Eur. Phys. J. Special Topics 229, 179-185 (2020)
https://doi.org/10.1140/epjst/e2019-900113-5

Regular Article

Stability of vacancy-free crystalline phases of titanium monoxide at high pressure and temperature

¹ Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, 108840 Troitsk, Moscow, Russia
² Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia
³ Ural Federal University, Ekaterinburg 620002, Russia
⁴ Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg 620016, Russia
⁵ Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia

^a e-mail: n.chtchelkatchev@gmail.com

Received: 17 June 2019
Published online: 11 February 2020

Abstract

There has 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.