The Microstructure and Electronic Properties of Yttrium Oxide Doped With Cerium: A Theoretical Insight.

Trivalent Cerium (Ce) doped Yttrium Oxide (YO) host crystal has drawn considerable interest due to its popular optical 5d-4f transition. The outstanding optical properties of YO:Ce system have been demonstrated by previous studies but the microstructures still remain unclear. The lacks of YO:Ce microstructures could constitute a problem to further exploit its potential applications. In this sense, we have comprehensively investigated the structural evolutions of YO:Ce crystals based on the CALYPSO structure search method in conjunction with density functional theory calculations. Our result uncovers a new rhombohedral phase of YO:Ce with R-3 group symmetry. In the host crystal, the Y ion at central site can be naturally replaced by the doped Ce, resulting in a perfect cage-like configuration. We find an interesting phase transition that the crystallographic symmetry of YO changes from cubic to rhombohedral when the impurity Ce is doped into the host crystal. With the nominal concentration of Ce at 3.125%, many metastable structures are also identified due to the different occupying points in the host crystal. The X-ray diffraction patterns of YO:Ce are simulated and the theoretical result is comparable to experimental data, thus demonstrating the validity of the lowest energy structure. The result of phonon dispersions shows that the ground state structure is dynamically stable. The analysis of electronic properties indicate that the YO:Ce possesses a band gap of 4.20 eV which suggests that the incorporation of impurity Ce ion into YO host crystal leads to an insulator to semiconductor transition. Meanwhile, the strong covalent bonds of O atoms in the crystal, which may greatly contribute to the stability of ground state structure, are evidenced by electron localization function. These obtained results elucidate the structural and bonding characters of YO:Ce and could also provide useful insights for understanding the experimental phenomena.