Effect of ammonium fluoride doping on the ice III to ice IX phase transition

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