Electronic structure and luminescence assets in white-light emitting Ca(2)V(2)O(7), Sr(2)V(2)O(7) and Ba(2)V(2)O(7) pyro-vanadates: X-ray absorption spectroscopy investigations

Ca(2)V(2)O(7), Sr(2)V(2)O(7), and Ba(2)V(2)O(7) pyro-vanadates were synthesized using a modified chemical precipitation method and annealing. Detailed crystal structure, morphology, electronic structure and optical properties were investigated by XRD, UV-visible absorption, FTIR, Raman, FE-SEM, XANES, and photoluminescence spectroscopy. Rietveld refinement on the XRD patterns of Ca(2)V(2)O(7), Sr(2)V(2)O(7), and Ba(2)V(2)O(7) has confirmed the triclinic structure (space group; P1̄(2)) of the pyro-vanadates. The band gap energy of Ca(2)V(2)O(7), Sr(2)V(2)O(7), and Ba(2)V(2)O(7) is estimated to be ∼2.67 eV, ∼2.97 eV and ∼3.09 eV, respectively. XANES spectra at the Ca L-edge, Sr K-edge and Ba L-edge have confirmed the Ca(2+), Sr(2+) and Ba(2+) ions in the Ca(2)V(2)O(7), Sr(2)V(2)O(7) and Ba(2)V(2)O(7) compounds, respectively. V K-edge XANES spectra have strengthened the presence of sub-pentavalent V ions in all of the pyro-vanadates. O K-edge XANES spectra of Ca(2)V(2)O(7), Sr(2)V(2)O(7) and Ba(2)V(2)O(7) have shown dominating tetrahedral symmetry of the V ions which is also corroborated with the V K-edge XANES. Broad-band emission spectra, ranging from 400 nm to 700 nm, have been observed from the charge-transfer transitions of VO(4) tetrahedra. (3)T(1) → (1)A(1) and (3)T(2) → (1)A(1) transitions, from the VO(4) tetrahedra, have provided two distinct emission peaks from the compounds which exhibit a red-shift with the decreasing ionic-radii of alkali-earth metal ions. The mixed compounds, with equal weight proportions, have shown remarkable emission characteristics towards the realization of rare-earth element free white-light-emitting devices.