La[AsO₃]: lanthanum oxoarsenate(III) with K[ClO₃]-type crystal structure

Colorless, transparent single crystals of the lanthanum oxoarsenate(III) La[AsO₃] emerged after exposing an equimolar mixture of La₂O₃ and As₂O₃ to high pressure (11.5 GPa) and high temperature (1000 C). The air- and water-stable compound crystallizes isotypically with K[ClO₃] and thus can be described in the monoclinic space group P2₁/m with the lattice constants a = 442.91(4), b = 531.40(5), c = 622.83(6) pm and β = 105.564(3) for Z = 2. Just like the K⁺ cations in K[ClO₃], the crystallographically unique La³⁺ cations in La[AsO₃] are surrounded by nine oxygen atoms (d(La³⁺-O^2-) = 254-287 pm). The corresponding [LaO₉]^15- polyhedra get first interconnected by two edges to form infinite ¹_∞{[LaO₇]^11-} chains running along the a axis. These chains generate ¹_∞{[LaO₆]^9-} layers spreading out parallel to the ab plane by vertex-sharing, which are further connected along [001] by common edges to build double layers. The(O1)^2- anions remain terminal and protrude from these double layers in c direction, but finally serve for their condensation in order to erect a three-dimensional anionic ¹_∞{[LaO₃]^3-} network. Charge compensation is achieved by incorporation of As³⁺ cations in creating isolated, ψ¹-tetrahedral [AsO₃]^3- units (d(As³⁺-O^2-) = 176-177 pm, [ang](O-As-O)= 96-99 ). Their lone pairs align along the c axis targeting at opposing La³⁺ cations. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)