Prototype Sandia octahedral molecular sieve (SOMS) Na₂Nb ₂O₆·H₂O: Synthesis, structure and thermodynamic stability

A 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) Å; b = 5.0293(2) Å; c = 16.4921(9) Å; β = 113.942(2)°). This phase belongs to the recently synthesized Sandia octahedral molecular sieves (SOMS) family, Na₂Nb_2-xM _xO_6-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 Na₂-Nb₂O₆· H₂O from the constituent oxides and from the elements have been determined to be -295.4 ± 4.8 and -2895.5 ± 6.4 kJ/mol, respectively. From the drop-solution calorimetric data for Na₂Nb ₂O₆·H₂O and its dehydrated perovskite phase, the enthalpy of the dehydration reaction, Na₂Nb ₂O₆·H₂O → 2NaNbO₃ + H₂O, has been derived, and its implications for phase stability are discussed.