A high pressure FT-IR spectroscopic study of phase transitions in 1-chloro- and 1-bromoadamantane

The infrared spectra of two orientationally disordered adamantane derivatives, 1-chloro- and 1-bromoadamantane, have been obtained as a function of pressure by use of a diamond anvil cell. Phase transitions were detected from the splittings of vibrational bands and from changes in the slopes on plots of frequency versus pressure. In 1-chloroadamantane, a disorder-order phase transition was detected at approximately 5 kbar. This high-pressure phase is identical to the ordered phase obtained at low temperature. A second phase transition at high pressure was identified, for the first time, at approximately 17 kbar. The new high-pressure phase is an ordered phase. For 1-bromoadamantane, a semi-ordered to ordered phase transition was detected at about 5 kbar, and no other transitions were observed up to a pressure of 35 kbar. For both derivatives, the phase changes were reversible.