ε-TiO, a Novel Stable Polymorph of Titanium Monoxide

For the Ti/O system, three titanium monoxide (TiO) phases (α, β, and γ) with defective NaCl-type structures and a high-temperature hexagonal phase (H) have been known for decades. In this work, single crystals of a novel polymorph, ε-TiO, were synthesized by using a bismuth flux. X-ray diffraction (XRD) revealed a hexagonal crystal structure (a=4.9936(3) Å, c=2.8773(2) Å, P6? 2m) that is isotypic with ε-TaN. While the Ti atoms are surrounded by trigonal prismatic (sixfold coordination) and trigonal planar (threefold coordination) arrangements of O atoms, the O atoms are found in a pseudo-square-pyramidal arrangement of Ti atoms. First-principles calculations of the formation enthalpy and the electron and phonon density of states and crystal orbital Hamilton population (COHP) analysis revealed that ε-TiO is more stable than α-TiO, which had previously been regarded as the most stable phase at low temperatures.