• • SnCN2 achieves a birefringence of 0.73 at 1064 nm, the highest among the four compounds, attributed to maximal [NCN] density (0.048 Å−3) and Sn-polyhedra density (0.024 Å−3). This value is 6.1 times that of commercial α-BBO, enabling significant device miniaturization for polarization-selective beam splitting and nonlinear optical phase matching.
• • Sn(SCN)2 exhibits a birefringence of 0.64 at 1064 nm, four times greater than CaCO3, despite a lower density of birefringence-active units. This demonstrates that spatial configuration and optimal lone pair orientation, rather than density alone, dominate optical anisotropy, providing a design rule for high-performance materials with reduced material consumption.
• • Sn2OCN2 shows a birefringence of 0.63 at 1064 nm; oxygen incorporation expands interlayer spacing from 4.1 to 6.7 Å, decreasing active unit density and slightly reducing birefringence. This trade-off highlights the need to balance structural expansion against polarization efficiency in layered architectures.
• • Sn(SCN)F exhibits a birefringence of 0.44 at 1064 nm, the lowest among the series, due to stereochemical inactivity of Sn and omnidirectional polarization contributions from linear [SCN] units. This underscores that LPE stereochemical activity is critical for generating anisotropic polarization gradients, with direct implications for material selection in wavelength-specific applications.