VUV–Vis Luminescent Properties of BaCaBO3F Doped with Ce3+ and Tb3+

Abstract

The BaCaBO3F wide band gap host and the charge-compensated phosphors BaCaBO3F:Ln3+ (Ln = Ce, Tb, Gd) and BaCaBO3F:Ce3+,Tb3+ have been synthesized by a solid-state reaction method at high temperature. Their spectroscopic properties in the vacuum ultraviolet (VUV)–vis range have been investigated. The band gap of the host lattice is estimated at 7.8 eV, which is considerably higher than the value found from its diffuse reflectance spectrum. The 5d crystal field level locations, Stokes shift, and Huang–Rhys factor have been determined from the VUV excitation and near-UV emission spectra of BaCaBO3F:Ce3+. The 5d1 decay lifetime is 29 ns at x ∼ 0.005. Concentration quenching occurs for Ba1–2xCexNaxCaBO3F with maximum intensity at x ∼ 0.035 and with critical distance Rc = 17.7 Å. An unusual broad, intense band in the excitation spectrum of BaCaBO3F:Gd3+ is assigned to a near-defect exciton. Very weak emission from 5D3, and intense green emission from 5D4, has been assigned for BaCaBO3F:Tb3+. In the VUV region, the spin-allowed and the spin-forbidden 4f8 → 4f75d transitions are observed. The codoped system BaCaBO3F:Ce3+,Tb3+ exhibits emission color tunability when varying the excitation wavelength or the dopant ion concentration. The energy transfer from Ce3+ to Tb3+ takes place in the fast migration regime with kET ∼ 106–107 s–1 and with energy-transfer efficiency up to 23% for the samples investigated. The mechanism is revealed from energy level and decay measurements. This codoped system exhibits absorption bands near the major Xe discharge wavelengths.