| Summary: | Two polytypic modifications of new non-centrosymmetric iodate Rb<sub>3</sub>Sc(IO<sub>3</sub>)<sub>6</sub> are synthesized hydrothermally. Their structures demonstrate different degrees of ordering and are solved in the same space group, <i>Pc</i>, with differing choice of b-axis: either 20 or 40 Å is selected from the same diffraction data, measured on crystals with better ordering. The difference in NLO properties of two polytypes is disclosed. Topology–symmetry analysis of separated blocks and layers allowed determination of local symmetry corresponding to mirror plane m, which defines strong optical nonlinearity. Layer alternation indicates a polytypic nature of Rb, Sc-iodate and its similarity to K, In- and K, Sc-nonlinear-optical iodate crystals, space group <i>Fdd</i>2, belonging to the proposed common family A<sub>3</sub>M(IO<sub>3</sub>)<sub>6</sub> (A = K, Rb; M = Sc, In), whose properties are explained on a symmetry basis for every known member. Hypothetical structural variant is predicted; structural disorder and crystal growth conditions are discussed as factors affecting properties.
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