Blue absorption and red emission of Bi2+ in solids: strongly spin–orbit coupled 6p levels in low symmetry fields

Abstract

Wave function embedded cluster ab initio calculations on a (BiO₈)¹⁴⁻ cluster under the effects of a high symmetry O_h confinement potential are used to study the energies of the ²P_1/2, ²P_3/2(1), and ²P_3/2(2) spin–orbit coupling levels of the 6s²6p configuration of Bi²⁺ in O_h, D_4h, D_2h, D₄, D_2d, D₂, S₄, C_4v, C₄, C_3v, C_2v, C₂, C_s, and C₁ fields, together with the ²P_1/2 → ²P_3/2(1) and ²P_1/2 → ²P_3/2(2) absorption oscillator strengths and the ²P_3/2(1) radiative lifetime. These levels are responsible for the blue absorptions and the red-orange emissions produced when Bi²⁺ is doped in borates, phosphates, sulphates, and other hosts. It is found that the splitting of ²P_3/2 is mainly due to the tetragonal D_4h and orthorhombic D_2h components of the actual field. It is enhanced by Bi going towards two or four ligands. The intensities of the ²P_1/2 → ²P_3/2(1) and ²P_1/2 → ²P_3/2(2) absorptions are mostly induced by the Bi displacements and by tetragonal scalenoidal D_2d fields. The most favorable fields for a large splitting of the ²P_3/2 level that can drive a red shift of the ²P_3/2(1) → ²P_1/2 emission are the C_2v and C_s fields resulting from the combination of D_2h orthorhombic fields and Bi approaching two or four ligands on the main orthorhombic planes.