Phase Transitions in Solid Hydrogen at Megabar Pressures

Solid H_2 was studied using Raman scattering in a diamond anvil cell up to pressures of 167 GPa and from temperatures of 4.3 to 150 K. This thesis reports the observation of the orientationally ordered phase in para-H_2 at 1.1 megabars and 8 K. New results are also presented about the recently observed transition by Hemley and Mao at 77 K and 1.5 megabars in a ~50% ortho-para mixture. This thesis shows that, contrary to expectations, the new phase is not the continuation of the low pressure orientationally ordered phase. This new phase called hydrogen-A (H-A) was determined to exist only at high pressures. Other interesting properties include orientational ordering transitions within the H-A phase and apparent insensitivity of the phase line to ortho-para concentration. Lastly, we present evidence indicating that the H-A phase line will have a critical point at a pressure of about 173 GPa and 151 K.