  There have existed for a long time a paradigm that TiO phases at ambient
conditions are stable only if structural vacancies are available. Using an
evolutionary algorithm, we perform an ab initio search of possible
zero-temperature polymorphs of TiO in wide pressure interval. We obtain the
Gibbs energy of the competing phases taking into account entropy via
quasiharmonic approximation and build the pressure-temperature diagram of the
system. We reveal that two vacancy-free hexagonal phases are the most stable at
relatively low temperatures in a wide range of pressures. The transition
between these phases takes place at 28 GPa. Only above 1290 K at ambient
pressure the phases with vacancies (B1-derived) become stable. In particular,
the high-pressure hexagonal phase is shown to have unusual electronic
properties, with a pronounced pseudo-gap in the electronic spectrum. The
comparison of DFT-GGA and GW calculations demonstrates that the account for
many-body corrections significantly changes the electronic spectrum near the
Fermi energy.
