High-purity Crystals Research Articles

Autodiffusion en volume dans la phase plastique de l'acide pivalique (acide 2.2 dimethyl propanoique)—I: Mesure des coefficients de diffusion et de l'effet isotopique

Self-diffusion studies have been performed in the orientationally disordered or the so-called plastic phase of pivalic acid. Single crystals of high purity (99.9999%) containing 10 9−10 10 dislocations m −2 have been used. Thin layers of pivalic acid labelled with 14C or tritium were deposited on sample surfaces. Concentrationpenetration curves were established by serial sectioning. Lattice self-diffusion coefficients D, were measured from 281 to 304.75K. At 281K, the value of D is independent of time. From 281 to 301K, D is given by: D(m 2S −1) = (4.9 ± 0.3)10 −4 exp [− (59± 1) kJ mole −1/ RT]. The activation enthalpy of the lattice self-diffusion is roughly equal to the heat of sublimation ( L s = 57 kJ mole −i) and in good agreement with values obtained from NMR. The mass factor f Δ K, where f is the correlation factor and Δ K is a correction factor, has been measured using isotope effect studies. Between 281 and 301K the value obtained is fΔ K* 0.1 +0.2 −0.1

Far-infrared observation of cyclotron and spin-flip resonances in HgSe

Far-infrared (FIR) magnetotransmission experiments were carried out at 4.2 K on high-purity HgSe crystals, with the use of a C${\mathrm{O}}_{2}$-pumped FIR laser and magnetic fields up to 14 T. Measurements were performed in the Voigt geometry, at three FIR wavelengths: 96.5, 119, and 163 \ensuremath{\mu}m. In addition to the usual plasma-shifted cyclotron resonance occurring in the transverse Voigt geometry, we have observed electric-dipole-excited electron-spin-resonance (EDE-ESR) and combined-resonance transitions. These resonances are made possible by the relaxation of selection rules due to spin-orbit coupling, as a consequence of $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ interaction and/or inversion asymmetry. Our data suggest that the inversion-asymmetry mechanism is stronger in HgSe than in other materials studied by FIR magnetotransmission, e.g., InSb. We have also observed a small but unambiguous magnetoplasma shift in EDE-ESR occurring in the transverse Voigt geometry. The positions of the observed resonances at the three laser frequencies were used to obtain band-structure parameters for HgSe within the framework of the Pidgeon-Brown model.

Dragging of domains by an electric current in ferromagnetic bulk cobalt at 4 K

The electrical resistivity of cobalt single crystals of high purity has been measured for current densities j up to 400 A/mm2, at 4.2 K. Large deviations from Ohm’s law are observed when j exceeds ≃12 A/mm2, in agreement with theoretical predictions. The electron gas in translation exerts hydromagnetic forces on the domain walls. The deviations from Ohm’s law happen when these forces are large enough to overcome the pinning forces and to set the walls in motion, in a direction parallel to the drift speed of the charge carriers. The dragging forces also tend to reorient the walls in a direction parallel to the current. Magnetic fields parallel to the current are used to minimize this reorientation.

Preparation of high purity yttrium single crystals by electrotransport

The possibility of obtaining yttrium crystals of high purity by the method of solid state electrotransport (SSE) was investigated in the present work. The behaviour of low contents of iron, aluminium, silicon, tantalum, copper, silver and vanadium as metallic impurities was studied using mass spectrometry. It is shown that all the impurities investigated, except copper, migrate to the anode. During electrotransfer a purification with respect to these impurities by a factor of 4–6 is obtained. It is proposed that the diffusion coefficients of the metallic impurities investigated are anomalously high and that the behaviour of the impurities during SSE in adapters necessitates further investigation. By using a three-stage process with intermediate removal of the anode end we produced yttrium single crystals with a resistance ratio ρ 293 ρ 4.2 = 570 .

Preparation of high-purity InP by the synthesis, solute diffusion technique

High-purity InP crystals were prepared by the synthesis, solute diffusion (SSD) technique. By vacuum baking of raw indium at 900 °C for 6 h, the first-to-freeze part of InP had a carrier concentration of ND−NA = 6.4×1014 cm−3 at 300 K and a mobility of μ = 79 100 cm2/V sec at 77 K. It is concluded that S and/or Si contained in In raw material are dominat donor impurities in the SSD-InP crystal, because their contents can be decreased by the vacuum baking.

The Phase Transition Phenomenon in Metal Phthalocyanine Crystals

The study was carried out on the gas adsorption and the α→β phase transition in metal phthalocyanine (Pc) crystals at different degrees of purity. The uptake of , , or on crystals decreased remarkably with the degree of purification, but the uptake of did not change. The α→β phase transition of crystals was accelerated by purifying, and it was considerably affected by the ambient gases in the low purity crystal, and hardly at all affected in the high purity crystal. The α→β phase transition of crystals in gas began upon decomposition of the addition complex (near 220°C) which formed readily at 25°C. The results of crystals were mostly similar to that of crystals.

