A new compound, Cs₄(SeO₄)(HSeO₄)₂(H₃AsO₄), was synthesized from aqueous solution of CsHSeO₄/Cs₃H(SeO₄)₂/H₃AsO₄ (80.5%). The compound was characterized by X-ray single-crystal analysis, with the following crystal data: monoclinic, space group <i>P</i>2₁, <i>a</i> = 5.973(1) Å, <i>b</i> = 13.691(3) Å, <i>c</i> = 11.910(3) Å and <i>β</i> = 94.867(1)°. The compound has a unit cell volume 970.39(4) Å³ and two formula units per cell, giving a calculated density of 3.780 g cm⁻³. The structure was solved from 5493 independent reflections and refined with 218 parameters yielded weighted residuals of 0.1243 and 0.0474 based on <i>F</i>² and values, respectively. This compound have a coordination number of caesium of 11, with Cs–O distances from 2.945(7) to 3.915(16) Å. As and Se atoms form the individual tetrahedra with different H-bonding connectivity in this structure. AsO₄–SeO₄ layers with attached SeO₄ groups are present. The infrared spectra of the new compound Cs₄(SeO₄)(HSeO₄)₂(H₃AsO₄) recorded at room temperature in the frequency range 4000–400 cm⁻¹ confirm the presence of AsO₄³⁻ and SeO₄²⁻ groups in the same crystal. The differential scanning calorimetry (DSC) and the thermogravimetry (TG) curves show two phase transitions at 411 and 418 K in this material.</p>< Réduire