Brillouin spectroscopic studies of the elastic properties of ice up to 10 kbar

Gagnon, Robert (1986) Brillouin spectroscopic studies of the elastic properties of ice up to 10 kbar. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The technique of high resolution Brillouin spectroscopy has been used to determine accurately, and for the first time, the hydrostatic pressure dependence of the elastic constants of ice Ih in the full pressure range of phase stability and metastability, 0-2.8 kbar, at -35.5°C. The temperature dependence of the elastic constants has also been determined in the temperature range -4°C to -35°C. -- A thermoelectrically cooled cryostat and a 10 kbar optical cell were constructed to house rotatable cylindrical specimens. Samples were prepared from large single crystals of Mendenhall glacial ice. Prior to the laser scattering experiments the c-axis orientations of the monocrystalline specimens were determined to within 0.5° by a unique optical birefringence device. -- The elastic constants were determined by analyzing the Brillouin frequency shifts as a function of crystal orientation. The absolute zero pressure values agreed to about 1% with other Brillouin spectroscopic studies of ice Ih. The percentage changes in the elastic constants c₁₁, c₁₂, c₁₃, c₃₃, c₄₄ and bulk modulus over the full pressure range, are 3.0, 7.4, 8.0, 2,8, -1.5 and 5.1 percent per kbar respectively. The negative pressure dependence of c₄₄, which is softening of the shear modes, indicates decreasing crystal stability as the phase transition to ice III (at ~3 kbar) is approached. -- Longitudinal and transverse acoustic velocities have also been determined for isotropic, finely polycrystalline, aggregates of four high pressure ice phases. The bulk moduli (±5%) derived from the velocity data, for ice II, III, V and VI are 138.9, 98.7, 141.9 and 181.4 kbar respectively. -- To obtain ice densities, so that refractive indices could be calculated for use in the Brillouin equation, a technique has been developed whereby the variation of sample volume with pressure has been measured directly in the pressure range 0-10 kbar. Isothermal bulk moduli derived from this data agree to within 5% with the adiabatic values calculated from the velocity data. -- A scheme has been worked out to classify the various ice phases, based on knowledge of the hydrogen bonds, which can qualitatively explain the observed velocity data and can furthermore be used to predict approximate velocities for phases for which no velocity data presently exist. -- There is excellent agreement between the present longitudinal velocity results and other published data. There are discrepancies between previous and present shear wave velocity results, however. The present values appear to be more accurate, at the pressures of the midpoints of the phases at least, as indicated by the consistency of the values for the bulk moduli derived using the present density data and the present velocity data.

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