Ce3+ Luminescence in Hexagonal Aluminates Containing Large Divalent or Trivalent Cations

Abstract

By analogy with earlier studies on Eu2+ phosphors the luminescence of the Ce3+‐doped hexagonal aluminates, containing large, divalent or trivalent cations, was also studied in relation to the crystal structure of the host lattices. The u.v. band ascribed to the luminescence of Ce3+ ions is found at a relatively short wavelength in the magnetoplumbites Ca‐, Sr‐, and aluminate: Ce and at a relatively long wavelength in the β‐aluminas Ba‐ and aluminate. The distorted magnetoplumbites which include La‐ and aluminate and Ce‐ and aluminates themselves occupy an intermediate position in between the above‐mentioned groups. The existence of various types of host lattices is also reflected in the shape and position of the excitation spectra of the Ce3+ luminescence. Differences in bond strength between Ce3+ ions and its surrounding oxygen ions, however, complicate the discussion of the position of the excited 5d levels of the Ce3+ ion. Apart from the u.v. band, a blue luminescence band was found in several Ce‐activated aluminates. This band occurs in phosphors where oxygen ions are thought to have partially replaced the large cations of the host lattice and is ascribed to Ce‐O associates. Next to direct excitation of these centers, the u.v.‐emitting Ce3+ ions transfer part of their energy of the associates. If Mg2+ ions are present in the lattice, the oxygen ions at large cation sites seem to be "screened" and no blue luminescence is found. The quantum efficiency of most Ce‐activated aluminate phosphors is 50–70%. Due to ineffective mutual energy transfer between u.v. emitting Ce3+ ions and between Ce‐O associates no concentration quenching occurs in the systems considered. For the same reason the influence of the preparation method on the luminescence properties is generally less drastic than that of the corresponding Eu2+ phosphors.