Influence of impurities on chemical etch pits in gypsum single crystals

Basal cleavages of gypsum are etched in nitric acid and potassium hydroxide solution. Terraced pits are commonly observed independent of the etchants used. Varying degrees of widths of terraces are observed on etching matched cleavage pairs as well as thin flakes. The terraced nature of etch pits has been attributed to the inhibitive action of the etchant due to the precipitation of impurities along dislocation lines. On prolonged etching of a thin flake holes are formed at some dislocation sites while at other dislocation sites, holes are not formed. Preferential etching of fission tracks gives rise to winged etch pits. The implications are discussed. Contribution No, 549.     

Influence of a state of impurities on the interaction between a dislocation and impurities in KCl single crystals

The strain-rate cycling test during the Blaha effect measurement at 80 to 240 K has been carried out for two kinds of single crystals: quenched and annealed specimens of KCl:Sr²⁺. It was found that the force-distance profile, which expresses the interaction between a dislocation and impurities, cannot be approximated by the Fleischer's model when I–V dipoles turn into aggregates for KCl:Sr²⁺, and that the activation energy for the break away of a dislocation from impurities becomes small for the annealed specimen, compared with the quenched one. Furthermore, the critical temperature,T _c, for the annealed KCl:Sr²⁺ is slightly smaller in comparison with that for the quenched one.     

Influence of sodium impurities on arf excimer-laser-induced absorption in CaF2 crystals

The formation of color centers induced by irradiation with ArF excimer lasers in CaF(2) crystals was found to depend strongly on the sodium impurity concentration. Sodium-related color centers were generated by two-photon absorption because the slope of the induced absorption coefficient just after irradiation started was proportional to the square of the laser fluence. The saturation absorption also depended on laser fluence, and a photobleaching induced absorption phenomenon was observed. We concluded that the saturation absorption level was determined by the equilibrium between two-photon excitation and one-photon reverse reaction.     

Dislocation etching of GaSe single crystals

Dislocation etching of GaSe single crystals was investigated by using a dilute chromic-sulphuric acid mixture, when conical etch pits were revealed on the (0001) surfaces. At the apices of spiral growth hills, bunches of spiral dislocations were revealed, proving that it is not a single screw dislocation of large Burgers vector, but a bunch of co-operating screw dislocations that were responsible for the spiral growth formations of large step-height. In the case of GaSe crystals, grown by vapour transport methods, dislocation densities of 10² to 10⁶ cm⁻² were found. The Bridgman crystals investigated were completely free from non-basal dislocations.     

Studies on etching of gel-grown barium oxalate dihydrate crystals

Etch pits produced by selective etchants such as 1 M HCl, 1 M HNO₃, 4 M BaCl₂, 4 M NH₄Cl and 4 M NH₄Cl-1 M HCl solutions on the as-grown {110} faces of barium oxalate dihydrate crystals are illustrated and explained. The kinetics of etching is studied. From Arrhenius plots, the activation energy of etching and the pre-exponential factor are computed. An empirical equation governing the kinetics has been suggested.     

Effect of impurities on the strengthening of CaF2 single crystals

The critical resolved shear stress (CRSS) of pure and aliovalently doped (with Y⁺³ and Na⁺) CaF₂ single crystals have been measured in compression along [1 1 1] axis between room temperature and 873 K. Strain rate cycling and/or stress relaxation experiments were performed to obtain the necessary parameters for the determination of rate-controling mechanism for plastic flow. The measured temperature and concentration dependences of the activation parameters suggest that the elastic interaction of dislocations with impurity cation-lattice anion defect pairs determine the CRSS as in alkali halide crystals. The CRSS for CaF₂ single crystals doped with trivalent Y⁺³ were found an order of magnitude larger than those doped with the monovalent Na⁺. Based on the valence of dopant cations and the magnitude of hardening produced, solution hardening in CaF₂ could be divided into two groups: rapid and gradual hardening.     

Distribution of impurities in lithium iodate crystals

The ability of single crystals α - lithium iodate to transmit polarized light in the wavelength band 0.25 to 0.7 μm has been studied. An impurity absorption band has been observed near the fundamental optical absorption edge. At the chosen conditions for crystal growth, the extent to which impurities were absorbed into the crystal lattice was a function of distance from the original seed crystal and so was dependent on the position of the crystal relative to the beam and the polarization of the beam.     

Effect of impurities on radiation-induced defect generation in BaFI crystals

We have studied radiation-induced defect formation processes in gamma-irradiated BaFI crystals doped with Eu, W, and Tl and compared them with those in unirradiated BaFI. The results demonstrate that the radiation-induced defect formation processes in the Eu- and W-doped crystals differ insignificantly from those in the undoped samples. In the Tl-doped crystals, the radiation-induced formation of native and extrinsic defects undergoes drastic changes because of the competition between native anion vacancies and Tl²⁺ dopant ions for electron capture.     

