Structure and Optical Properties of LiAgGaSe and LiAgInSe.

Complete substitution of Li atoms for Ag atoms in AgGaSe and AgInSe was achieved, resulting in the solid solutions LiAgGaSe and LiAgInSe. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS-type structure (space group 4̅2) was retained within the entirety of the Ga-containing solid solution LiAgGaSe, which is noteworthy because the end-member LiGaSe normally adopts the orthorhombic β-NaFeO-type structure (space group 2). 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 LiAgInSe, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in LiAgGaSe and from 1.2 to 2.5 eV in LiAgInSe, 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. 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.