Non-Monotonic Structural Evolution and Enhanced Nonlinear Optical Performance in LixAg1-xGaTe2 Solid Solutions.

A series of LixAg1-xGaTe2 (x = 0.1 ∼ 0.6) solid-solution single crystals was grown by the Bridgman method. The inherent instability of LiGaTe2 and the narrow bandgap of AgGaTe2 are simultaneously surmounted, and the structure and nonlinear optical performance are thereby regulated in a coordinated manner. At critical compositions x = 0.3 and 0.5, a distinct lattice relaxation occurs, characterized by anomalous expansion along the a-axis and concurrent contraction along the c-axis. This relaxation relieves in-plane stress while strengthening polar bonds along the c-axis, which is expected to enhance microscopic hyperpolarizability and optimize NLO performance. Consequently, the second-harmonic generation (SHG) intensity increases monotonically with increasing Li content, reaching 7 times that of AgGaTe2 at x = 0.6. Comprehensive multimodal characterizations provide robust confirmation of the underlying mechanisms. Raman spectroscopy is consistent with strengthened chemical bonds and suggests local stress heterogeneity within the lattice. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis indicates superior oxidation resistance in the relaxed structures at x = 0.3, suggesting enhanced material stability. This work validates synergistic cation engineering as an effective strategy for developing high-performance chalcogenide NLO crystals.