Climate and the Eclipse of the Ancient Cities of the Indus

Abstract

© 2018 The Royal Society of Chemistry. The vanadate-based phosphors Sr 2 V 2 O 7 :Eu 3+ (SV:Eu 3+ ), Sr 9 Gd(VO 4 ) 7 :Eu 3+ (SGV:Eu 3+ ) and Sr 9 Gd(VO 4 ) 7 /Sr 2 V 2 O 7 :Eu 3+ (SGV/SV:Eu 3+ ) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu 3+ based on the bond valence model. By comparing the change of bond energy when the Eu 3+ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu 3+ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu 3+ , SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu 3+ can be obtained through changing the concentration of Eu 3+ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O 2- → V 5+ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f-f transitions of Eu 3+ . The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O 2- → V 5+ CT of SV, and a sharp characteristic spectrum ( 5 D 0 - 7 F 2 ) at about 615 and 621 nm.