Synthesis and characterization of [PtIn6](GeO4)2O and its solid solution [PtIn6](GaO4)2-x(GeO4)xOx/2 (0 ≤ x ≤ 2):: Gradual color change of the solid solution from black (x=0) to yellow (x=2) as a consequence of quantum dot effect

A new phase [Ptln(6)](GeO4)(2)O, a filled variant of [Ptln(6)](GaO4)(2), and the solid solution [Ptln(6)](GaO4)(2),(GeO4)(x)O-x/2 (0 <= x <= 2) were prepared and characterized. Single-crystal structure refinements show that [Ptln(6)](GeO4)(2)O is isotypic with the mineral, sulfohalite Na6FCl(SO4)(2), and crystallizes in the space group Fm3m (Z = 4) with a = 1006.0(1) pm. The building units of [Ptln(6)](GeO4)(2)O are isolated [Ptln(6)](10+) octahedra and (GeO4)(4-) tetrahedra, and the isolated O2- ions occupy the centers of the Ins octahedra made up of six adjacent Ptln(6) octahedra. The lattice parameter of the solid solution [Ptln(6)](GaO4)(2-x)(GeO4)(x)O-x/2 (0 <= x <= 2) varies gradually from a = 1001.3(1) pm at x = 0 to a = 1006.0(1) pm at x = 2, and the color of the solid solution changes gradually from black (x = 0) to red (x = 1) to yellow (x = 2). The cause for the gradual color change was examined by performing density functional theory electronic structure calculations for the end members [Ptln(6)](GaO4)(2) and [Ptln(6)](GeO4)(2)O. Our analysis indicates that an oxygen atom at the center of a Ins octahedron cuts the In 5p/In 5p bonding interactions between adjacent [Ptln(6)](10+) octahedra thereby raising the bottom of the conduction bands, and the resulting quantum dot effect is responsible for the color change.