Ductile fracture initiation in pure α-Fe: Part II. Microscopic observations of an initiation mechanism

α-Fe single crystals of high purity were strained to fracture byin-situ high voltage electron microscopy. This method permitted recording of events leading to the failure of the crystal at high magnifications. Specifically, observations were made regarding the first step in the fracture mechanism, namely the initiation of voids, which occurred after a dislocation pattern of cells had developed. The cell size was found to be a few tens of nanometers in diameter and, because of the small scale of the dislocation patterns, a direct observation of void initiation was not possible. However, the misorientation between cells could be determined by electron diffraction techniques leading to the conclusion that void initiation took place at cell walls and at the boundaries of deformation microtwins. The high energy of these interfaces is another factor supporting the above conclusion.

The effect of radiation on the OH− infrared absorption of quartz crystals

The effect of 1.6-MeV-electron irradiation on the infrared properties of high-purity quartz crystals has been studied. The infrared bands assciated with OH− impurities in SiO2 crystals are strongly temperature dependent and must be studied at 77 K or below. Prolonged electron irradiation at low temperature suppresses all of the OH− bands, but these bands recover when the crystals are annealed to approximately 740 K. If as-grown crystals are irradiated at 300 K, the intensity of the OH− bands decreases and two new bands appear at 3367 and 3306 cm−1. The relative absorption intensity of these two bands depends on radiation temperature and dose.

A Comparison of the High Resoluton Crystal Structures of the π- Molecuar Complexes of Anthracene and Phenothiazine with Pyromellitic Dianhydride

Abstract The π-molecular complexes formed by aromatic molecules with three rings tend to form crystals of high purity and are therefore ideal for solid state spectroscopic and photocurrent studies. We have therefore undertaken an investigation of the detailed structures of a number of such complexes in order to facilitate the interpretation of such data and other physical properties and in order to better understand the large changes in physical properties generated by small changes in composition and molecular packing. The piezoreflection spectra of the anthracene-pyromellitic dianhydride complex (A-PMDA) has been studied by Eckhardt and Merski (1973). The photo-current excitation spectra of A-PMDA was reported by Karl and Ziegler (1975) and more recently the preparation, optical properties, absorption and photoconductivity properties of phenothiazine-pyromellitic dianhydride have also been studied. (Anthonj, Karl, Robertson and Stezowski, 1979).

X-ray stimulated luminescence in MgO

Studies have been made of the emission spectrum of MgO crystals induced by X-irradiation at 90 K. Two bands (half-widths ∼0.8 eV) were observed to peak at 4.95 and 3.2 eV, respectively, in high purity crystals. Doping with 100 ppm or greater of Fe, Co, Cr, Cu, Mn, and Ni suppressed the luminescence, though in the MgO:Ni crystal the 2.3 eV Ni 2+ band due to the 1 T 2g → 3 A 2g transition was observed. In deuterium-doped crystals the ratio of the intensity of 3.2–4.95 eV emission was found to be 1.2 as compared to 8 for the undoped crystals. Prior exposure of the pure crystals to ionizing radiation enhances the 4.95 eV band by a factor of three while not affecting the 3.2 eV band. This enhancement of intensity decays in several stages upon standing at room temperature in a way that reflects the thermal stability of the various components of the composite V-band absorption. These facts together with the observation that the 210 K thermoluminescence peak is composed entirely of 4.95 eV emission indicate that this luminescence band is associated with the recombination of an electron with a hole located in a V-type center, i.e. O−□ + e → (O 2 −□)∗ → O 2 −□ + 4.95 eV, where the square indicates that the perturbing positive ion vacancy is adjacent to the oxugen ion which has captured the hole. In MgO:Li + , which exhibits no V-type centers upon irradiation, the 4.95 eV band was absent and a 2.9 eV emission which may be associated with recombination at the [Li] 0 center was observe.

Horizontal Zone-Melting of Indium Antimonide in Vacuum. I. Basic Technique

A new approach has been developed to prepare high-purity InSb crystals; all processes, including synthesis of the compound and zone-melting procedures, were carried out in a vacuum evacuated continuously by a diffusion pump or by an ion pump. Details of the apparatus and zone-refining procedures are described, laying stress on the minimization of contamination from the possible source. n-type crystals with a carrier concentration range of 1013–1014/cm3 were easily obtained, and the maximum mobility as 77 K was 8×105 cm2/V·sec.

