The relation of physical properties to the symmetry of potassium iodate

The symmetry properties of potassium iodate have been investigated in order to determine whether it might be a useful nonlinear optical material. In order to resolve the confusion over the point symmetry of this material, several experimental techniques have been used. These are single-crystal X-ray diffraction (room temperature), optical examination using conoscopic methods, second harmonic generation powder test, and pyroelectric measurements (the latter three techniques covering various temperature ranges). Correlating these observations with previously reported properties of KIO₃ yields the result that below 212 °C, KIO₃ is monoclinic Cm with a₀ = 8.86 (5), b₀ = 12.54 (6), c₀ = 7.70 (5) Å and β = 125°05′±20′. The material is negative biaxial with 2V = 20°30′±30′ and pyroelectric with dP_s/dT = 5.8 × 10⁻⁹ Coul. cm⁻²°C⁻¹ (at room temperature). Above 212 °C this material is rhombohedral R3m with a₀≃7.70 and α ≃ 109°28′. The nature of the twinning below 212 °C and a method of detwinning are discussed. Type one phase matching angles were calculated.