Synthesis and Characterization of [PtIn₆](GeO₄)₂O and Its Solid Solution [PtIn₆](GaO₄)_2-<i>_x</i>(GeO₄)<i>_x</i>O<i>_x</i>_/2 (0 ≤ <i>x</i> ≤ 2): Gradual Color Change of the Solid Solution from Black (<i>x</i> = 0) to Yellow (<i>x</i> = 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)xOx/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 O2- 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)xOx/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.