Structural and optical properties of pekovite and maleevite phosphor for white led applications

Abstract

Polycrystalline pekovite (SrB2Si2O8) and maleevite (BaB2Si2O8) phosphor doped with Eu and/or Dy ions of nominal composition (25-a-b)MO-25B2O3-50SiO2: aEu2O3, bDy2O3 where M = (Sr, Ba) and a, b = 1 or 0 mol% were synthesized via solid state reaction to study their structural and optical properties for white LED applications. The maleevite and pekovite phosphor were sintered at temperature of 950 and 1000 oC respectively for 5 hours in air and CO. The XRD results revealed that the incorporations of dopants caused the mean crystallite sizes to increase from 18.01 – 115.93 nm indicating improved crystallinity which is in orthorhombic structure and this was confirmed by SEM. The rare earth dopants were incorporated in the crystal and were confirmed by EDAX. FT-IR and FT-RAMAN spectra revealed the presence of borosilicate superstructure. Both danburite breathing mode along with multiple vibrational modes in FT-IR were found to be red-shifted when strontium were replaced with barium due to the larger ionic radii which increased Ba-O bond length. UV-VIS-NIR diffused reflectance spectra were collected and the optical band gaps of the samples were determined which were in the ranges 3.23 – 4.25 eV in pekovite and 3.42 – 3.7 eV in maleevite samples. Photoluminescence spectra of Eu3+ and Dy3+ ions were obtained from UV excitation which showed intense emission peaks at 483 and 575 nm from Dy3+ along with 590 and 613 nm from Eu3+ and the dopant site symmetry ratios were calculated. Combinations of blue, green and red emission from Eu3+ and Dy3+ showed its potential as a tune-able white light phosphor. Emissions at ~435 nm from Eu2+ were detected in all CO sintered phosphor and air sintered pekovite, exhibiting the unusual reduction of Eu3+ to Eu2+ in air. Overall, low cost phosphor materials which would be suitable for white LED applications were synthesized.