(NH(4))Ga(HAsO(4))(2) and TlAl(HAsO(4))(2) - two new RbFe(HPO(4))(2)-type M (+) M (3+) arsenates

The crystal structures of hydro-thermally synthesized (T = 493 K, 7-9 d) ammonium gallium bis-[hydrogen arsenate(V)], (NH(4))Ga(HAsO(4))(2), and thallium aluminium bis-[hydrogen arsenate(V)], TlAl(HAsO(4))(2), were solved by single-crystal X-ray diffraction. Both compounds crystallize in the common RbFe(HPO(4))(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.