Two successive amorphous‐to‐amorphous phase transformations in TiO2

Abstract

AbstractBased on constant pressure ab initio simulations, we propose, for the first time, two successive amorphous‐to‐amorphous phase transformations for TiO2. The first one is a gradual phase transformation from a low‐density amorphous phase to a high‐density amorphous phase, whereas the second one is a first‐order phase transformation from the high‐density amorphous phase to a very high‐density amorphous phase. The low‐density amorphous to high‐density amorphous phase change is irreversible, whereas the high‐density amorphous to very high‐density amorphous phase transformation is reversible. The high‐density amorphous and very high‐density amorphous phases consist of differently coordinated configurations. The sevenfold and ninefold‐coordinated arrangements formed in amorphous TiO2 under pressure are similar to the main building motif of the baddeleyite and cotunnite polymorphs of TiO2, respectively, while the eightfold‐coordinated configuration is different from the local structure of the cubic TiO2 phase. The electronic structure calculations suggest that both dense amorphous phases present a semiconducting character with a band gap energy less than that of the original low‐density amorphous phase.