ε-La(IO): 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(IO), is obtained by a supercritical hydrothermal method with high temperature and high pressure ( = 400 °C and ≈ 25 MPa). Different from the known phases, ε-La(IO) crystallizes in the chiral space group 2, which features a three-dimensional framework with multiple IO groups stacked along different directions around the LaO polyhedra. ε-La(IO) 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(IO) is second-harmonic-generation (SHG)-active and the powder SHG response is measured to be 11.1 × KHPO 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.