Measurement of the ortho-to-para hydrogen conversion in the high-pressure compound Ar(H-2)(2)

Ortho-para hydrogen conversion kinetics has been measured in the stoichiome tric Ar(H-2)(2) compound, at pressures from 3 to 35 GPa, by analysing the i ntensity of the rotational Raman transitions. The validity of this method t o measure ortho-para composition for high-pressure solids is discussed, and verified for the pressures used lee this work. The conversion rate increas es with pressure of almost a factor of 50 in the investigated pressure rang e, but always remains lower than that measured for pure solid hydrogen at t he same pressure. The different crystal structure, in particular the lower number of nearest neighbours, plays a significant role in this difference.