Brillouin scattering experiments near the α-β phase transition in single crystals of the N₂ and CO

Askarpour, Vahid (1986) Brillouin scattering experiments near the α-β phase transition in single crystals of the N₂ and CO. Masters thesis, Memorial University of Newfoundland.

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Abstract

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants of large cooled single crystals of β-N₂ and β-CO in an effort to gain insight into the α-β phase transition of these two unique examples of diatomic molecules which are extremely similar in many physical and chemical properties. The crystals were grown in a quartz cell inside a liquid helium cryostat. Laue x-ray diffraction analysis provided for the determination of the orientations of these crystals relative to the laboratory frame of reference. An argon ion laser beam was incident on the crystal and the scattered beam was analysed at 90° relative to the incident beam by a piezo-electrically scanned triple-pass Fabry-Perot interferometer. The signals were then collected by a photomultiplier tube, amplified through an amplifier/discriminator assembly and finally recorded by the data acquisition and stabilization system. -- The values of elastic constants of β-N₂ at 47.5 K in units of 10⁹ N/m² (subject to an absolute uncertainty of about 2%) are: c₁₁-2.307 c₁₂-1.454 c₁₃-1.274 c₃₃-2.488 c₄₄-0.378. -- The values of elastic constants of β-CO at 62 K in units of 10⁹ N/m² (subject to an absolute uncertainty of about 4%) are: c₁₁-2.013 c₁₂-1.212 c₁₃-0.998 c₃₃-2.227 c₄₄-0.402. These values were compared to those at the corresponding triple points. There is an approximate elastic constant increase of 1% per 1 K of temperature decrease and there seems to be no evidence for mode softening in either N₂ or CO. -- Three large single crystals of β-CO were cooled slowly through the phase transition resulting in three single crystals of α-CO. The orientations of these face centered cubic crystals were determined and correlated to those of the hexagonal close packed phase. It was noted that the hexagonal basal planes appear to prefer to transform to planes of the form {110} of the cubic structure. A possible mechanism is discussed and the result is compared to previous works on metals and molecular crystals that undergo h.c.p. to f.c.c. phase transition.

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