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

Abstract Single crystals of the alkali‐metal hypodiphosphates K 4 P 2 O 6 · 8H 2 O ( 1 ) and Na 2 K 2 P 2 O 6 · 8H 2 O ( 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 [P 2 O 6 ] 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 [P 2 O 6 ] 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 H 2 O loss in case of slow heating avoiding the formation of a hydrate melt.