A novel monoclinic phase of impurity-doped CaGa2S4 as a phosphor with high emission intensity

In the solid-state synthesis of impurity-doped CaGa2S4, calcium tetrathiodigallate(III), a novel phosphor material (denominated as the X-phase), with monoclinic symmetry in the space group P21/a, has been discovered. Its emission intensity is higher than that of the known orthorhombic polymorph of CaGa2S4 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 orthorhombic phase are surrounded by eight and four S atoms, respectively. The photoluminescence peaks from Mn2+ and Ce3+ in the doped X-phase, both of which are supposed to replace Ca2+ ions, have been observed to shift towards the high energy side in comparison with those in the orthorhombic phase. This suggests that the crystal field around the Mn2+ and Ce3+ ions in the X-phase is weaker than that in the orthorhombic phase.

Powder XRD patterns of the X-phase are shown in Fig. 1. This phase happened to be generated when doped with one of the impurities such as Mn, Ge, Sn, La, Ce, Nd, Sm, Dy, intentionally incorporated as an activator of fluorescence or phosphorescence. Thus it is expected that the X-phase is strongly related with the dopant. However, detailed conditions how to prepare the X-phase from a directed preparation route have not been clarified yet. From the analysis of the singlecrystal XRD data, the chemical formula of the compound in the X-phase was shown to be CaGa 2 S 4 . The simulation of the powder XRD pattern corresponds well to the experimental pattern as shown in Fig. 1. Hence we can conclude that the Xphase is a newly found polymorph of CaGa 2 S 4 .
The unit cell of the X-phase is shown in Figs. 2 and 3; the asymmetric unit consists of two Ca sites, four Ga sites and seven S sites. Figs. 4 and 5 show the environment of the cation sites in the unit cell. In the X-phase, each Ca site is surrounded by seven sulfur atoms forming a distorted enneahedron. Each of the involved sulfur atoms is shared by the adjacent one or two Ca sites and by two Ga sites. The Ca-S distances range from 2.8261 (10) to 3.0823 (13) Å. Each Ga site is tetrahedrally surrounded by sulfur atoms which are also shared by adjacent two Ca sites and two Ga sites. The Ga -S distances and the S-Ga-S angles range from 2.2254 (11) to 2.3116 (11) Å and from 93.54 (3) to 124.12 (3) °, respectively. In the orthorhombic phase, each of the three Ca sites and the two Ga sites is in an eightfold and fourfold coordination environment, respectively, by sulfur atoms. Each of the sulfur atoms of these sites is shared by adjacent two Ca sites and two Ga sites. The Ca-S distances in this polymorph range from 2.970 to 3.130 Å. The Ga-S distances and the S-Ga-S angles range from 2.240 to 2.318 Å and from 96.6 to 125°. It should be noted that the coordination number of the Ca sites in the X-phase is reduced by one compared to that of the orthorhombic phase, while the Ga sites are tetrahedrally surrounded in both phases. Nevertheless, the framework built up from the GaS 4 tetrahedra and their connection modes (corner and edge-sharing) is quite different in the two phases.
The intensities of photoluminescence from Mn 2+ and Ce 3+ in the X-phase were about four and eight times higher than those in the orthorhombic phase. These emission peaks shifted toward the high energy side in comparison with those in the orthorhombic phase. Mn 2+ and Ce 3+ ions are known to be very sensitive with respect to the crystal field (Peters &  Baglio, 1972;Provenzano & White, 1991;Barthou et al., 1994;Wang et al., 2003). Thus the observed shifts of the emission peaks are assumed to be caused by a weakened crystal field originating from the decrease in the coordination number of the Ca sites in the X-phase.

Experimental
A mixture of CaS (4 N) and Ga 2 S 3 (6 N) was used as a starting material added with a small amount of a rare earth element (REE) or a transition metal (TM) as an activator. It was weighed to 0.5 g in total according to the formula Ca 1-x M x Ga 2 S 4 (M = REE or TM) (Boitier et al., 2009;Obonai et al., 2009). The mixture was carefully stirred to be homogenized under Ar atmosphere, and then sealed in a silica glass capsule under vacuum of 10 -4 Pa, where the inner surface of the capsule was coated with a carbon film to prevent the reaction between the starting materials and the container. It was sintered at 1411 K for 1 h. The X-phase samples were sometimes and unexpectedly synthesized and the detailed condition how and when the X-phase emerges has not been clarified yet.

Refinement
Freeing of the site occupation factors for the metal and the sulfur atoms revealed no noticeable incorporation of the dopant metals or of vacancies at the S positions. The highest difference peak of 2.64 e Å -3 is located 1.40 Å from the Ga4 atom; the deepest hole (-3.11 e Å -3 ) is 1.39 Å from the Ga4 atom.      The environment of the cation sites in the X-phase: Ca sites.

Figure 5
The environment of the cation sites in the X-phase: Ga sites. Special details Geometry. ENTER SPECIAL DETAILS OF THE MOLECULAR GEOMETRY Refinement. Refinement was performed using reflections with F 2 > 2.0 σ(F 2 ). The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 σ(F 2 ) is used only for calculating R-factor (gt).