ε-La(IO3)3: A Polar Iodate with High Thermal Stability and a Large Second-Harmonic-Generation Response Obtained by a Supercritical Hydrothermal Method

The synthesis conditions at high temperature and high pressure are favorable for exploring new compounds with novel structures and properties. In this work, a new polar iodate, ε-La(IO₃)₃, is obtained by a supercritical hydrothermal method with high temperature and high pressure (T = 400 °C and P ≈ 25 MPa). Different from the known phases, ε-La(IO₃)₃ crystallizes in the chiral space group P2₁, which features a three-dimensional framework with multiple IO₃^- groups stacked along different directions around the LaO_x polyhedra. ε-La(IO₃)₃ possesses high thermal stability up to 525 °C and exhibits a wide band gap of about 4.05 eV. Attributed to its noncentrosymmetric arrangement, ε-La(IO₃)₃ is second-harmonic-generation (SHG)-active and the powder SHG response is measured to be 11.1 × KH₂PO₄ at 1064 nm in the 26-50 μm particle size range. This work has enriched the structural diversity of iodates and would further promote the materials' exploration under a supercritical hydrothermal method.