A chiral sodium lanthanum sulfate for second-order nonlinear optics and proton conduction

Abstract

Chiral metal sulfates have demonstrated potential applications in various areas such as ferroelectricity and nonlinear optics (NLO) due to their non-centrosymmetric structural features. However, the synthesis of chiral metal sulfates remains a formidable challenge, as tuning the polarizability and anisotropy of the highly symmetric tetrahedral sulfate group is particularly difficult. In this context, we have rationally synthesized a chiral sodium lanthanum sulfate, NaLa(SO₄)₂(H₂O), through the combination of alkali and rare-earth cations. The compound exhibits NLO response with a short absorption cutoff edge (<192 nm). Theoretical calculations suggest that the NLO response mainly originates from the synergistic cooperation of [LaO₉] polyhedra and [SO₄] tetrahedra. Furthermore, NaLa(SO₄)₂(H₂O) may potentially achieve birefringence phase matching, as predicated by high birefringence (0.13 @ 1064 nm) in theoretical calculations. Additionally, NaLa(SO₄)₂(H₂O) shows a high proton conductivity of 3.22 × 10⁻³ S cm⁻¹ under 85 °C and 98% relative humidity. The complementary cation strategy of this work offers a new paradigm for the controllable synthesis of chiral metal sulfates and development of functional materials.