Muon spin rotation in superconductors

By means of the muon spin rotation technique (μ+SR), the temperature dependence of the magnetic field inside the normal-conducting domains of high-purity tantalum crystals in the intermediate state has been measured in the temperature range 2.36K<T<3.5K. The results agree well with macroscopic measurements of the thermodynamical critical field. It is also shown that the distribution of magnetic fields in the mixed state of niobium single crystals may be determined by μ+SR. Possible applications of these findings to the study of long-range diffusion of positive muons at low temperatures are indicated.

Intrinsic photorefractive effect of LiNbO3

The photorefractive effect of undoped LiNbO3 crystals of high purity is studied by means of two-photon excitation of picosecond light pulses. We show that two-photon photorefractive recording is accompanied by characteristic changes of the optical absorption and electron spin resonance spectra due to the formation of color centers. The role of these centers for the photorefractive process in discussed.

A memory‐effect in KCl crystals

AbstractIt is shown that the photoelectric effect produced by far UV light in KCl crystals of high purity „remembers”︁ the light energy in the F‐band obtained previously by the crystal.

Growth and characterization of titanium suboxide crystals

The titanium and vanadium suboxides MO x (where 1.5 < x < 2 and M is Ti or V) are particularly interesting compounds because many show metal-insulator transitions which result in interesting electrical, magnetic and spectroscopic phenomena. We have grown high purity crystals of the congruently melting compounds Ti 2O 3, Ti 3O 5 and V 2O 3 and mixed compounds using the tri-arc melting technique. Crystals were grown at speeds of 1-1.5 cm/h. The Magneli phases M n O 2 n-1 ( n⩾4) do not melt congruently and therefore we have grown a number of these compounds using chemical vapor transport with TeC1 4 or NH 4C1. Twinned needles typically larger than 1 cm long are grown by pulling transport in 3–5 days. The crystals have been characterized by X-ray diffraction, atomic absorption and gamma-ray activation analysis. In addition, the use of a scanning microcalorimeter made it possible to determine phase transitions on needles weighing as little as 2–10 mg. The stoichiometry was determined using X-ray diffraction, pycnometric density measurements and thermogravimetry.

The effect of nonmagnetic inclusions on the easy direction of gadolinium

It is shown that the presence of nonmagnetic inclusions in gadolinium crystals has a significant effect on the first anisotropy constant K1, leading to a variation of easy direction with inclusion content. The observed concentration of hexagonal oxide platelets is sufficient to explain differences in the values of anisotropy constants and easy direction reported by various workers. The authors conclude that the most reliable measurements are those of Corner and Tanner (1976) and Smith et al. (1977) on characterised, high-purity gadolinium crystals produced by solid state electrotransport.

Anomalous slip in high-purity vanadium crystals deformed in compression

High-purity vanadium single crystals, having resistivity ratios in the range 1 100 – 1 800, were compression tested at 295 K, 173 K and 77 K. Samples having orientations near the [011] - [ 1 11] tie line slip on expected planes. Samples having a central, or near [001] orientation slip on the anomalous (0 1 1) plane at lower temperatures.

Simultaneous synthesis and growth by THM of large HgTe crystals of high purity

Large mercury telluride crystals are grown, by the travelling heater method directly from mercury and tellurium as source material. Their purity is attested by the electrical characteristics measured after appropriate annealing.

The influence of material purity and irradiation temperature on self-ion damage in molybdenum

Abstract An electron microscope study has been made of vacancy loops formed as a result of displacement cascade collapse in molybdenum irradiated with 60 keV self-ions. The experiments were directed towards exposing two aspects of this process. Firstly, the influence of material purity was investigated by comparing the structures generated in three crystals, one being of high purity, the second containing a significantly higher level of substitutional impurities and the third doped specifically with nitrogen. The results demonstrated that nitrogen in solution was the most effective in reducing both the number of visible vacancy loops formed by cascade collapse and the fraction of vacancies surviving in the cascades to form loops. The second major aspect studied was the influence of irradiation temperature on vacancy loop formation in the high purity crystal. In this case the number of visible loops decreased sharply with temperature over the range studied, i.e. up to 535°C. The decrease is shown to follow a different form to that observed in copper and possible mechanisms to account for this are considered. Comparisons of the experimentally observed images in the high purity crystal with those predicted theoretically for pure edge and non-edge loops showed that all of the loops are perfect with b = ½〈111〉. Moreover, the loops are mostly non-edge, lying on {110} planes in the room temperature irradiated foils, but they rotate towards the pure edge configuration as irradiation temperature increases.

Preparation of cadmium telluride by a programmed solution growth technique

A new method of crystallization of cadmium telluride is described. The growth is carried out, by normal freezing of a tellurium-rich solution, in a vertical Bridgman furnace. Crystals of high purity (45 mm in diameter-150 mm in length) are obtained with this process.