  The D2O ice VI to ice XV hydrogen ordering phase transition at ambient
pressure is investigated in detail with neutron diffraction. The lattice
constants are found to be sensitive indicators for hydrogen ordering. The a and
b lattice constants contract whereas a pronounced expansion in c is found upon
hydrogen ordering. Overall, the hydrogen ordering transition goes along with a
small increase in volume which explains why the phase transition is more
difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4
GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier
studies, the ice XV obtained after slow-cooling at ambient pressure is best
described with P-1 space group symmetry. Using a new computational approach, we
achieve the atomistic reconstruction of a supercell structure that is
consistent with the average partially ordered structure derived from Rietveld
refinements. This shows that C-type networks are most prevalent in ice XV but
other structural motifs outside of the classifications of the fully
hydrogen-ordered networks are identified as well. The recently proposed Pmmn
structural model for ice XV is found to be incompatible with our diffraction
data and we argue that only structural models that are capable of describing
full hydrogen order should be used.
