Crystal structure of strontium potassium arsenate octahydrate

The crystal structure of strontium potassium arsenate octahydrate, SrKAsO₄·8H₂O, has been determined by single crystal X-ray diffraction. The crystals are tetragonal, a = 7.144(1), c = 23.613(2) Å, space group I4₁md (No. 109), Z = 4, V = 1205.1(1) ų, and d _c = 2.258 g cm^–3. All cations and anions in this compound are completely surrounded by water molecules. Sr²⁺ and K⁺ ions are each coordinated to eight water molecules arranged in an approximately square antiprism. The Sr(H₂O)₈]²⁺ polyhedron shares one face of four water molecules with a K(H₂O)₈]⁺ polyhedron forming O₄-Sr-O₄-K-O₄ polyhedra. The O₄-Sr-O₄-K-O₄ units are linked together through a pair of edge-sharing linkages of the outer water molecules, alternating along the a-axis and b-axis. The hydrated cation polyhedra appear to define the host lattice, which accommodates the AsO ₄ ^3- ion in the interstitial space. The environment of the AsO ₄ ^3- ion consists of 16 water molecules; each oxygen atom is the acceptor in hydrogen bonds from four water molecules. The pair distances associated with this environment may be used in investigations of the structure of hydrated ₄ ^n- ions in solutions. Each water molecule is coordinated to one Sr and one K, and hydrogen bonded to two O atoms of two different AsO ₄ ^3- ions. There is no hydrogen bonding between water molecules.