Investigation of the Elastic Properties of Rb₄LiH₃(SO₄)₄ as a Function of Temperature and Pressure

Wu, WenLong (2004) Investigation of the Elastic Properties of Rb₄LiH₃(SO₄)₄ as a Function of Temperature and Pressure. Masters thesis, Memorial University of Newfoundland.

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Abstract

A high resolution acoustic interferometer is used to study the elastic properties of Rb₄LiH₃(SO₄)₄ as a function of temperature and pressure. The temperature and pressure ranges in our investigation were 4 K-300 K and 0 kbar-10 kbar, respectively. At ambient pressure, a phase transition at Tc = 132 ± 0.3 K is observed using the sound velocity measurements as a function of temperature. At room temperature, the occurrence of a phase transition at a pressure of Pc = 8.4 ± 0.2 kbar is discovered through sound velocity measurements as a function of pressure. Attempt has also been made to distinguish the x and y direction in the monoclinic phase. The results turn out to be still equivalent along these two directions. Using the sound velocity measurements as a function of temperature under different pressures, the temperature-pressure phase diagram of Rb₄LiH₃(SO₄)₄ is derived. It is found that Tc increases rapidly with pressure at a rate of 19±1K/kbar. Using the sound velocity measurements, the temperature and pressure dependence of five of the seven independent elastic constants of Rb₄LiH₃(SO₄)₄ are obtained. To our knowledge, we are the first to investigate the elastic constants as a function of pressure. We observed that the elastic constant (C₁₁-C₁₂)/2 shows incomplete softening at Tc or Pc. In the high symmetry phase, this elastic constant exhibits a nonlinear temperature or pressure dependence. A Landau model involving both temperature and pressure as external parameters is proposed. Our calculations agree well with our experimental results. Our model predicts that the actual soft acoustic mode shows complete softening at Tc. It also predicts that the actual soft acoustic mode propagates along the direction away [110] by -5⁰.

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