Prototype Sandia Octahedral Molecular Sieve (SOMS) Na2Nb2O6·H2O: Synthesis, Structure and Thermodynamic Stability

new microporous phase Na2Nb2O6·H2O, which transforms to NaNbO3 perovskite on heating, has been synthesized by the hydrothermal method. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that the structure comprises a framework of [NbO6] and [NaO6] octahedra with other Na+ being located in the channels (space group C2/c; a = 17.0511(9) A; b = 5.0293(2) A; c = 16.4921(9) A; =113.942(2)). This phase belongs to the recently synthesized Sandia octahedral molecular sieves (SOMS) family, Na2Nb2xMxO6-x(OH)x·H2O (M = Ti, Zr) and is the archetype for the substituted structures. Using drop-solution calorimetry into molten 3Na2O·4MoO3 at 974 K, the enthalpies of formation of Na2Nb2O6·H2O from the constituent oxides and from the elements have been determined to be -295.4 (plus_minus) 4.8 and -2895.5 (plus_minus) 6.4 kJ/mol, respectively. From the drop-solution calorimetric data for Na2Nb2O6·H2O and its dehydrated perovskite phase, the enthalpy of the dehydration reaction, Na2Nb2O6·H2O 2NaNbO3 + H2O, has been derived, and its implications for phase stability are discussed.