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

Abstract

We report on the identification and survey of the Pb4Ln2O7 series (Ln = La, Gd, Sm and Nd) which turn out to be real van der Waals 2D oxides. In the neutral layers, strong covalent Pb–O bonds together with external stereoactive Pb2+ lone pairs, which act as sensitizers, lead to an ideal matrix for enhanced and tunable luminescence by lanthanide emitters, tested here for Sm3+ and Eu3+ doping. DFT calculations and preliminary ex-solution experiments validate the weak bonding between the layers separated by 3.5 Å and suggest a indirect to direct crossover realized by isolating the layers