Structural features of two novel alluaudite-like arsenates Cd1.16Zn2.34(AsO4)1.5(HAsO4)(H2AsO4)0.5 and Cd0.74Mg2.76(AsO4)1.5(HAsO4)(H2AsO4)0.5

► Two novel arsenates represent new structure type among alluaudite-like compounds. ► The structures were studied by single-crystal X-ray diffraction. ► Unlike the most of the alluaudite-like compounds, these crystallize in the C2 S. G. ► Main features are short hydrogen bonds and interesting crystal-chemical properties. Two new compounds, Cd1.16Zn2.34(AsO4)1.5(HAsO4)(H2AsO4)0.5 (1) and Cd0.74Mg2.76(AsO4)1.5(HAsO4)(H2AsO4)0.5 (2), have been prepared hydrothermally. Their crystal structures consist of chains of edge-sharing M1O4(OH0.5)2, M1aO4(OH0.5)2, M2O5(OH0.5), and M2aO5(OH0.5) octahedra (M1, M1a=Zn, Cd; M2, M2a=Zn for 1, and M1, M1a=Mg, Cd; M2, M2a=Mg for 2) that are stacked parallel to (101) and are connected by the [(AsO4)0.5(AsO3(OH))0.5]2.5− and [(AsO4)0.5(AsO2(OH)2)0.5]2− tetrahedra. These chains produce two types of channels parallel to the c-axis. Cd atoms are located in channels 2, while in channels 1 are situated hydrogen atoms of OH groups. The infrared spectra clearly show the presence of broad OH stretching and bending vibrations centred at 3236, 2392 1575 and 1396cm−1 in (1), and 3210, 2379 1602 and 1310cm−1 in (2). The OH stretching frequency is in good agreement with O⋯O distances. Furthermore, structural characteristics of compounds with similar alluaudite-like structures were discussed.