High-pressure Raman spectroscopy has been used to study tris(hydroxymethyl)aminomethane (C(CH(2)OH)(3)NH(2), Tris). Molecules with globular shapes such as Tris have been studied thoroughly as a function of temperature and are of fundamental interest because of the presence of thermal transitions from orientational order to disorder. In contrast, relatively little is known about their high-pressure behavior. Diamond anvil cell techniques were used to generate pressures in Tris samples up to approximately 10 GPa. A phase transition was observed at a pressure of approximately 2 GPa that exhibited relatively slow kinetics and considerable hysteresis, indicative of a first-order transition. The Raman spectrum becomes significantly more complex in the high-pressure phase, indicating increased correlation splitting and significant enhancement in the intensity of some weak, low-pressure phase Raman-active modes.

The pressure/temperature phase diagram of LiAlH 4 has been constructed by using Raman spectroscopy data. In situ high pressure-temperature experiments were carried out using resistively heated diamond anvil cells up to 150 °C and 7 GPa. Room temperature phase transitions of monoclinic R-LiAlH 4 f δ-LiAlH 4 were observed at ∼3.2 GPa. As the temperature is increased to ∼100 °C, both the R and δ phases transform to β-LiAlH 4 and remain stable up to 5.5 GPa. At temperatures greater than 300 °C, a new γ-LiAlH 4 phase forms. Data of Konovalov (1995) has been used to define the phase boundary between βand γ-LiAlH 4 phases. We present a pressure-temperature phase diagram of LiAlH 4 based using diamond anvil cells coupled with Raman spectroscopy.

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