Chemical Pressure Effects on the Stokes Shift of Bi3+ Luminescence in Orthorhombic Perovskites

The relationship between the Stokes shift of Bi3+ emission and the volume of the unit-cell in two series of orthorhombic perovskites, LnB O-3+(3) (Ln = La, Gd, Y; B (3+) = Al, In, Ga) and AB O-4+(3) (A = Ca, Sr; B (4+) = Zr, Sn) is explored. The Stokes shift increases linearly with increasing cell volume. This is explained qualitatively by the lattice chemical pressure acting on the Bi3+ ion. The degree of Bi3+ ion off-centering displacement, which is due to the stereochemical activity of the lone-pair electrons (6 s(2)), is controlled by the chemical pressure. A small cell suppresses the off-centering displacement to produce a small Stokes shift of emission by limiting the excited state structural distortion. In large cell, the off-centering displacement is more easily accommodated. The elimination of ground state distortion in the excited state gives larger Stokes shift of emission. These qualitative arguments are supplemented by recent first-principles calculations on Bi3+ luminescence in these perovskites. The Bi3+ luminescence in SrZrO3, previously assigned to emission from the D-state, is now assigned to the localized P-3(0,1) -> S-1(0) transition. The energy of the S-1(0) -> P-3(1) transition is correlated with the covalence of the BO6/2 perovskite framework. Discussion on the effective ionic radius of the Bi3+ ion in these perovskites is presented.

Automated summary

The relationship between the Stokes shift of Bi3+ emission and the volume of the unit-cell in orthorhombic perovskites has been explored. It was found that the Stokes shift increases linearly with increasing cell volume, which can be explained by the lattice chemical pressure acting on the Bi3+ ion. The off-centering displacement of the Bi3+ ion is controlled by the chemical pressure, and a larger cell allows for a larger off-centering displacement, resulting in a larger Stokes shift of emission.