La[AsO 3 ]: Lanthanum Oxoarsenate(III) with K[ClO 3 ]‐Type Crystal Structure

Colorless, transparent single crystals of the lanthanum oxoarsenate(III) La[AsO 3 ] emerged after exposing an equimolar mixture of La 2 O 3 and As 2 O 3 to high pressure (11.5 GPa) and high temperature (1000 °C). The air‐ and water‐stable compound crystallizes isotypically with K[ClO 3 ] and thus can be described in the monoclinic space group P 2 1 / 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[ClO 3 ], the crystallographically unique La 3+ cations in La[AsO 3 ] are surrounded by nine oxygen atoms ( d (La 3+ –O 2– ) = 254–287 pm). The corresponding [LaO 9 ] 15– polyhedra get first interconnected by two edges to form infinite $\rm^{1}_{\infty}$ {[LaO 7 ] 11– } chains running along the a axis. These chains generate $\rm^{1}_{\infty}$ {[LaO 6 ] 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 $\rm^{1}_{\infty}$ {[LaO 3 ] 3– } network. Charge compensation is achieved by incorporation of As 3+ cations in creating isolated, ψ 1 ‐tetrahedral [AsO 3 ] 3– units ( d (As 3+ –O 2– ) = 176–177 pm, [ang](O–As–O)= 96–99°). Their lone pairs align along the c axis targeting at opposing La 3+ cations.