Energy estimates of twinning processes in cubic single crystals

The role of twinning processes giving rise to the fragmentation and rotation of a structure caused by single-crystal deformation, as well as the joining of grains during the manufacturing of optical ceramics, is investigated. Models describing the twins in the single crystals under study, which are formed by rotating a cubic lattice about the threefold symmetry axis through an angle of 60°, are constructed. The twin formation energies are estimated.     

Anisotropy in two-photon direct creation of biexcitons in CuBr cubic crystals

The polarization dependence of two-photon absorption is considered in the case of CuBr crystals. Geometrical selection rules for direct creation of Γ₁, Γ₃ and Γ₅ biexcitons are provided. Using the fact that the absorption to Γ₃ and Γ₅ final states exhibits anisotropy with respect to the crystal axes, several geometrical situations for separating experimentally biexciton lines are discussed.     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed. Original Russian Text ? V.I. Marchenko, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, p. 841.     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed.     

Energy storage in heavily doped zinc sulfide crystals with laminar morphology

We have measured the optical and structural properties of zinc sulfide crystals doped with 0.8% indium and 0.1% copper. These crystals store electronic energy at low temperatures and exhibit a broad structureless thermoluminescence. Their properties are compared with those of crystals with lower dopant concentrations. The thermoluminescence of the heavily-doped crystals can be excited by their own photoluminescent emission. This results in energy storage throughout the volume of the sample, even with the primary photoexcitation at spectral regions of strong absorption. We relate the energy storage to the laminar morphology and attribute the storage to the separation of electrons at the lamina surfaces from recombination sites in the interior of the laminae.     

Sensitivity of self-activated zinc sulfide single crystals to infrared light

Journal of Applied Spectroscopy -     

Reversible changes in the structure of zinc sulfide crystals during elastic deformation

EPR is used to detect the effect of a reversible decrease of the number of stacking faults during elastic uni-axial compression of microtwin zinc-sulfide crystals. The result is explained by the behavior during deformation of the crystal of incoherent twin boundaries consisting of sloped walls formed by ensembles of partial dislocations. Fiz. Tverd. Tela (St. Petersburg) 39, 1230–1233 (July 1997)     

A new classification of twinning in crystals

A comprehensive and informative classification of twinning in crystals is proposed. It is based on the nature of the twin mapping operation. If the mapping operation is a symmetry element of a certain prototype space group (in Aizu's sense), the twin is called an “Aim twin”. Otherwise it is called a “Bollmann twin”. Aim twins are essentially transformation twins. They may be further divided into ferroic twins and translation twins. Ferroic twins, in turn, can be of two types: ferroelastic or F-twins (e.g. the 90° twins of BaTiO₃), and nonferroelastic-ferroic or N-twins (e.g. the Dauphiné twins of quartz). The antiphase domains in Cu₃Au are a typical example of translation twins (T-twins). The three types of Aim twins (F, N and T) have distinctive macroscopic physical properties. Bollmann twins are divided into two main categories: C-twins and M-twins, where C stands for coincidence lattice and M for miscellaneous. C-twins are further categorized into two types, depending on the “total” or “partial” nature of the coincidence sublattice. M-twins can be of three types, depending on the dimensionality of the dichromatic pattern being 0, 1 or 2. Illustrative examples are discussed. A compact and informative twin symbol is introduced.     

Effect of a dislocation forest on the bauschinger effect during the twinning of zinc single crystals

A study was made of the effect of a dislocation in the {{11¯22}} 11¯23 system (pyramidal slip) on the characteristics of the Bauschinger effect during the twinning of single crystals of zinc and of the alloy Zn + 0.6% Cu. The dislocation forest retards the backward motion of the twinning dislocations, leading to the effect.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 101–104, May, 1971.In conclusion the authors thank candidates of physicomathematical sciences F. F. Lavrent'yev for useful discussion.     

Bulk and surface ordering in cubic crystals

The order-disorder transition of face and body centred cubic crystals is studied by a Monte Carlo simulation of a two-component lattice gas with nearest neighbour interactions, and of different compositions. It is shown that the degree of surface order may both be higher and lower than that of the bulk. General guide lines are drawn from the results to predict such behaviour in experimental situations.     

Excitation spectra and structure of luminescence centers of manganese ions in single crystals of zinc sulfide

We investigate the photoluminescence excitation spectra in ZnS:Mn single crystals at room temperature and at the temperature of liquid nitrogen with a different concentration of Mn²⁺ ions. The strongest bands peaking at 557, 578, 600, and 637 nm are associated with a different position of the Mn ion in the lattice of the crystals under investigation. The difference obtained in the excitation spectra can be explained by the resonance transfer of energy between the Mn ions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 208–210, March–April, 2000.     

Mechanical twinning of yttrium oxide single crystals

The deformation structure of an yttrium oxide single crystal under point load was studied by microhardness and electron microscopy. It was concluded that mechanical twinning is the main mechanism of plastic deformation of these single crystals. A twin was simulated by rotating the structure around the three-fold axis. It was shown that the average change of the distance between anions was about 5% and that between cations was 15%. The comparison with the structure of the regions of the fluorite single crystals and optical ceramics deformed in the analogous manner was performed.