Europium doped thiosilicate phosphors of the alkaline earth metals Mg, Ca, Sr and Ba: structure and luminescence

or Ce 3+ as an activator on the other side. Also, the quantum efficiency of the thiosilicates is acceptable (e.g. 30% for Sr2SiS4, 35% for Ca2SiS4) and open to further improvement, e.g. by finetuning the synthesis conditions [1]. The thermal quenching temperature T0.5 (defined as the temperature for which the emission intensity is half of the intensity at low temperature) in Ca2SiS4:Eu is sufficiently high (470K), but lower for Sr2SiS4:Eu (380K) [2,3]. From an application point of view, the stability against moisture of the alkaline earth thiosilicates is a matter of concern, but techniques to improve this have been proposed [4]. The luminescent characteristics of the Euions in the thiosilicates are linked to the structural characteristics. This was demonstrated with the study of (Ca,Sr)2SiS4 [3], where the two emission bands of these thiosilicates originate from the two inequivalent sites of the europium ions. Due to the different crystallographic structures of Ca2SiS4 and Sr2SiS4, phase segregation occurs in Ca2xSr2-2xSiS4 when 0.5 < x < 0.9. This was observed by x-ray diffraction analysis and cathodoluminescence studies at the microscopic level [3]. Here we also report on the (Ca,Mg)2SiS4system. In the orthorhombic Ca2SiS4-phase, a substitution of a small percentage of Ca 2+ by Mg 2+ shifts the emission spectrum to longer wavelengths. Similarly, in the Mg2SiS4-phase a small percentage of Mg 2+ can be substituted by Ca 2+ , shifting the emission peaks. Intermediate multiphase compositions exhibit more complex PL spectra.