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

A new microporous phase Na 2 Nb 2 O 6 ·H 2 O, which transforms to NaNbO 3 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 [NbO 6 ] and [NaO 6 ] octahedra with other Na + being located in the channels (space group C 2/ 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, Na 2 Nb 2 - x M x O 6 - x (OH) x ·H 2 O (M = Ti, Zr) and is the archetype for the substituted structures. Using drop-solution calorimetry into molten 3Na 2 O·4MoO 3 at 974 K, the enthalpies of formation of Na 2 Nb 2 O 6 ·H 2 O 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 Na 2 Nb 2 O 6 ·H 2 O and its dehydrated perovskite phase, the enthalpy of the dehydration reaction, Na 2 Nb 2 O 6 ·H 2 O → 2NaNbO 3 + H 2 O, has been derived, and its implications for phase stability are discussed.