Dehydration, polymorphism and ionic conductivity of sodium hypodiphosphates

A series of 12 compounds: Na(H₃P₂O₆) (1 and 2), Na(H₃P₂O₆)·H₂O (3), Na₂(H₂P₂O₆) (4 and 5), Na₂(H₂P₂O₆)·2H₂O (6), Na₂(H₂P₂O₆)·6H₂O (7 and 8), Na₅(H₂P₂O₆)(HP₂O₆)·21H₂O (9), Na₃(HP₂O₆) (10 and 11) and Na₃(HP₂O₆)·9H₂O (12) has been characterized by variable-temperature optical microscopy, thermogravimetry, single-crystal and powder X-ray diffraction. Crystalline salts 1-4, 6-10 and 12 have been obtained in typical solution crystallization processes. Dehydrations of Na(H₃P₂O₆)·H₂O (3), Na₂(H₂P₂O₆)·6H₂O (both 7 and 8) and Na₃(HP₂O₆)·9H₂O (12) were monitored by TGA, variable-temperature microscopy and powder or microsample powder X-ray diffraction, and turned out to be one-step, destructive transformations, resulting in almost pure triclinic form of Na(H₃P₂O₆) (1), α-Na₂(H₂P₂O₆) (4) and α-Na₃(HP₂O₆) (10), respectively. Sodium ionic conductivity was calculated using the bond valence sum method for all obtained anhydrous substances and measured for the best candidate, compound 4 in the temperature range 300-420 K, reaching 10^-4 S m^-1.