Magnesium Fluoride Crystals Research Articles

Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications

Results of an investigation of acousto-optic (AO) cells using single crystals of magnesium fluoride (MgF2) are presented. Two acousto-optic tunable filter (AOTF) cells for imaging application have been designed and tested in the visible and ultraviolet (UV) regions of the spectrum from 190 to 490 nm. The two imaging filters were developed by using the wide-angle AO interaction geometry in the (010) and (11 0) planes of the crystal. These filters were used to obtain spectral images at the shortest wavelengths achieved so far. Advantages and drawbacks of this crystal are discussed and photoelastic, acoustic, and AO properties of MgF2 are examined. The investigation confirmed that MgF2-based AOTF cells can be used in the deep UV region up to 110 nm.

Possible application of compound Fresnel lens for neutron beam focusing

We have developed a Fresnel-type focusing device for cold neutrons fabricated using single crystals of magnesium fluoride. This stacked-lens device using 50 elements (with 50 beam–bending interfaces) demonstrated a focal length of 5 m with a good transmission of 0.829 for 1.14 nm neutrons. The focused beam was 5 times more intense than a beam of the same spot size with the same final flight path length produced with the traditional pinhole collimation. The background-scattering noise from lens was an order of magnitude larger than that of the traditional pinhole collimation in the momentum range of 0.01 –0.05 nm −1 , almost 10 −3 of focused direct beam at the lowest momentum transfers measurable. This device produced an intensity gain in measured SANS data of more than 10 times when compared with the conventional pinhole geometry with the same minimum momentum transfer.

Ultrashort pulses at 130 nm by third-harmonic generation in a magnesium fluoride crystal

The use of third-harmonic generation of visible pulses in a magnesium-fluoride wedge has been proposed in order to obtain ultrashort pulses with photon energy in the vacuum ultraviolet range. Starting from a 200 fs blue pulse with 18 \ensuremath{\mu}J of energy (which is the second harmonic of the IR pulses produced by a regenerative Ti-Sapphire laser system), ${10}^{3}$ photons were obtained at 9.4 eV, with ${10}^{\ensuremath{-}9}$ conversion efficiency. From the measurements it was inferred that ${\ensuremath{\chi}}^{(3)}(\ensuremath{-}3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega})\ensuremath{\approx}0.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}{\mathrm{m}}^{2}/{\mathrm{V}}^{2}$ for the third-order susceptibility of magnesium fluoride. The simplicity of the arrangement makes this approach ideal for the generation of the ultraviolet probe required by time and angle-resolved photoemission spectroscopy.

Dissolution of magnesium fluoride crystals in the presence of some minerals salts

Dissolution of magnesium fluoride seed crystals has been studied in the absence and presence of some mineral salts under conditions in which the under-saturation was maintained constant and controlled by a specific fluoride ion electrode. Over a range of relative under-saturation, 0.3–0.75, the dissolution reaction appears to be governed by a surface polynucleation process which is markedly inhibited by the presence of additives. The action of mineral salts on the rate of dissolution of magnesium fluoride crystals can be interpreted in terms of a Langmuir-type adsorption isotherm.

ChemInform Abstract: Stress Corrosion of Ionic and Mixed Ionic/Covalent Solids.

Slow crack growth data, surface reaction studies, and solubility results for sapphire and magnesium fluoride crystals are used to identify stress corrosion mechanisms for each material. Results suggest that stress corrosion in sapphire proceeds by the dissociative chemisorption of environmental species on strained crack tip bonds. Crack growth rates measured in nonaqueous environments and isotopically labeled water suggest that ion solvation rather than dissociative chemisorption dominates the crack tip interaction in magnesium fluoride.

Stress Corrosion of Ionic and Mixed Ionic/Covalent Solids

Slow crack growth data, surface reaction studies, and solubility results for sapphire and magnesium fluoride crystals are used to identify stress corrosion mechanisms for each material. Results suggest that stress corrosion in sapphire proceeds by the dissociative chemisorption of environmental species on strained crack tip bonds. Crack growth rates measured in nonaqueous environments and isotopically labeled water suggest that ion solvation rather than dissociative chemisorption dominates the crack tip interaction in magnesium fluoride.

