Ice III is a hydrogen-disordered phase of ice that is stable between about 0.2 and 0.35 GPa. Upon cooling, it transforms to its hydrogen-ordered counterpart ice IX within the stability region of ice II. Here, the effect of ammonium fluoride doping on this phase transition is investigated, which is followed for the first time with in situ neutron diffraction. The a and c lattice constants are found to expand and contract, respectively, upon hydrogen ordering, yielding an overall negative volume change. Interestingly, the anisotropy in the lattice constants persists when ice IX is fully formed, and negative thermal expansion is observed. Analogous to the isostructural keatite and β-spodumenes, the negative thermal expansion can be explained through the buildup of torsional strain within the a–b plane as the helical “springs” within the structure expand upon heating. The reversibility of the phase transition was demonstrated upon heating. As seen in diffraction and Raman spectroscopy, the ammonium fluoride doping induces additional residual hydrogen disorder in ice IX and is suggested to be a chemical way for the “excitation” of the configurational ice-rules manifold. Compared to ice VIII, the dopant-induced hydrogen disorder in ice IX is smaller, which suggests a higher density of accessible configurational states close to the ground state in ice IX. This study highlights the importance of dopants for exploring the water’s phase diagram and underpins the highly complex solid-state chemistry of ice
