  The reversible phase transition from hydrochloric-acid-doped ice VI to its
hydrogen-ordered counterpart ice XV is followed using differential scanning
calorimetry. Upon cooling at ambient pressure fast hydrogen ordering is
observed at first followed by a slower process which manifests as a tail to the
initial sharp exotherm. The residual hydrogen disorder in H2O and D2O ice XV is
determined as a function of the cooling rate. We conclude that it will be
difficult to obtain fully hydrogen-ordered ice XV by cooling at ambient
pressure. Our new experimental findings are discussed in the context of recent
computational work on ice XV.
