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

TL;DR

This study uses ab initio methods and evolutionary algorithms to explore the stability of vacancy-free TiO phases under high pressure and temperature, revealing new stable phases and their electronic properties.

Contribution

It identifies stable vacancy-free TiO phases at high pressures and temperatures, challenging the existing paradigm of vacancy necessity for stability.

Findings

Two stable vacancy-free hexagonal phases at low temperatures and wide pressure range.

Phase transition between these phases occurs at 28 GPa.

High-pressure phase exhibits a pseudo-gap in electronic spectrum.

Abstract

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.