Bonding Scheme and Optical Properties in BiM2 O2 (PO4 ) (M=Cd, Mg, Zn); Experimental and Theoretical Analysis.

Luminescence properties of the Bi(M,M')2 PO6 (M=Mg, Zn, Cd) series have been rationalized as a function of the M element using optical spectroscopy, as well as empirical and first principles calculations. The latter yielded indirect band gaps for all compounds with energies between 2.64 and 3.62 eV, whereas luminescence measurements exhibit bright warm white emission luminescence even at room temperature assigned to Bi3+ transitions with, for example, 22.8 % quantum yield for M=Mg. The energies of the excitation maxima are shifted with the covalent character of the Bi-O bond by inductive effects of the neighboring M-O bonds. This is discussed on the basis of empirical and electronic structure calculations. Strikingly, in all the investigated compounds, an excitation process occurring at energies higher than the band gaps is observed, which seems to be intrinsic to the s2 →sp electronic transitions of the Bi3+ ions. Concerning the emission process, a direct correlation between the lone pair (LP) activity and the emission energy upon change of the lattice parameters was established governing the LP stereo-activity in the BiMg2-x Cdx PO6 system. As a result, the possibility for tunable optical properties appears realistic in the Bi2 O3 -MO-X2 O5 (X=P, V, As, etc.) systems taking into account the diversity of reported or novel crystal structures that can be designed using well-established rules of crystal chemistry.