Bonding Scheme and Optical Properties in BiM₂O₂(PO₄) (M=Cd, Mg, Zn); Experimental and Theoretical Analysis

Abstract

Luminescence properties of the Bi(M,M') PO (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 Bi 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 s →sp electronic transitions of the Bi 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 BiMg Cd PO system. As a result, the possibility for tunable optical properties appears realistic in the Bi O -MO-X O (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.

Identification and optical features of the Pb4Ln2O7 series (Ln = La, Gd, Sm, Nd); genuine 2D-van der Waals oxides

“…4a. The broad excitation bands match well the reflectance spectrum shown figure 4c and correspond to the host-lattice absorption for which a contribution of a (Pb)-O(2p) → Ln (4f) charge transfer is active 25 . From the PDOS calculations, it was shown that the 6s and 6p states of Pb are respectively localized at the top of the VB and the bottom of the CB, leading to host absorption mainly driven by the Pb 2+ : 6s 2 → 6sp transitions and so it can be assumed that Pb 2+ play the role of sensitizer.…”

“…The local charge were calculated using the Henry's model implemented in Winpacha, using non empirical scales of atomic electronegativity and hardness. 24,25 It gives results of the Table S2 with in average ca. -0.8 and -0.5 on core and external oxygen respectively and ca.…”

“…If the former play for a redshift of the excitation energy due to both the nephelauxetic effect, the LP stereoactivity tune the stoke shift shifting the excited-state potential relative to the ground state, during the 6s 2 → 6sp transition, see ref. 16,25 iii) In addition, one benefits of the large absorption section and broad spectra of ns 2 cations able to charge transfer via oxygen ligands. Pb4La2O7 contains Pb 2+ ions that can act as activators, 10 but no room temperature emission can be detected, most probably due to a concentration quenching.…”

Direct Bi3+–Bi3+ contacts mediated by lone pairs in the HP-BiNiO(PO4) polymorph

“…The main features of the LDA+U density of states (DOS, Fig. 3b,c) are typical for bismuth transition metal phosphates [4] and conserve the same topology for both the AP and HP-polymorphs. The coordinates deduced from the relaxation are presented in Table 1 and compared with those deduced from the Rietveld refinement.…”

“…[1] The quasi-systematic polymerization of O(Bi,M)4 anti-tetrahedra (A-Td) into versatile (0D to 3D) types of structural units is allowed by the flexibility of the A-Tds able to co-admit, Bi 3+ ions, alkali, alkali-earths, transition metals or heavy metals… into more or less ordered frameworks. [2] Among the simplest ordered terms, the BiMO(XO4) and BiM2O2(XO4) series (M = Ca, Cd, Co, Mg, Mn, Ni, Pb and X = As, P,V) [1] were mainly studied for their optical [3], [4] or magnetic properties. [5]- [8] For instance BiCu2O2(PO4) (BCPO) is a frustrated two-leg S = 1/2 ladder with a cascade of field induced transitions.…”