The crystal structures of hydro-thermally synthesized (T = 493 K, 7-9 d) ammonium gallium bis-[hydrogen arsenate(V)], (NH4)Ga(HAsO4)2, and thallium aluminium bis-[hydrogen arsenate(V)], TlAl(HAsO4)2, were solved by single-crystal X-ray diffraction. Both compounds crystallize in the common RbFe(HPO4)2 structure type (R c) and share the same tetra-hedral-octa-hedral framework topology that houses the M + cations in its channels. One of the two Tl sites is slightly offset from its ideal position. Strong O-H⋯O hydrogen bonds strengthen the network.
Keywords: (NH4)Ga(HAsO4)2; TlAl(HAsO4)2; crystal structure.