[en] HUO2PO4.4H2O, P4/ncc, a=6.995 (2), c=17.491 (4) A, Z=4, V=855.84 A3, Dsub(c)=3.399 g cm-3. The structure has been refined using 695 Mo Kα (lambda=0.710688 A,μ=166cm-1) intensities to R=0.025. The two-dimensional network of hydrogen bonds serves as the path for H+ ion conduction in this material. (Auth.)