title: Origin of Blue-Green Emission in α-Zn2P2O7and Local Structure of Ln3+Ion in α-Zn2P2O7:Ln3+(Ln = Sm, Eu): Time-Resolved Photoluminescence, EXAFS, and DFT Measurements.
abstract: Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm3+and Eu3+, in Zn2P2O7. Interestingly, it was oberved that pure Zn2P2O7displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to VO1+and VO2+defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the blue-green region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-Zn2P2O7. The fact that α-Zn2P2O7has two different coordination numbers for zinc ions, i.e. five- and six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium- and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between five- and six-coordinated Zn2+sites and it is the six-coordinated Zn2+site which is the most favorable for lanthanide ion doping. Extended X-ray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from Zn2P2O7to Eu3+, whereas this transfer is inefficient and almost incomplete in case of Sm3+, which is indeed important information for the realization of pyrophosphate-based tunable phosphors.
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Santosh Kumar
authors.initials: SK
authors.lastname: Gupta
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Partha Sarathi
authors.initials: PS
authors.lastname: Ghosh
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Ashok Kumar
authors.initials: AK
authors.lastname: Yadav
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Shambhu Nath
authors.initials: SN
authors.lastname: Jha
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Dibyendu
authors.initials: D
authors.lastname: Bhattacharyya
authors.affiliation: Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai 400085, India.
authors.forename: Ramakant Mahadeo
authors.initials: RM
authors.lastname: Kadam