Ice-nine (Ice IX)
Ice-nine (ice IX)
is the low-temperature equilibrium, slightly denser, structure of ice-three (Space group P41212, cell dimensions
6.692 Å (a) and 6.715 Å (c) at 165 K and 280 MPa
). It is metastable in the
ice-two phase space and converts to ice-two, rather than back to ice-three, on warming. The change from proton disordered is a partial process starting within ice-three that is only completed at lower temperatures, but with a first order transition near 126 K. The hydrogen bonding is mostly proton-ordered
as ice-three undergoes a proton disorder-order transition
to ice-nine when rapidly cooled in liquid nitrogen (77 K,
so avoiding ice-two formation, see Phase
Diagram); ice-three and ice-nine having identical structures
apart from the proton ordering .
The ice-nine, described by Kurt Vonnegut in 'Cat's Cradle'
, with a freezing
point well above ambient under normal atmospheric pressure
is fortunately a completely fictitious material, reportedly
invented by the Nobel prize winner Irving Langmuir to entertain
H. G. Wells.