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

Colourless, water- and air-stable single crystals of cerium(III) oxoarsenate(III) Ce[AsO3] were prepared by the reaction of cerium metal (Ce) and arsenic sesquioxide (As2O3) in the presence of cesium chloride (CsCI) as fluxing agent at 750 degrees C in an evacuated silica ampoule. Ce[AsO3] crystallizes monoclinically (a = 902.89(8), b = 782.54(7), c = 829.68(7) pm, beta = 103.393(3)degrees, Z = 8) in the space group P2(1)/c and is isotypic with alpha-Pb[SeO3]. 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 psi(1) 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 Ce[AsO3] with the lattice parameters a = 439.32(4), b = 529.21(5), c = 617.34(6) pm and beta = 105.369(3)degrees with Z = 2 was obtained by high-pressure synthesis (11 GPa, 1200 degrees 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 P2(1)/m, crystallizing isotypically with La[AsO3], beta-Pb[SeO3], Pb[SO3] (scotlandite) or K[ClO3] 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 Ce[AsO3] 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). (C) 2014 Elsevier Masson SAS. All rights reserved.