Surface recrystallization in calcium-doped NaCl crystals

Exposure of cleavage surfaces of calcium-doped NaCl to water vapour produced surface recrystallization at sites from which condensed water had evaporated. The crystals, initially stable in air, became hygroscopic following further recrystallization. Examination of the habit, properties and growth conditions of these crystals pointed to an interaction between moisture and calcium that resulted in hydrated calcium chloride. This interaction proceeds much more rapidly along grain boundaries in NaCl bicrystals. In general, the calcium in NaCl separates out into second-phase in a crystal cooled to room temperature, and any subsequent exposure to moisture produces erosive attack of the matrix adjacent to precipitates, resulting in a macroscopically defective substructure.     

Anisotropic etching of silicon crystals in KOH solution

Theoretical two-dimensional etched shapes are derived from numerical simulations involving the equation of the dissolution slowness surface related to silicon crystals etched in aqueous KOH solutions. Theoretical changes in cross-sectional shapes of starting circular sections and in x₁ and [001] profilometry traces with the angle of cut, _o, are analysed in terms of the geometrical features of the slowness surface. The important role played by extrema in the dissolution slowness in determining the final two-dimensional etched shapes, is outlined. Theoretical etched shapes are systematically compared with the experimental shapes and the adequation of the proposed slowness surface is discussed.     

Anisotropic etching of silicon crystals in KOH solution

The anisotropic etching behaviour of various crystalline silicon plates in an aqueous KOH solution was studied. Variations of the etch rate with the angle of cut related to singly-rotated plates have been determined and orientation effects in the out-of-roundness profiles related to various {h k 0} sections have been analysed in terms of crystal symmetry. In addition, changes in the surface texture with crystal orientation were systematically investigated. All the experimental results furnished evidence for a dissolution process governed by the crystal orientation. A procedure has been proposed to derive the dissolution slowness surface from experiments starting from a tensorial representation of the anisotropic etching.     

Selective etching and dissolution of BaF2 crystals

The selective etching and dissolution of BaF₂ crystals have been studied in aqueous solutions of inorganic salts, inorganic and organic acids. The effect of additives on selective etch rates is demonstrated. It is observed that chemical exchange reactions determine the selective etching and dissolution of BaF₂ crystals. The form of etch pits corresponds to the real dissolution form of BaF₂ crystals (rhombododecahedron). It has been established that the process of dissolution in HNO₃ and HCl is diffusion controlled. The activation energy is independent of acid concentration and has the same value for both HNO₃ and HCl. The pre-exponential factor is concentration dependent. An empirical equation for dissolution kinetics has been obtained.     

Laser implantation of impurities in cadmium telluride crystals

Processes of laser implantation of shallow donors (aluminum and indium) and an acceptor (antimony) in CdTe crystals (n,p∼10¹⁵ cm⁻³) are investigated. Thin dopant films vacuum deposited on the etched surface of the crystals are irradiated by ruby (λ=0.694 μm) and Nd:YAG (λ=1.06 μm) laser pulses (pulse duration 20 ns) over a wide energy interval (0.1–1.8 J/cm²). The irradiated surfaces are studied by x-ray microanalysis, Auger spectroscopy, and the thermopower method. It is it is shown that irradiation by a Nd:YAG laser produces a uniform doping of a subsurface layer of the crystal by aluminum. The implantation of indium leads to the formation of a precipitate. The concentration of implanted impurities reaches 10¹⁹–10²¹ cm⁻³. Pis’ma Zh. Tekh. Fiz. 24, 1–6 (June 12, 1998)     

High-temperature anomaly in the yield stress of doped NaCl crystals

The yield stress of NaCl crystals doped with divalent metals was found to rise with temperature in the range 293-600 K, the effect being most pronounced around 573 K. At higher temperatures, the yield stress decreases substantially.     

Microstructural,etching and hardness studies on flux-grown KNiF3 crystals

Surface structures on {100} faces of flux-grown KNiF₃ crystals are reported. Etching experiments establish HNO₃ to be a dislocation etchant for the crystals. The etching behaviour of the HNO₃-KNiF₃ surface system is investigated. The results obtained on the effect of etching time and etchant concentration on lateral extension and depth of dislocation etch pits are reported. It is observed that the etchant is rendered passive after some period of initial etching. Indentation-induced hardness testing studies suggest a Vickers microhardness value in the range of (2.93 to 3.50)×10² kg mm^–2, and the response of indentation to load is in accordance with Kick's Law.