Structure and Optical Properties of LiAg_1-GaSe₂ and LiAg_1-InSe₂

Complete substitution of Li atoms for Ag atoms in AgGaSe₂ and AgInSe₂ was achieved, resulting in the solid solutions LiAg_1-GaSe₂ and LiAg_1-InSe₂. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state ⁷Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS₂-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution LiAg_1-GaSe₂, which is noteworthy because the end-member LiGaSe₂ normally adopts the orthorhombic β-NaFeO₂-type structure (space group Pna2₁). 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 LiAg_1-InSe₂, 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 LiAg_1-GaSe₂ and from 1.2 to 2.5 eV in LiAg_1-InSe₂, 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 AgGaS₂. 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.