Luminescence of Sb3+ in rare earth orthoborates LnBO3 (Ln = Sc,Y, La,Gd, Lu)

The luminescence of several Sb³⁺-activated rare earth orthoborates (LnBO₃Sb³⁺; Ln = Sc, Y, La, Gd, Lu) are reported. In all compositions the Stokes shift of the Sb³⁺ luminescence is rather large, resulting in rather low quenching temperatures (200 K or lower). The Stokes shift appears to be dependent on the coordination number and on the radius of the host lattice cation. This is explained from the assumed tendency of the Sb³⁺ ion to occupy an off-center position which becomes more apparent when the space available for the Sb³⁺ ion increases. The present results are compared with those on LnBO₃Bi³⁺. It appears that the Stokes shift of the Bi³⁺ luminescence is more sensitive to the host lattice and is smaller than the Stokes shift of the Sb³⁺ luminescence. This is explained by the large radius of the Bi³⁺ ion compared to the Sb³⁺ ion. In GdBO₃Sb³⁺ thermally activated energy transfer is observed from Gd³⁺ to Sb³⁺.