A novel monoclinic phase of impurity-doped CaGa(2)S(4) as a phosphor with high emission intensity.

In the solid-state synthesis of impurity-doped CaGa(2)S(4), calcium tetra-thio-digallate(III), a novel phosphor material (denominated as the X-phase), with monoclinic symmetry in the space group P2(1)/a, has been discovered. Its emission intensity is higher than that of the known ortho-rhom-bic polymorph of CaGa(2)S(4) crystallizing in the space group Fddd. The asymmetric unit of the monoclinic phase consists of two Ca, four Ga and eight S sites. Each of the Ca and Ga atoms is surrounded by seven and four sulfide ions, respectively, thereby sharing each of the sulfur sites with the nearest neighbours. In contrast, the corresponding sites in the ortho-rhom-bic phase are surrounded by eight and four S atoms, respectively. The photoluminescence peaks from Mn(2+) and Ce(3+) in the doped X-phase, both of which are supposed to replace Ca(2+) ions, have been observed to shift towards the high energy side in comparison with those in the ortho-rhom-bic phase. This suggests that the crystal field around the Mn(2+) and Ce(3+) ions in the X-phase is weaker than that in the ortho-rhom-bic phase.