Hydrogen bonding in the crystalline state. Crystal structure of CaHAsO₄.3H₂O

Calcium hydrogen arsenate trihydrate (CaHAsO₄.3H₂O) crystallizes in the orthorhombic system, space group Pbca. Unit-cell parameters are: a_o = 11.195 (1), b_o = 10.713 (2), c_o = 11.179 (2) Å; Z = 8. The crystal structure was refined anisotropically to R = 0.034, using 1321 non-zero reflexions measured on an automatic three-circle diffractometer (Cu Kα radiation). Corrections for absorption, secondary extinction and anomalous scattering were applied. The hydrogen atoms were located from a difference map, and one of the three water molecules appears very weakly hydrogen bonded. CaHAsO₄.3H₂O is iso-structural with MgHPO₄.3H₂O (newberyite) and with MnHPO₄.3H₂O; its crystal structure is built up from isolated Ca octahedra sandwiched between As tetrahedra. The resulting layers, parallel to (100), are connected by infinite zigzag chains of hydrogen bonds along [100]. Some common features of the related compounds CaHAsO₄ (weilite), CaHAsO₄.H₂O (haidingerite) and CaHAsO₄.2H₂O (pharma-colite) are discussed.