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

Abstract

The relationship between the Stokes shift of Bi3+ emission and the volume of the unit-cell in two series of orthorhombic perovskites, LnB 3+O3 (Ln = La, Gd, Y; B 3+ = Al, In, Ga) and AB 4+O3 (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 s2), 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 3P0,1 → 1S0 transition. The energy of the 1S0 → 3P1 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.