ABiO 2 X (A = Cd, Ca, Sr, Ba, Pb; X = halogen) Sillen X1 Series: Polymorphism Versus Optical Properties

Overview & Synopsis

The Sillen X1 series of Bi 3+ A 2+ O 2 X (A = Cd, Ca, Sr, Ba, Pb; X = Cl, Br, I) compounds is composed of three main crystallographic types, namely, the tetragonal form (space group (S.G.) I4/mmm), the orthorhombic form (S.G. Cmcm), and the monoclinic form (S.G. P2 1 /m). Because of Bi 3+ /A 2+ disorder the Bi 3+ based photoluminescence (PL) of the tetragonal polytypes is quenched at room temperature (RT). In the two other ordered forms, the Bi−O−Bi connectivity is different but limited, such that bluish/ greenish emission occurs at RT in the monoclinic CdBiO 2 Cl and CaBiO 2 Cl and orthorhombic SrBiO 2 Cl and BaBiO 2 Cl phases. The crystal structure of BaBiO 2 Br was refined in the orthorhombic Cmcm space group and also shows RT emission. Focusing on the RT luminescent activity as a key parameter, the PL active compounds were investigated by means of density functional theory calculations and UV−visible reflectance spectroscopy. The influence of A and X ions on the excitation energy is discussed by analyzing the A−O−Bi and Bi−X bonding schemes and gives some insights for rational tuning of both the excitation and emission energies.