Crystal structures and thermal decomposition of permanganates AE[MnO4]2·n H2O with the heavy alkaline earth elements (AE=Ca, Sr and Ba)
AbstractReexamination of the syntheses and crystal structures as well as studies of the thermal decomposition of the heavy alkaline earth metal permanganates Ca[MnO4]2·4 H2O, Sr[MnO4]2·3 H2O and Ba[MnO4]2 are the focus of this work. As an alternative to the very inelegant Muthmann method, established for the synthesis of Ba[MnO4]2 a long time ago, we employed a cation-exchange column loaded with Ba2+ cations and passed through an aqueous potassium-permanganate solution. We later used this alternative also with strontium- and calcium-loaded columns and all the compounds synthesized this way were indistinguishable from the products of the established methods. Ca[MnO4]2·4 H2O exhibiting [CaO8] polyhedra crystallizes in the orthorhombic space group Pccn with the lattice parameters a=1397.15(9), b=554.06(4) and c=1338.97(9) pm with Z=4, whereas Sr[MnO4]2·3 H2O with [SrO10] polyhedra adopts the cubic space group P213 with a=964.19(7) pm and Z=4. So the harder the AE2+ cation, the higher its demand for hydration in aqueous solution. Consequently, the crystal structure of Ba[MnO4]2 in the orthorhombic space group Fddd with a=742.36(5), b=1191.23(7) and c=1477.14(9) pm with Z=8 lacks any crystal water, but contains [BaO12] polyhedra. During the thermal decomposition of Ca[MnO4]2·4 H2O, the compound expels up to two water molecules of hydration, before the crystal structure collapses after the loss of the third H2O molecule at 157°C. The crystal structure of Sr[MnO4]2·3 H2O breaks down after the expulsion of the third water molecule as well, but this already occurs at 148°C. For both the calcium and the strontium permanganate samples, orthobixbyite-type α-Mn2O3 and the oxomanganates(III,IV) AEMn3O6 (AE=Ca and Sr) remain as final decomposition products at 800°C next to amorphous phases. On the other hand, the already anhydrous Ba[MnO4]2 thermally decomposes to hollandite-type BaMn8O16 and BaMnO3 at 800°C.
Die Kristallstrukturen von Strontium- und Bariumhexacyanoferrat(III)-Hexamethylentetramin-Hydraten. Strukturmodell für Additionsverbindungen der Alkali- und Erdalkalimetallhexacyanoferrate mit Hexamethylentetramin / The Crystal Structures of Hydrates of Strontium and Barium Hexacyanoferrate(III) with Hexamethylenetetramine. Structural Model for Adducts of the Hexacyanoferrates of Alkaline and Alkaline Earth Metals with Hexamethylenetetramine