Dimorphic cerium(III) oxoarsenate(III) Ce[AsO3]

Colourless, water- and air-stable single crystals of cerium(III) oxoarsenate(III) CeAsO₃] were prepared by the reaction of cerium metal (Ce) and arsenic sesquioxide (As₂O₃) in the presence of cesium chloride (CsCl) as fluxing agent at 750 °C in an evacuated silica ampoule. CeAsO₃] crystallizes monoclinically (a = 902.89(8), b = 782.54(7), c = 829.68(7) pm, β = 103.393(3)°, Z = 8) in the space group P2₁/c and is isotypic with α-PbSeO₃]. There are two crystallographically different Ce³⁺ positions. (Ce1)³⁺ is coordinated by nine oxygen atoms (d(Ce–O) = 244–286 pm) and (Ce2)³⁺ by only eight (d(Ce–O) = 239–273 pm). Both crystallographically different As³⁺ cations form discrete ψ¹ tetrahedra AsO₃]^3? (d(As–O) = 174–179 pm), which are attached to the Ce³⁺ cations via edges and corners. The second monoclinic modification of CeAsO₃] with the lattice parameters a = 439.32(4), b = 529.21(5), c = 617.34(6) pm and β = 105.369(3)° with Z = 2 was obtained by high-pressure synthesis (11 GPa, 1200 °C) and has both a higher density (6.31 vs. 6.13 g · cm^?3) and a higher calculated Madelung part of the lattice energy (15,155 vs. 15,132 kJ · mol^?1). It adopts the space group P2₁/m, crystallizing isotypically with LaAsO₃], β-PbSeO₃], PbSO₃] (scotlandite) or KClO₃] and exhibits nine-fold coordinated Ce³⁺ cations exclusively (d(Ce–O) = 254–287 pm) along with tripodal AsO₃]^3? anions (d(As–O) = 175–176 pm). Raman spectroscopy on both phases of CeAsO₃] shows stretching vibrations between 769 and 731 cm^?1 as well as asymmetric vibrations in the range of 659–617 cm^?1. The symmetric bending mode vibrations emerge in an interval from 340 to 410 cm^?1 and the asymmetric bending modes range between 230 and 290 cm^?1.