Structure and Hindered Vibration of Bi2+ in the Red-Orange Phosphor SrB4O7:Bi

The emission band profile of bismuth-doped SrB4O7 is measured with high resolution, and distinct vibronic features are observed at 4.2 K. The energy intervals show an irregular distribution. It is interpreted in the light of ab initio calculations as due to two off-center totally symmetric normal vibrational modes of Bi2+, with harmonic vibrational frequencies omega(1a') = 73 cm(-1) and omega(2a') = 89 cm(-1), the former being hindered by an energy barrier of about one-third of its vibrational frequency. Wave function-based ab initio calculations on the potential energy surfaces corresponding to the off-center displacements of Bi2+ are performed in the three lowest states of the (BiO9)(16-) cluster embedded in SrB4O7. Bi2+ is found to move off-center with respect to the Sr site of C-s point symmetry and to stay in the symmetry plane in the three states. It is also found to have two symmetric and one antisymmetric vibrational modes of frequencies spanning a small window around 100 cm(-1). Low energy barriers are found in the symmetric mode of lowest vibrational frequency in the ground state and in the first excited state. On the basis of the ab initio potential energy surface scenario, model calculations on the vibrational levels of two perturbed harmonic oscillators give the values of the empirical energy barrier and the vibrational frequencies.