Polymorphism in LiLa(WO₄)₂: synthesis, local structure distortion induced nonlinear optical properties and neutron PDF study

Scheelite-like functional materials are potentially used in energy-related applications such as photoluminescence, photocatalysis, batteries, supercapacitors, and laser hosts due to their diverse and tunable crystal structures. We report a soft chemical approach to synthesize polycrystalline powder samples of LiLa(WO₄)₂ polymorphs at a relatively low temperature and time. This study demonstrates a detailed structural investigation of the LiLa(WO₄)₂ polymorphs by a dual-space approach that combines Rietveld and pair distribution function analysis (PDF) using neutron scattering data. In situ, powder neutron diffraction followed by thermal analysis and high-energy ball milling studies confirms the irreversible structural transformation from the triclinic (P1̄) to the tetragonal (I4₁/a) structure around ∼700 °C. Below this temperature, the formation of the triclinic phase is thermodynamically favorable and phase transition is irreversible. The direct bandgap, superior laser threshold damage, and local geometric distortions lead to second harmonic generation in LiLa(WO₄)₂ polymorphs. Hence, we report a simple, reliable and low-cost method to synthesize β- and α-LiLaW polymorphs, demonstrating the associated irreversible phase transition along with in-depth average and local structure variations. This study also emphasizes the critical role of local structural distortions in the observed optical nonlinear (NLO) properties such as second harmonic generation (SHG), which can be extended to other scheelite-like materials to achieve high performance NLO in the near future.