New Quaternary Alkali-Metal Hypodiphosphate Hydrates: Preparation, Crystal Structure, Vibrational Spectrum, and Thermal Behavior of K4P2O6·8H2O and Na2K2P2O6·8H2O in Comparison to Na4P2O6·10H2O

Single crystals of the alkali‐metal hypodiphosphates K4P2O6·8H2O (1) and Na2K2P2O6·8H2O (2) could be obtained and their crystal structures determined. The compounds 1 and 2 crystallize isotypic in the orthorhombic space group Pbca (no. 61) with four formula units in the unit cell. The crystal structures are built up by discrete [P2O6]4– units in an ethane‐like staggered conformation, by the corresponding alkali‐metal cations and water molecules. FT‐IR/FIR and FT‐Raman spectra of the crystalline title compounds were recorded and a complete assignment for the [P2O6]4– modes is proposed. Raman spectra of aqueous hypodiphosphate solutions deliver additional polarization data supporting the band assignment. Compounds 1 and 2 show a complete H2O loss in case of slow heating avoiding the formation of a hydrate melt.