Synthesis and Crystal Structures of a Stable, a Metastable and a High Temperature Modification of Pb 2 NaIO 6

In analogy to the synthesis of polycrystalline M 2 NaIO 6 ( M = Ca, Sr, Ba) by precipitation in water at 90 °C, the title compound was first prepared as a metastable compound. The stable modification of Pb 2 NaIO 6 was obtained by a heat treatment to 400 °C followed by cooling to room temperature. The crystal structure was refined from powder diffraction data [space group P 2 1 / c (14), a = 5.9040(2), b = 5.7526(2), c = 10.1104(3) Å, β = 125.341(1)°]. On heating, at ca. 125 °C, a phase transition to a cubic high temperature modification was observed. The crystal structure was refined from XRD data measured at 200 °C [space group Fm $\bar{3}$ m (225), a = 8.2678(1) Å]. Depending on the precipitation temperature between 90 °C and 0 °C, several metastable modifications were obtained, which can be distinguished by significantly different lattice parameters. The XRD pattern of a powder precipitated at room temperature is pseudocubic. The crystal structure was refined at room temperature in P 2 1 / c with a = 5.8201(4), b = 5.8473(4), c = 10.0798(5) Å, β = 125.074(3)°. This modification behaves almost as a cubic lattice on heating as found from XRD and DSC measurements.