  The removal of residual hydrogen disorder from various phases of ice with
acid or base dopants at low temperatures has been a focus of intense research
for many decades. As an antipode to these efforts, we now show using neutron
diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for
the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice
VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice
VIII with ~31% residual hydrogen disorder illustrating the long-range
hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by
~20 K with respect to the hydrogen-ordering temperature of pure ice VII after
which the hydrogen-ordering took place slowly over a ~60 K temperature window.
These findings demonstrate that ND4F-doping slows down the hydrogen-ordering
kinetics quite substantially. The partial hydrogen order of the doped sample is
consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue
that local ferroelectric domains must exist between ionic point defects of
opposite charge. In addition to the long-range effect of NH4F-doping on
hydrogen-ordered water structures, the design principle of using topological
charges should be applicable to a wide range of other 'ice-rule' systems
including spin ices and related polar materials.
