Structure and Optical Properties of Li x Ag 1- x GaSe 2 and Li x Ag 1- x InSe 2 .

Complete substitution of Li atoms for Ag atoms in AgGaSe 2 and AgInSe 2 was achieved, resulting in the solid solutions Li x Ag 1- x GaSe 2 and Li x Ag 1- x InSe 2 . The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state 7 Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS 2 -type structure (space group I 4̅2 d ) was retained within the entirety of the Ga-containing solid solution Li x Ag 1- x GaSe 2 , which is noteworthy because the end-member LiGaSe 2 normally adopts the orthorhombic β-NaFeO 2 -type structure (space group Pna 2 1 ). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution Li x Ag 1- x InSe 2 , the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in Li x Ag 1- x GaSe 2 and from 1.2 to 2.5 eV in Li x Ag 1- x InSe 2 , enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS 2 . All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.