The geometrical structure and electronic properties of trivalent Ho 3+ doped Y 2 O 3 crystals: a first-principles study.

Trivalent rare-earth holmium ion (Ho 3+ ) doped yttrium oxide (Y 2 O 3 ) has attracted great research interest owing to its unique optoelectronic properties and excellent performances in many new-type laser devices. But the crystal structures of the Ho 3+ -doped Y 2 O 3 system (Y 2 O 3  : Ho) are still unclear. Here, we have carried out a first-principle study on the structural evolution of the trivalent Ho 3+ doped Y 2 O 3 by using the CALYPSO structure search method. The results indicate that the lowest-energy structure of Ho 3+ -doped Y 2 O 3 possesses a standardized monoclinic P 2 phase. It is found that the doped Ho 3+ ion are likely to occupy the sites of Y 3+ in the host crystal lattice, forming the [HoO 6 ] 9- local structure with C 2 site symmetry. Electronic structure calculations reveal that the band gap value of Ho 3+ -doped Y 2 O 3 is approximately 4.27 eV, suggesting the insulating character of Y 2 O 3  : Ho system. These findings could provide fundamental insights to understand the atomic interactions in crystals as well as the information of electronic properties for other rare-earth-doped materials.