Brillouin scattering in single crystals of fcc and hcp argon alloys

Ahmad, Syed Faiz (1980) Brillouin scattering in single crystals of fcc and hcp argon alloys. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The fcc arid hcp adiabatic elastic constants of the alloys of Ar containing 0₂ and N₂ as impurity have been determined to an accuracy of better than 2% by the high resolution Brillouin scattering technique. Large single crystals of Ar-0₂ and Ar-N₂ were grown in a cylindrical quartz tube of 3 mm internal diameter situated in a liquid helium cryostat. The quality of the crystal, whether It was hcp or fcc, and its orientation with respect to the lab co-ordinates were determined by analyzing X-ray Laue transmission photographs. The light of 5145 Å from an Ar⁺ laser was incident along the cylindrical axis of the sample cell, and scattered light at 90⁰ was analyzed by a piezoelectrically scanned triple-passed Fabry-Perot Interferometer. The spectrum was recorded by a photon counting apparatus consisting of a photomultiplier tube, an amplifier/discriminator and a commercial data acquisition and stabilization system (Burleigh, DAS-1). -- It was found that the fcc phase of Ar accepts up to about 4% of 0₂ and remains stable, contrary to the published phase diagrams. Hcp single crystals could only be grown with about 6½% 0₂ in the solid; above 7½% dendritic growth was observed. To within the experimental error, the fcc elastic constants do not vary with a change in the concentration of 0₂ up to about 4%. The fcc elastic constants of Ar-2% 0₂ were determined at 83.0 ± 0.3 K to be (units: 10⁹ Nm⁻2): c₁₁ = 2.40 ± 0.03 , c₁₂ = 1.59 ± 0.02 , c₄₄ = 1.11 ± 0.02 . For fcc Ar-4% 0₂, the elastic constants at 82.2 ± 0.3 K were: c₁₁ = 2.39 ± 0.03 , c₁₂ = 1.59 ± 0.02 , c₄₄ = 1.09 ± 0.02 . The fcc elastic constants of Ar-5% N₂ at 77.8 ± 0.3 K were: c₁₁ = 2.43 ± 0.05 , c₁₂ = 1.61 ± 0.04 , c₄₄ = 1.07 ± 0.03 . The decreasing tendency in c₄₄ may well reflect mode softening. -- The hcp elastic constants of Ar-6½% 0₂ crystals at 81.3 ± 0.3 K were determined to be: c₁₁ = 2.90 ± 0.04 , c₁₂ = 1.50 ± 0.03 , c₁₃ = 1.18 ± 0.02, c₃₃ = 3.24 ± 0.05 , c₄₄ = 0.65(6) ± 0.011 . -- A comparison of the pure Ar fcc elastic constants (transformed to trigonal orientation and converted to hcp values by taking internal strains into account) and hcp Ar-6½% 0₂ elastic constants revealed that the two sets of values are equal within the experimental error except for c₁₃ which is greater in the latter case by about 8%. An attempt has been made to explain this significant change. It is, however, still not well understood as to whether this change in c₁₃ is a structure or an impurity effect. Nevertheless, it is clear that future theoretical calculations should concentrate on c₁₃ and should include anisotropies in the potentials.

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