The polymorphous transition that occurs during the process of metasomatic replacement was observed in the experiment on the retgersite paramorph formation after nickelhexahydrite crystals. This process runs through a solution film formed on the primary crystal surface and then penetrated into the crystal along with the replacement front. One or several secondary mineral grains have ingrown into a comparatively perfect crystal up to the complete replacement; these grains are randomly oriented with regard to the crystal. A finegrained mass replaces the imperfect crystals. An aggregate of mineral grains incorporated into the fine-grained mass is an intermediate case. Single-crystal pseudomorphs have been synthesized for the first time as products of the nonisomorphic interaction. Conditions of the nickelhexahydrite metastable equilibrium and the retgersite stable equilibrium are closely related and determine the structural features of the replacement reactions for these two compounds. A relatively low supersaturation with retgersite in the vicinity of the eutonic point resulted in the nucleation and growth of secondary single crystals. A deviation from the eutonic point gives rise to the increasing supersaturation and mass precipitation of the secondary phase.
This paper discusses the characteristics of ZnO and ZnO:Ga ceramics fabricated by uniaxial hot pressing. The short-wavelength transmission limit of zinc oxide ceramics is in the 370-nm region; the long-wavelength limit is determined by the free-charge-carrier concentration and lies in the interval from 5 to 9 μm. The total transmittance of such ceramics in the visible and near-IR regions is about 70% when the sample is 0.5 mm thick. The luminescence spectrum is represented by a broad emission band with maximum at 580 nm, having a defect nature. The introduction of 0.03-0.1 mass % gallium into the zinc oxide structure inhibits grain growth and increases the free-charge-carrier concentration to 3.44 × 10 19 cm −3. As the gallium concentration increases in the range 0.05-0.1 mass % in a ceramic of composition ZnO:Ga, the defect luminescence band is suppressed and a characteristic exciton luminescence is formed with a maximum corresponding to 389 nm and a damping time constant of 1.1 ns.
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