In our ongoing investigations of heteropolyniobate chemistry, a phase featuring decorated, A-type trivacant alpha-Keggin ions linked by their charge-balancing sodium cations has been isolated and structurally characterized. This is the first heteropolyniobate reported that has a true lacunary structure type. Na-15[(PO2)(3)PNb9O34]center dot 22H(2)O (1) [triclinic space group P (1) over bar (No. 2); a = 12.242 (2) angstrom, b = 12.291 (3) angstrom, c = 22.056 (4) angstrom; alpha = 93.12 (3)degrees, beta = 99.78 (3)degrees, gamma = 119.84 (3)degrees; Z = 4, V = 2799.2 (10) angstrom(3)] is composed of bilayers of the heteropolyanions alternating with layers of hydrated Na+ cations. Sodium cations also bridge the clusters within their layers through Na-O-t-Nb, Na-O-b-Nb-2, and Na-O-t-P bonds (t = terminal and b = bridging). This phase is poorly soluble in water, suggesting that it is more characteristic of a framework of linked heteropolyanions rather than a water-soluble heteropolyanion salt. Two-dimensional solid-state Na-23 multiple-quantum magic angle spinning (MAS) NMR of 1 reveals five distinctive chemical and structural environments for sodium, which agrees with the crystallographic data. The Na-23 and H-1 MAS NMR studies further illustrate the rigid and immobile nature of this framework of cations and anions.