First-principles study of ${\mathrm{Bi}}^{3+}$-related luminescence and traps in the perovskites $\mathrm{Ca}M{\mathrm{O}}_3\left(M=\mathrm{Zr},\mathrm{Sn},\mathrm{Ti}\right)$

The ${\mathrm{Bi}}^{3+}$ ion is an excellent activator and sensitizer for luminescent materials. However, the complexity and variety of the ${\mathrm{Bi}}^{3+}$-related transitions bring a great challenge to the study of luminescence processes of ${\mathrm{Bi}}^{3+}$ doped materials. Here, we presented first-principles calculations to determine the excitation, relaxation, and emission processes of ${\mathrm{Bi}}^{3+}$ activated materials by using $\mathrm{Ca}M{\mathrm{O}}_3:{\mathrm{Bi}}^{3+}\left(M=\mathrm{Zr},\mathrm{Sn},\mathrm{Ti}\right)$ as prototype systems, where ${\mathrm{Bi}}^{3+}$ substitutes ${\mathrm{Ca}}^{2+}$ in similar coordinate environments but presents tremendously different excitation and emission spectra. The equilibrium geometric structures of excited states were calculated based on density-functional theory (DFT), with appropriately constraining the electron occupation and including the spin-orbit couplings. Then the hybrid DFT calculations were carried out to obtain the electronic structures and defect levels. Different metastable excited states and Stokes shift were obtained for $M=\mathrm{Zr}$, Sn, and Ti, which explain the remarkable differences in the measured emission spectra. The energies of three types of transitions are obtained from the calculations, including intra-${\mathrm{Bi}}^{3+}$ bands transition and charge transfer between ${\mathrm{Bi}}^{3+}$ ions and the band edges. This leads to a clear and reliable interpretation of all the excitation spectra in the series. The method and its applications to $\mathrm{Ca}M{\mathrm{O}}_3:{\mathrm{Bi}}^{3+}$ show the potential of first-principles calculations in analyzing and predicting luminescent properties of ${\mathrm{Bi}}^{3+}$ activated materials.

Figure 1

Left: schematic configuration coordinate diagram of the excitation and emission involving $A$ band, the lowest MMCT and CT states; right: thermodynamic charge transition levels, with double directional arrows showing the transitions zero-phonon lines.

Figure 2

(a) Crystalline structure of $\mathrm{Ca}M{\mathrm{O}}_3$ orthorhombic distorted perovskite. (b) Polyhedron diagram of ${\mathrm{Ca}}^{2+}$ site in $\mathrm{Ca}M{\mathrm{O}}_3$, which is coordinated with 12 oxygen atoms, with four oxygen ligands of bond lengths are more than 3 Å showing in green.

Figure 3

PBE calculated band structures, with band gaps taken from hybrid DFT calculation for pristine hosts: (a) ${\mathrm{CaTiO}}_3$, 3.72 eV; (b) ${\mathrm{CaSnO}}_3$, 4.85 eV; (c) ${\mathrm{CaZrO}}_3$, 5.94 eV, and the corresponding total or partial densities of states shown as DOS and colored PDOS, respectively.

Figure 4

The VRBE of ${\mathrm{Bi}}^{3+}$ induced defect levels and their zero-phonon transitions relative to the VBM in $\mathrm{Ca}M{\mathrm{O}}_3$ perovskites $\left(M=\mathrm{Zr},\mathrm{Sn},\mathrm{Ti}\right)$ calculated with hybrid DFT, where solid lines represent thermodynamic charge transition levels and dash lines represent optical charge transition levels. The VBM by hybrid DFT calculation is aligned relative to the vacuum state.

Figure 5

Configurational coordinate diagrams of the defects’ potential surfaces as a function of the generalized configuration coordinate for $\mathrm{Ca}M{\mathrm{O}}_3$ perovskites $\left(M=\mathrm{Zr},\mathrm{Sn},\mathrm{Ti}\right)$. Notes: (1) ${\mathrm{Bi}}^{3+}$ and ${\left({\mathrm{Bi}}^{3+}\right)}^{*}$ denote the ground and ${}^{3}P_{0,1}$ excited states of ${\mathrm{Bi}}^{3+}$ in the host, respectively. (2) ${\mathrm{Bi}}^{4+}+e_{\mathrm{CBM}}$ simulates ${\mathrm{Bi}}^{4+}$ with an electron at CBM, representing the lowest MMCT excited state. (3) ${\mathrm{Bi}}^{2+}+h_{\mathrm{VBM}}$ simulates ${\mathrm{Bi}}^{2+}$ with a loose hole at VBM, representing the lowest CT excited state. (4) Energy surfaces for ${\left({\mathrm{Bi}}^{3+}\right)}^{*}$ and ${\mathrm{Bi}}^{2+}+h_{\mathrm{VBM}}$ in ${\mathrm{CaTiO}}_3$ are estimated by referring to ${\mathrm{CaZrO}}_3$ for energy relaxation and are drawn in dashed lines for reference only.