K<sub>2</sub>MM‘<sub>3</sub>Se<sub>6</sub> (M = Cu, Ag; M‘ = Ga, In), A New Series of Metal Chalcogenides with Chain−Sublayer−Chain Slabs:  <sub>∞</sub><sup>1</sup>[M‘Se<sub>4</sub>]−<sub>∞</sub><sup>2</sup>[(MSe<sub>4</sub>)(M‘Se<sub>4</sub>)]−<sub>∞</sub><sup>1</sup>[M‘Se<sub>4</sub>]

A new series of novel isostructural metal chalcogenides, K<sub>2</sub>CuIn<sub>3</sub>Se<sub>6</sub> (<b>1</b>), K<sub>2</sub>CuGa<sub>3</sub>Se<sub>6</sub> (<b>2</b>), and K<sub>2</sub>AgIn<sub>3</sub>Se<sub>6</sub> (<b>3</b>), were obtained by a reactive flux technique and structurally characterized. Compounds <b>1</b>, <b>2</b>, and <b>3</b> crystallize in the space group <i>C</i>2/<i>c</i> of the monoclinic system with eight formula units in a cell:  <i>a</i> = 11.445(2) Å, <i>b</i> = 11.495(2) Å, <i>c</i> = 21.263(4) Å, <i>β</i> = 97.68(3)°, <i>V</i> = 2772(1) Å<sup>3</sup>, R1/wR2 = 0.0676/0.1652 for <b>1</b>; <i>a</i> = 11.031(2) Å, <i>b</i> = 11.050(4) Å, <i>c</i> = 20.808(7) Å, <i>β</i> = 97.71(2)°, <i>V</i> = 2513(1) Å<sup>3</sup>, R1/wR2 = 0.0301/0.0511 for <b>2</b>; and <i>a</i> = 11.633(1) Å, <i>b</i> = 11.587(1) Å, <i>c</i> = 21.355(1) Å, <i>β</i> = 98.010(8)°, <i>V</i> = 2850.4(4) Å<sup>3</sup>, R1/wR2 = 0.0471/0.0732 for <b>3</b>. These isostructural compounds are characterized by a chain−sublayer−chain slab structure. The sublayer, composed of alternative corner-sharing mixed-metal tetrahedra, is sandwiched by parallel corner-sharing tetrahedral chains. Optical absorption spectra of compounds <b>1</b>, <b>2</b>, and <b>3</b> reveal the presence of a sharp optical gap of 1.68, 1.72, and 1.64 eV, respectively, suggesting that these materials are semiconductors and suitable for efficient absorption of solar radiation in solar cell applications. IR spectra show no obvious absorption in the range 800−4000 cm<sup>-1</sup>.