Publications
Octahedral microporous phases Na2Nb2-xTixO6-x(OH)xH2O and their dehydrated perovskites : crystal chemistry, energetics and stability relations
A family of microporous phases with compositions Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O (0 {le} x {le} 0.4) transform to Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5x} perovskites upon heating. In this study, we have measured the enthalpies of formation of the microporous phases and their corresponding perovskites from the constituent oxides and from the elements by drop solution calorimetry in 3Na{sub 2}O {center_dot} 4MoO{sub 3} solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = {approx}0.2 but then more exothermic thereafter. In contrast, the formation enthalpies for the corresponding perovskites become less exothermic across the series. The energetic disparity between the two series can be attributed to their different mechanisms of ionic substitutions: Nb{sup 5+} + O{sup 2-} {yields} Ti{sup 4+} + OH{sup -} for the microporous phases and Nb{sup 5+} {yields} Ti{sup 4+} + 0.5 V{sub O}** for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O {yields} Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5X} + H{sub 2}O, have been derived, and their implications for phase stability at the synthesis conditions are discussed.