Polar lanthanide copper iodates LnCu(IO3)5 (Ln = La, Ce, Pr, and Nd): Synthesis, crystal structure and characterization

Four new polar lanthanide copper iodates LnCu(IO3)5 (Ln ​= ​La, Ce, Pr, and Nd) were synthesized by the hydrothermal method at 220 ​°C. The four compounds are isostructural and they can be viewed as derivation from two parts: the [Ln(IO3)3] layers from the Ln(IO3)3 with 3D framework and the [Cu(IO3)2] chain from the α-Cu(IO3)2 with 2D layer structure. As the compounds crystallize in the noncentrosymmetric orthorhombic space group Pna21, they may find interesting applications as nonlinear optical crystals. The second-harmonic generation (SHG) response of powder LaCu(IO3)5 is measured to be around 0.5 times that of KDP. The measurement of SHG response versus particle sizes of samples indicates that LaCu(IO3)5 can achieve phase matching. In order to interpret the origination of SHG effects, the dipole moment of LaO9, CuO6 and IO3 groups were calculated. It is suggested that the IO3 groups contribute mostly (∼90.7%) and the CuO6 groups contribute little (∼1.6%) to the SHG response of LaCu(IO3)5. The results of this research show that the combination of lanthanide and copper elements with the electron lone-pair I5+ cations can be a feasible way to fabricate new polar materials in the rare-earth and transition metal iodate system.