Improved vacuum seals for magnesium fluoride windows

Improvements and extensions have been made to the techniques of sealing oriented magnesium fluoride crystals to metals, with a suitable glass solder. The technique for sealing a 38 mm diameter window in a 90 mm diameter stainless steel flange, 12 mm thick is described. The sealing of windows to all-glass devices is relatively simple using standard graded seals to soda glass and then with the solder glass to the window. The method of cleaning the mounted windows is also described.

The magnetic susceptibility of γ-irradiated single crystals of magnesium fluoride

Abstract Data of static magnetic susceptibility measurements on γ-irradiated single crystals of magnesium fluoride are reported for doses up to 1 × 1011 rad. The susceptibility was interpreted to consist of three parts: (1) a diamagnetic part xd =-0.384 × 10−6 emu/g, due to the normal MgF2 matrix; (2) a temperature-dependent paramagnetic part, obeying the Curie law, arising from unpaired electrons; and (3) a temperature‐independent paramagnetic part which is attributed to the presence of metallic magnesium. The results show that the radiation damage process in MgF2 is strikingly similar to that in LiF.

Magnetic Properties of Lattice Imperfections in Alkali Halide Single Crystals

Using a torque magnetometer, experimental results are reported on some new magnetic properties in high-purity, optical-quality single crystals of lithium fluoride, potassium chloride, and sodium chloride. Since the torque observations are not produced by the gross diamagnetism of these salts, the magnetic centers may be along dislocations because the magnetic susceptibility is observed to possess uniaxial anisotropy. Soft x irradiation increased the observed magnetization. In addition, there is a cooperative interaction among the magnetic centers which gives a remanent magnetization. The temperature dependence of the remanent magnetization is similar to the Bloch spin-wave concept of ferromagnetism. These new magnetic properties are an order of magnitude smaller when measured in crystals of magnesium fluoride and of calcium fluoride.

Electron Microscopy of Irradiation‐Induced Defect Clusters in Magnesium Fluoride

AbstractThe passage of electrons through thin crystals of magnesium fluoride produces point defects. These defects are mobile at temperatures normally attained under the electron beam, and they can give rise both to dislocation loops and to precipitate platelets. The aggregation of these radiation‐induced defects can be controlled by using a cooled specimen stage in the electron microscope. This allows the growth and characteristics of the defect clusters to be studied. Dislocation loops lie on {011} planes and they sometimes show stacking fault contrast; their Burgers vectors are 1/2 〈011〉 and 〈011〉. Prolonged irradiation can produce precipitates on and within the loops. Higher beam intensities produce precipitates directly. The nature of the loops and precipitates is discussed in relation to the production of point defects by ionization mechanisms and irradiation‐stimulated migration.

Crystal Imperfections in Magnesium Fluoride

Single crystals of magnesium fluoride, grown by the Czochralski technique, have been examined by means of chemical etching, and optical and electron microscopy. Dislocations in as-grown crystals are found to be decorated with precipitates; the nature of the precipitates has been investigated. Grown-in dislocations and undecorated dislocations introduced by slip can be etched in hot sulfuric acid. Etch pits at grown-in dislocations often exhibit characteristic ``beaks'' or ``tails.'' Chemical etching has been used to establish the existence of three slip systems, namely {110}〈001〉, {001}〈100〉, and {101̄}〈101〉. Specimens for examination by transmission electron microscopy were prepared by chemical dissolution after mechanical shaping. Grown-in dislocations are normally removed by unavoidable selective etching. In the microscope the samples are damaged by the electron beam. Point defects are produced which aggregate and result in both dislocation loops and precipitate platelets.

Some Properties of Magnesium Fluoride crystallized from the Melt

Magnesium fluoride crystals can be grown in vacuo using the Stockbarger technique. The melting point is 1255 ± 3°C. The crystal structure is of the tetragonal SnO2 type, with lattice constant a = 4.621 ± 0.001 A and axial ratio 1: 0.6601 ± 0.0001 giving c = 3.050 A at 18°C. The density is 3.1766 ± 0.0002 g cm-3 at 18°C. MgF2 is a uniaxial positive crystal, and the refractive indices are given for the ordinary and extraordinary rays at various wavelengths in the visible spectrum. The transparent region extends from 1100 A to 1360 cm-1. Infra-red reflectivity begins to rise at about 620 cm-1, and peaks around 500 cm-1. The mean dielectric constant is m = 5.26.