Robust Giant Tetragonal Distortion Coupled with High-Spin Co in Electron-Doped BiCoO.

BiCoO is a PbTiO type of perovskite oxide with a giant tetragonal distortion (/ = 1.27) that shows a pressure-induced transition from tetragonal to orthorhombic phases accompanied by a large volume shrinkage at 3 GPa. In this study, we carried out electron doping of BiCoO by substituting Ti for Co in order to destabilize the tetragonal phase and observe a giant negative thermal expansion (NTE) at ambient pressure. BiCoTiO ( = 0, 0.1, 0.2, and 0.25) was successfully obtained by using high-pressure synthesis. However, the / ratio of the tetragonal phase was almost constant against (≤0.2), and NTE was not observed at any , suggesting that the tetragonal distortion coupled with high-spin Co is robust against electron doping. In = 0.25, a metastable orthorhombic phase was obtained by the high-pressure synthetic process, while it partially transformed into a tetragonal phase after annealing at 600 K. The stability of the giant tetragonal phase is strongly connected with the spin state of Co.