According to the published phase diagram, Nb18W8O69 is obtained only in a narrow temperature range from ca. 1265-1375 ºC. Like all of the metastable niobium tungsten oxides, it requires a reconstructive phase transition and mobility of highly charged niobium and tungsten cations to convert to the neighboring phases and is thus kinetically stable at room temperature. However, WO3 sublimes via the gaseous molecules W3O9 and W4O12 at these temperatures. There is competition between (i) reaction kinetics and product stability favored at higher temperatures and (ii) reactant stability favored at lower temperatures. Roth and Waring noted in their study of Nb2O5-WO3 phase equilibria that "[T]he binary equilibrium stability regions of this structural group of compounds tend to decrease with increasing W 6+ content. This phenomenon is probably due to the increasing size of the basic "building block" unit, and therefore to the greater energy needed to maintain long range ordering." 23 Nb18W8O69 presents synthetic challenges but its relation to Nb16W5O55 and its status as the largest known block phase motivate further study as a new electrochemical energy storage material.