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 degrees C). The air- and water-stable compound crystallizes isotypically with K[ClO3] and thus can be described in the monoclinic space group P2(1)/m with the lattice constants a = 442.91(4), b = 531.40(5), c = 622.83(6) pm and beta = 105.564(3)degrees 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+-O2-) = 254-287 pm). The corresponding [LaO9](15-) polyhedra get first interconnected by two edges to form infinite (1)(infinity){[LaO7](11-)} chains running along the alpha axis. These chains generate (2)(infinity){[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 (3)(infinity){[LaO3](3-)} network. Charge compensation is achieved by incorporation of As3+ cations in creating isolated, psi(1)-tetrahedral [AsO3](3-) units (d(As3+-O2-) = 176177 pm, angle(O-As-O)= 96-99 degrees). Their lone pairs align along the c axis targeting at opposing La3+ cations.