Phase transitions in solid DCl have been investigated by Raman spectral mea
surement up to 70 GPa at 298 K. The I-III phase transition accompanied by o
rientational ordering of the molecules took place at 19.0 +/-0.5 GPa, exact
ly the same pressure as observed for solid HCl. A further transition with h
ydrogen bond symmetrization (III-IV transition) was observed at about 56+/-
2 GPa higher by 5 GPa than the symmetrization pressure of HCl previously de
termined. The bond symmetrization is realized when the midpoint barrier sep
arating two potential minima along the hydrogen-bonding axis is depressed a
nd smeared out quantum mechanically. The observed isotope effects on the sy
mmetrization pressure are hence interpreted in terms of proton (deuteron) t
unneling transfer in the hydrogen bonding potential, which gradually is def
ormed toward a single minimum shape with increasing pressures.