We use electron momentum density in ice as a tool to quantify order-disorder transitions by comparing Compton profiles differences of ice VI, VII, VIII, and XII with respect to ice Ih. Quantitative agreement is found between theory and experiment for ice VIII, which is the fully hydrogen ordered phase. Robust signatures of the oxygen disorder are identified in the momentum density for the VIII-VII ice phase transition. The unique aspect of this work is the determination of the fraction $n_e$ of electron directly involved in phase transitions as well as the use of position space signatures for quantifying oxygen site disorder.

• Received 6 July 2010

DOI:https://doi.org/10.1103/PhysRevB.83.094117