Synthesis and Characterization of [PtIn₆](GeO₄)₂O and Its Solid Solution [PtIn₆](GaO₄)_2-x(GeO₄)_xO_x/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 [PtIn₆](GeO₄)₂O, a filled variant of [PtIn₆](GaO₄)₂, and the solid solution [PtIn₆](GaO₄)_2-x(GeO₄)_xO_x/2 (0 ≤ x ≤ 2) were prepared and characterized. Single-crystal structure refinements show that [PtIn₆](GeO₄)₂O is isotypic with the mineral, sulfohalite Na₆FCl(SO₄)₂, and crystallizes in the space group Fm3̄m (Z = 4) with a = 1006.0(1) pm. The building units of [PtIn₆](GeO₄)₂O are isolated [PtIn₆]¹⁰⁺ octahedra and (GeO₄)^4- tetrahedra, and the isolated O^2- ions occupy the centers of the In₆ octahedra made up of six adjacent PtIn₆ octahedra. The lattice parameter of the solid solution [PtIn₆](GaO₄)_2-x(GeO₄)_xO_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 [PtIn₆](GaO₄)₂ and [PtIn₆](GeO₄)₂O. Our analysis indicates that an oxygen atom at the center of a In₆ octahedron cuts the In 5p/In 5p bonding interactions between adjacent [PtIn₆]¹⁰⁺ octahedra thereby raising the bottom of the conduction bands, and the resulting quantum dot effect is responsible for the color change.