Three new alluaudite-like protonated arsenates: NaMg<SUB>3</SUB>(AsO<SUB>4</SUB>)(AsO<SUB>3</SUB>OH)<SUB>2</SUB>, NaZn<SUB>3</SUB>(AsO<SUB>4</SUB>)(AsO<SUB>3</SUB>OH)<SUB>2</SUB> and Na(Na<SUB>0.6</SUB>Zn<SUB>0.4</SUB>)Zn<SUB>2</SUB>(H<SUB>0.6</SUB>AsO<SUB>4</SUB>)(AsO<SUB>3</SUB>OH)<SUB>2</SUB>

Three new arsenates NaMg3(AsO4)(AsO3OH)2 (1), NaZn3(AsO4)(AsO3OH)2 (2) and Na(Na0.6Zn0.4)Zn2(H0.6AsO4)(AsO3OH)2 (3) were synthesised under low-temperature hydrothermal conditions. 1 is the first example of a synthetic hydrous Na-Mg arsenate, and 2 and 3 are the first synthetic hydrous Na-Zn arsenates with octahedrally coordinated Zn atoms. They are all isostructural with the protonated members of the alluaudite-group compounds. Their crystal structures were determined from the single-crystal X-ray diffraction data collected at 293 K [space group C2/c, a = 11.972(2)/12.023(2)/12.458(3), b = 12.367(3)/12.385(3)/12.484(3), c = 6.743(2)/6.750(2)/6.839(2) Å, β = 112.67(3)/112.79(3)/113.83(3)°, V = 921.3(4)/926.7(4)/973.0(4) Å3, Z = 4, for 1, 2, and 3 respectively]. The crystal structures consist of chains of edge-sharing M1O4(OH)2 and M2O6, octahedra (M1, M2 = Mg for 1, M1, M2 = Zn for 2, and M1= Na, Zn, M2 = Zn for 3) that are stacked parallel to (101) and are connected by the AsO4 tetrahedra. These chains produce two types of channels parallel to the c-axis. Na atoms are located in the second-type channels, while hydrogen atoms of the OH groups are situated in the channels of the first-type. To obtain further information on the anion groups and especially on the short hydrogen bonds, infrared and single-crystal Raman spectra were measured and evaluated. The OH stretching frequency is in good agreement with the observed O···O distances. Furthermore, crystal chemistry of synthetic alluaudite-like arsenates is discussed