Raman study of phase transition and hydrogen bond symmetrization in solid DCl at high pressure

Abstract

Phase transitions in solid DCl have been investigated by Raman spectral measurement up to 70 GPa at 298 K. The I-III phase transition accompanied by orientational ordering of the molecules took place at $19.0\ifmmode\pm\else\textpm\fi{}0.5\mathrm{GPa},$ exactly the same pressure as observed for solid HCl. A further transition with hydrogen bond symmetrization (III-IV transition) was observed at about $56\ifmmode\pm\else\textpm\fi{}2\mathrm{GPa}$ higher by 5 GPa than the symmetrization pressure of HCl previously determined. The bond symmetrization is realized when the midpoint barrier separating two potential minima along the hydrogen-bonding axis is depressed and smeared out quantum mechanically. The observed isotope effects on the symmetrization pressure are hence interpreted in terms of proton (deuteron) tunneling transfer in the hydrogen bonding potential, which gradually is deformed toward a single minimum shape with increasing pressures.