Luminescence properties of silicate apatite phosphors M2La8Si6O26:Eu (M = Mg, Ca, Sr)

Europium ions doped M2La8Si6O26 (M = Mg, Ca, Sr) silicate apatite phosphors have been synthesized by using solid-state reaction method, post-treated in a reducing atmosphere and studied by optical spectroscopy techniques. It has been revealed that the emission spectra from all the synthesized samples contain a set of spectral lines due to the 4f - 4f transitions of Eu3+ ions, which predominantly occupy the A2 sites of Cs symmetry in the apatite structure. Efficient conversion of Eu3+ to Eu2+ has been achieved in a reducing H2/Ar atmosphere for the Mg2La8(SiO4)6O2 apatite due to the charge compensation mechanism by oxygen vacancies. This apatite possesses broadband yellow emission with the band maximum at 560 nm and the luminescence decay time τ < 1 μs due to the 5d − 4f Eu2+ transitions and this yellow luminescence band can be efficiently excited by near-UV to blue light (300-450 nm). It is assumed that by optimization of the europium concentration and Eu3+/Eu2+ ratio promising single-phase phosphors can be obtained for warm white LEDs having high color rendering indexes.