Use Of Single Crystals Research Articles

X-Ray Diffraction Studies of the Molecular Complexes of Tetracycline Salts

Abstract An examination of the salt-complexes of tetracycline and oxytetracycline by the X-ray powder method has shown the presence of at least four different kinds of crystal structures, i.e., the dα, dβ, hγ, and hδ forms. Among them, the dα form has been studied in detail by the use of single crystals. The monoclinic unit cell dimensions of the dα crystals do not vary much either with a salt anion or with a complexing agent, and the intensities of the various reflections so far examined are very similar. On the basis of these findings, it is suggested that the crystal structures of the dα-type salt-complexes consist of an almost identical framework of tetracycline or oxytetracycline molecules which encloses spaces capable of containing complexing agents of various types. The orthorhombic dβ form is a polymorphic modification of the dα form, probably arising from submicroscopically polysynthetic twinning of the latter.

A conversion of the Philips powder camera for single crystal use

An attachment is described which permits the use of a single crystal goniometer head with the Philips powder camera (114.59 mm in diameter). Use of the attachment does not require any modification of the powder camera beyond removal of the cover. This device permits accurate determination of the Bragg angles of the reflections obtainable in a zero layer Weissenberg photograph with only small orientation corrections.

Space-charge-limited solid-state devices

The history and development of interest in space-charge-limited (scl) current in solids is traced from early work on the electron atmosphere in insulators to present understanding of the physical mechanisms of current and awareness of device potentialities. SCL solid-state devices have achieved practical realization through the use of single crystals and evaporated layers of cadmium sulphide, but high-resistivity single crystal silicon is likely to be a preferred material for practical and reliable devices. The square-law dielectric diode is the fundamental scl solid-state device, and theoretical studies and experimental investigations of this device are described. The design and operation of the dielectric triode with negative and positive gate bias are discussed, the input resistance of this device is high; it is insensitive to changes of temperature, and its operating characteristics and parameters are described well by the concepts of perveance and amplification factor. It is now apparent that there is considerable scope in future electronics for scl solid-state devices as individual elements and in thin-film integrated circuits.

Corrosion of Corundum Crystals by Commercial Glasses

ONE of the most striking and least-understood aspects of refractory corrosion by molten glass is the relatively high rate of solution at the refractory-glass-gas interface, compared with that which occurs below the glass-level1,2. Any attempts to explain this phenomenon, or even measure it, are made exceedingly difficult by the complex nature of the materials involved3,4. This difficulty has been partially overcome by the use of single crystals of corundum as the refractory materials5,6, thus eliminating factors such as porosity, pore size, pore shape, texture, etc.

New Orthorhombic Phase in the Ru–B and Os–B Systems

By the use of single crystal and polycrystalline x-ray diffraction techniques, isomorphous orthorhombic lattices (space group No. 59) have been found in the Ru-B and Os-B systems having lattice parameters of a0=4.6443±0.0003 Å, b0=2.8668±0.0008 Å, c0=4.0449±0.0004 Å and a0=4.6832±0.0001 Å, b0=2.8717±0.0002 Å, c0=4.0761±0.0001 Å, respectively. The probable empirical formulas are RuB2 and OsB2, with two formula weights per unit cell.

Symposium on Solid State Conductivity IV. Solid State Radiation Detectors, with Particular Reference to Low Dose rates

The use of single crystals of photoconductor has been investigated at gamma dose rates of (a) 1 to 300 r/min for supervoltage beam measurements, (b) 10 to 300 r/hr for intracavitary probes, and (c) at very low dose rates of the order of 1 mr/hr for radiation protection or for the detection of isotopes in the human body. Results are presented of measurements on CdS and CdSe crystals from various industrial sources. It is possible to obtain crystals which pass 0·1 to 1 μA per r/min (60Co) with a dark current some 3 orders of magnitude smaller; a few have been tested which give up to 40 μA per r/min, but the more sensitive crystals are more difficult to obtain. Results of measurements on single crystals have shown that linearity with dose rate, reasonably short time constants, and small dependence on temperature are possible with CdS, but CdSe has not yet been found to be so good in the two last-mentioned properties. Radium dose rates of 1·3 to 2 mr/hr have been detected to an accuracy of about 20% within 5 to 10 min of beginning the exposure. Such dose rates would enable the distribution of a few tens of microcuries of 131I to be displayed, by an array of single crystals of CdS, and the methods of achieving this are discussed.Present photoconductors show considerable promise for these and other applications, with little improvement in properties but with supplies made more readily available.

THE GROWTH OF METAL SINGLE CRYSTALS

Before 1939, metal single crystals were of interest only to a small number of solid-state physicists and an equally small group of research metallurgists, whose principal aim was to understand how metals deform plastically. Single crystals had also been used for the determination of physical and mechanical properties in various crystallographic directions. Much of this pioneer work is reviewed in the well-known book “Kristallplastizität” by Schmid and Boas. Since the war, the number of physicists interested in the solid state has greatly increased, while in metallurgical circles the study of single crystals has assumed an air of almost complete conventionality. The surge in fundamental work on metals has led to the very widespread use of single crystals, and today equipment for their preparation is standard in a large number of laboratories. Whilst the principles of the methods used are still the same as in the twenties, in detail the methods are often more sophisticated and the products more satisfactory, particularly from the points of view of purity and crystal perfection.

The diffraction of thermal neutrons by single crystals

Experimental measurements of the intensity of neutron reflexion by large single-crystal slices are shown to be in agreement with previously published calculations. It is demonstrated that accurate comparison can be made of the structure factors of planes in a zone by the use of cylindrical or pillar crystals, sufficiently small in diameter to give negligible secondary extinction effects. The use of single crystals for structure analysis is discussed.

News and Views

THE discourse by Sir Harold Carpenter which we publish to-day as a special supplement provides a very complete survey of the important results which have been obtained by the study of single crystals of metals. Whilst ordinary metals consist of a large number of crystals with their axes directed more or less at random, means have been devised for producing specimens, large enough for the tests usually applied by the engineer to metallic specimens, but consisting of a single individual. An extensive field has thus been opened up for the study of the properties of solids. The use of single crystals eliminates one very obscure factor, the existence of boundaries of unknown properties between the crystals, and also, by limiting the directions in which slip can occur under the influence of stress, greatly simplifies the geometrical conditions of a mechanical test. The study of the electrical, and especially of the magnetic, properties is correspondingly simplified. It becomes possible to compare the properties of metals with those of ionic crystals, such as rock salt, which have so far proved most amenable to theoretical treatment. The work on metallic single crystals is thus certain to throw much light on a problem of the greatest interest to the engineer as well as to the physicist that of the cohesion of solids.