Redox and charge transfer processes and luminescence in CaSO4:Zn,Mn

CaSO4:Zn,Mn synthesized by solid state diffusion reaction in air exhibit intense thermoluminescent (TL) glow peaks at 140 °C and 210 °C. The heights of both the peaks were nearly the same in (Ca0.74Zn0.20Mn0.06)SO4. Optimal TL sensitivity was obtained after a 750 °C, 1 h pre-sintering treatment. ZnSO4 serves as an efficient flux for the incorporation of Mn in CaSO4. In contrast, the TL intensity of 140 °C TL peak in CaSO4:Mn made by similar recipe is less intense due to the partial oxidation of Mn. The relatively high electronegativity of Zn (1.7) when compared to Mn (1.6) facilitates the incorporation of Mn in (Ca,Zn)SO4 host. At high Mn concentrations, the 140 °C peak got quenched; as a result the Ca0.64Zn0.17Mn0.20 phosphor exhibits a single peak at 210 °C. Its TL output is nearly three times higher than that of the major TL peak in CaSO4:Dy. The photoluminescent (PL) and TL emission spectra of (Ca,Zn)SO4:Mn peak at 500 nm are characteristic of Mn2+ ions. The 210 °C TL peak in CaSO4:Mn,Zn is attributed to redox process involving O2i2− interstitial ions and anion vacancies (F+ centres) adjacent to Zn2+ as well as Mn2+ ions while the 140 °C TL peak is attributed to redox process involving O2i2− and anion vacancies adjacent to Mn2+ ions. Zn2+ has a higher second ionization potential and hence electrons trapped near Zn2+ ions will be more tightly bound than those trapped near Mn2+ ions. The high PL sensitivity of CaSO4:Zn,Mn is attributed to charge transfer processes involving Zn0 atoms and F+ centres near the vicinity of Mn2+ ions.