Ho2[As4O9] : the first ternary catena‐oxotetraarsenate(III) of the lanthanoids

Abstract

The pale yellow holmium(III) catena‐oxotetraarsenate(III) Ho2[As4O9] crystallizes needle‐shaped from a CsBr‐flux‐assisted metallothermic reaction between As2O3 and elemental holmium at 875 °C in the monoclinic space group P21/c with the lattice parameters a = 802.98(6) pm, b = 548.83(4) pm, c = 1101.62(9) pm and β = 107.519(3)° for Z = 2. The singular Ho3+ cation is surrounded by seven oxygen atoms in the shape of a capped distorted octahedron [HoO7]11-. These polyhedra link with each other twice via edge and twice via corner to form layers described by the Niggli formula , which spread out parallel to the (100) plane. Both distinct As3+ cations form ψ1‐tetrahedral [AsO3]3- units with three oxygen atoms, which are vertex‐condensed to a catena‐oxotetraarsenate(III) anion [As4O9]6-. All [As4O9]6- groups serve as linkers between the layers, while the stereochemically active free electron pairs at the As3+ cations arrange themselves to create lone‐pair channels along the [010] direction. Furthermore, Raman‐spectroscopic investigations, which frequently show Ho3+ luminescence, and effective coordination number calculations are carried out for the new crystal structure of Ho2[As4O9].