far observed during slow heat treatments. Considerable change in surface structure of the crystals grown at different temperatures is clearly reflected during the heat treatment. The rates of heating have marked influence on the resulting morphology of the crystalline superstructures.     

Problems, prospects, and alternatives for semiconductor single crystal growth in space

Experimental results for growing semiconductor single crystals under real microgravitation conditions aboard spacecraft have been analyzed. The causes for the formation of dopant distribution micro- and macroinhomogeneities in crystals have been studied. It has been shown that it is necessary to provide diffusion heat and mass transfer conditions in a melt to achieve a high homogeneity of properties in grown crystals. These conditions and expected optimal parameters of crystals can be obtained in the absence of thermogravitational convection, when a free surface of the melt is eliminated, and also under conditions of minimizing external quasi-static effects on the melt causing forced convective flows in them and, correspondingly, inhomogeneity of composition and properties for grown crystals due to the increasing gravitational sensibility of melts under microgravitation conditions.     

Generation modes of Eu<Superscript>2+</Superscript> nonequilibrium dimers and influence of a magnetic field on their reconstruction in NaCl: Eu crystals

Three generation modes have been experimentally revealed for impurity-vacancy Eu²⁺ dimers with the atomic structure determined by the spin-selective transitions to equilibrium configurations in a constant magnetic field of 5–15 T in NaCl: Eu crystals. It has been shown that, under conditions of linearly increasing mechanical stresses, the plastic strain of the NaCl: Eu crystals accelerates the aggregation of impurities and generates magnetically sensitive dimers several tens of times more rapidly than in the case of diffusion-controlled aggregation. Under conditions of linearly increasing strains, magnetically sensitive dimers arise as a result of the destruction of impurity precipitates by moving dislocations. The identification of dimers and the determination of their average spin in the process of spin-dependent reorganization of the atomic structure of the dimers have been performed using optical spectroscopy and SQUID magnetometry.     

Growth rate of potash alum crystals: comparison of silent and ultrasonic conditions

The influence of power ultrasound on the growth rate of potash alum was investigated. The experiments on growth of potash alum crystals were carried out in a stirred double jacket tank in silent conditions as well as in the presence of power ultrasound (20 kHz) at 32 degrees C, with different initial crystal sizes. It was observed that the mass growth rate of potash alum was faster under ultrasound compared to that under silent conditions. The shape was not modified by ultrasound but the size of crystals, which are grown under ultrasound, are smaller and with higher density compared to those grown under silent conditions.     

Synthesis, crystal growth and characterization of a metal-organic nonlinear optical tri-allylthiourea mercury chloride single crystals

Metal-organic nonlinear optical material tri-allylthiourea mercury chloride single crystal was grown using low temperature solution growth method. The growth parameters such as pH and temperature were optimized for getting colorless single crystals. The X-ray rocking curve measurement shows that the grown crystal was free from internal structural grain boundaries. The structural, spectral, optical and thermal properties of the grown specimen were studied using various characterization techniques. The defects present in the grown crystals were studied with chemical etching analysis. The results show that the grown crystals are suitable candidates for nonlinear optical applications.     

Photoluminescence and X-ray diffraction studies on Cu2O

Cuprous oxide (Cu₂O) crystals were grown by the two-step crystallization method in air atmosphere conditions from polycrystalline thin copper foils. The method comprises two stages; in the first one the copper plates are oxidized at 1020 °C by some hours in line with its initial thickness. In the second stage, the growth of large crystalline areas is promoted by annealing the Cu₂O samples at 1100 °C for long periods. Raman scattering an X-ray measurements demonstrates the existence of the single-phase Cu₂O. The effects on the crystalline structure and photoluminescence (PL) response were studied as a function of the conditions used in the second stage of the synthesis method. PL spectra were taken from 10 to 180 K to define the main radiative recombination paths. Besides the near band excitonic transitions, two strong emission bands at 720 and 920 nm associated with relaxed excitons at oxygen and copper vacancies were detected. Both excitonic-vacancy bond transitions presented similar intensities that are related to the growth method. X-ray and Raman scattering measurements help to assess the samples crystalline quality.     

Crystalline perfection, optical and dielectric studies on l-histidine nitrate: A nonlinear optical material

Single crystals of l-histidine nitrate (LHN), a recently investigated nonlinear optical material, were grown by conventional solution technique. Crystal structure and vibrational modes of the grown crystals were confirmed by powder X-ray diffractometry and FT-Raman spectrometry, respectively. Crystalline perfection of the grown crystals was evaluated by employing an in-house developed high-resolution X-ray diffractometer (HRXRD) and it was found that the grown crystals were free from structural grain boundaries and the perfection was reasonably good. However, HRXRD could reveal the fact that the crystals contain predominantly the interstitial point defects. The birefringence was measured over a range of wavelength between 5480 and 5630 Å and it was found that its value is nearly constant and 10 times higher than that of KDP. The optical band gap was found to be ∼3.73 eV. The photoluminescence excitation and emission spectra for single crystals were recorded. The SHG efficiencies of LHN samples of different particle sizes were measured by the Kurtz and Perry technique and they removed the ambiguity in the values reported differently in the literature. Dielectric properties were studied as a function of temperature over a wide range of frequency. The optical and dielectric studies along with the crystalline perfection reveal that the LHN crystal could be a good candidate for nonlinear optical devices.     

Phase transitions in crystals of protein amino acids with trapped drops of water solutions

It has been shown that the phase transitions observed at temperatures of about 250 K in crystals of protein amino acids and of their compounds grown from water solutions are related to drops of these solutions trapped by crystals in their growth.     

LaSrGaO4 substrate for epitaxial high-Tc superconducting thin films

We have shown lanthanum strontium gallate (LaSrGaO₄) has considerable potential as a substrate material for high-temperature superconducting thin films. We report on the growth and physical properties of LaSrGaO₄ crystal. Single crystals of LaSrGaO₄ have been grown by the Czochralski technique with diameter 32 mm and lengths of 110 mm. We found that this crystal had no structural phase transitions and was twin-free. We show that high-quality epitaxial Y-Ba-Cu-O films can be grown on LaSrGaO₄.     

Investigation of the phase transitions of ferroelectric KIO3 and KNbO3 crystals by optical diffraction

Optical diffraction is reviewed as a technique for investigation of the phase transitions in crystals with a multidomain structure. It has been used to study the phase transitions in KIO₃ and KNbO₃ single crystals. Strong optical diffraction bands resulted from electric domains in KNbO₃ crystals and their change with temperature were observed when a laser beam passed through the crystals. The diffraction patterns observed changed abruptly at 427°C, 223°C, and -50°C respectively, at which KNbO₃ crystals undergo structural phase transitions. It is considered that the change of the diffraction patterns with temperature is due to change of the electric domains in the crystals.