Dehydration, Polymorphism and Ionic Conductivity of Sodium Hypodiphosphates

A series of 12 compounds: Na(H3P2O6) (1 and 2), Na(H3P2O6)center dot H2O (3), Na-2(H2P2O6) (4 and 5), Na-2(H2P2O6)center dot 2H(2)O (6), Na-2(H2P2O6)center dot 6H(2)O (7 and 8), Na-5(H2P2O6)(HP2O6)center dot 21H(2)O (9), Na-3(HP2O6) (10 and 11) and Na-3(HP2O6)center dot 9H(2)O (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)center dot H2O (3), Na-2(H2P2O6)center dot 6H(2)O (both 7 and 8) and Na-3(HP2O6)center dot 9H(2)O (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), alpha-Na-2(H2P2O6) (4) and alpha-Na-3(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).