Einkristalle von La[AsO 4 ] im Monazit‐ und Sm[AsO 4 ] im Xenotim‐Typ

Single Crystals of La[AsO 4 ] with Monazite‐ and Sm[AsO 4 ] with Xenotime‐Type Structure Brick‐shaped, transparent single crystals of colourless monazite‐type La[AsO 4 ] (monoclinic, P2 1 /n, a = 676.15(4), b = 721.03(4), c = 700.56(4) pm, β =104.507(4)°, Z = 4) and pale yellow xenotime‐type Sm[AsO 4 ] (tetragonal, I4 1 /amd, a = 718.57(4), c = 639.06(4) pm, Z = 4) emerge as by‐products from alkali and rare‐earth metal chloride fluxes whenever the synthesis of lanthanide(III) oxoarsenate(III) derivatives from admixtures of the corresponding sesquioxides in sealed, evacuated silica ampoules is accompanied by air intrusion and subsequent oxidation. Nine oxygen atoms from seven discrete [AsO 4 ] 3− tetrahedra recruit the rather irregular coordination sphere of La 3+ (d(La 3+ −O 2− ) = 248 – 266 pm plus 291 pm) and even a tenth ligand could be considered at a distance of 332 pm. The trigonal dodecahedral figure of coordination consisting of eight oxygen atoms at distances of 236 and 248 pm (4× each) about Sm 3+ is provided by only six isolated tetrahedral [AsO 4 ] 3− units. Alternating trans ‐edge condensation of the latter with the [LaO 9+1 ] polyhedra of monazite‐type La[AsO 4 ] and the [SmO 8 ] polyhedra of xenotime‐type Sm[AsO 4 ] constitutes the main structural chain features along [100] or [001], respectively. The bond distances and angles of the complex [AsO 4 ] 3− anions range within common intervals (d(As 5+ −O 2− ) = 167 – 169 pm, ∢(O–As–O) = 100 – 116°) for both lanthanide(III) oxoarsenates(V) presented here.