ɛ‐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, ?‐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, P 2m) that is isotypic with ?‐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 ?‐TiO is more stable than α‐TiO, which had previously been regarded as the most stable phase at low temperatures.