  We report a comprehensive experimental investigation of the phase diagram of
ammonia hemihydrate (AHH) in the range of 2-30 GPa and 300-700 K, based on
Raman spectroscopy and x-ray diffraction experiments and visual observations.
Four solid phases, denoted AHH-II, DIMA, pbcc and qbcc, are present in this
domain, one of which, AHH-qbcc was discovered in this work. We show that,
unlike previously thought, the body-centered cubic (bcc) phase obtained on
heating AHH-II below 10 GPa, denoted here as AHH-pbcc, is distinct from the
DIMA phase, although both present the same bcc structure and O/N positional
disorder. Our results actually indicates that AHH-pbcc is a plastic form of
DIMA, characterized by free molecular rotations. AHH-qbcc is observed in the
intermediate P-T range between AHH-II and DIMA. It presents a complex x-ray
pattern reminiscent of the "quasi-bcc" structures that have been theoretically
predicted, although none of these structures is consistent with our data. The
transition lines between all solid phases as well as the melting curve have
been mapped in detail, showing that: (1) the new qbcc phase is the stable one
in the intermediate P-T range 10-19 GPa, 300-450 K, although the II-qbcc
transition is kinetically hindered for T < 450 K, and II directly transits to
DIMA in a gradual fashion from 25 to 35 GPa at 300 K. (2) The stability domain
of qbcc shrinks above 450 K and eventually terminates at a pbcc-qbcc-DIMA
triple point at 21.5 GPa-630 K. (3) A direct and reversible transition occurs
between AHH-pbcc and DIMA above 630 K. (4) The pbcc solid stability domain
extends up to the melting line above 3 GPa, and a II-pbcc-liquid triple point
is identified at 3 GPa-320 K.
