Intercalation induced emergent polar order in 2D van der Waals LiInP$_2$S$_6$

Hydration was employed as a tool to structurally engineer the optical properties in \ce{LiInP2S6}, inducing a new polar phase with giant non-resonant second harmonic generation (SHG). Modeling reveals an increase in susceptibility by 1500$\%$ and 500$\%$ for $d_{11}$ and $d_{14}$, respectively, in hydrated \ce{LiInP2S6}. In contrast to the anhydrated phase with the point group (PG) 32, hydrated phase has PG 3; stabilized by sub-$\AA$ lattice distortions (predicted $\sim$3.1 pm/f.u.) induced by water intercalation in the lattice.  Owing to the indistinguishable extinction conditions between the two phases, the symmetry shift was investigated through extensive nonlinear optical characterization, supported by first principles calculations, instead of solely relying on the conventional diffraction approach. Density functional theory (DFT) calculations suggest emergence of $\Gamma_2^-$ polar mode consistent with the PG 3 in the hydrated \ce{LiInP2S6}.

The phase transformation (PG 3 $\leftrightarrow$ PG 32) was further confirmed through temperature-dependent SHG, X-ray diffraction, and differential scanning calorimetry (DSC). 

Beyond enhancing nonlinear optical response (best in 3-4.0 eV) through a nominally detrimental hydration process, SHG emerges as an economically benign and ultrasensitive probe of subtle symmetry variations,  opening new avenues for symmetry-resolved structural characterization.