During systematic research on the mineral-related arsenates in the system CdO–MO–As2O5–H2O (M2+= Mg, Co, Ni, Cu, Zn), three new Cd-containing arsenates Cd4.65Ni0.35(AsO4)2(HAsO4)2·4H2O (1), Cd0.75Co2.75(H0.5AsO4)2(HAsO4) (2) and Cd1.25Zn0.75(HAsO4)2·2H2O (3) have been synthesized under hydrothermal conditions, while the Zn-arsenate Zn9(AsO4)6·4H2O (4) and again 2 have been prepared under ionothermal conditions using the ionic liquid 1-ethyl-3-methylimidazolium bromide, (C6H11N2)Br, as the solvent. Their crystal structures were determined using single-crystal X-ray diffraction data and refined to the reasonably low R-values: 1R1 = 0.030; 2R1 = 0.047; 3R1 = 0.029; 6R1 = 0.029. The compounds 1, 2 and 3 are structural analogues of three different arsenate mineral phases. Compound 4 was for the first time synthesized using ionic liquid as the solvent and the hydrogen atoms were found and refined. Infrared and Raman spectra were measured and evaluated in order to obtain further information on the anion groups and especially on the short hydrogen bonds. The OH stretching frequency is in good agreement with the observed O⋯O distances. The different approaches in the synthesis of mineral-related arsenates, the temperature treatment as well as the role of added water in ionothermal synthesis are also discussed.

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