TWO NEW STRUCTURE TYPES: KFe3(AsO4)2 (HAsO4)2 AND K(H2O)M3+ (H1.5AsO4)2 (H2ASO 4)(M3+ = Fe, Ga, In) - SYNTHESIS, CRYSTAL STRUCTURE AND SPECTROSCOPY

Abstract

The new arsenates KFe3(AsO4) 2(HAsO4)2 (C2/c; Z = 4; a = 18.975(4), b = 6.5850(13), c = 10.955(2) Å, β = 100.42(3)°, V = 1346.3(5) Å3) and K(H2O)M3+(H1.5AsO 4)2(H2AsO4) (P2/n; Z = 2; a = 4.700(1), 4.641(1), 4.798(10); b = 8.712(2), 8.696(2), 8.832(2); c = 13.828(3), 13.772(3), 13.971(3) Å β = 94.86(3), 94.71(3), 94.68(3)°; V = 564.2(2), 553.9(2), 590.1(2) Å3 for M3+ = Fe, Ga, In, respectively), were synthesized hydrothermally in Teflon-lined stainless steel autoclaves (493 K, 7-9 d, pH ≤ 1) from mixtures containing M 3+2O3, M1+2CO 3, H5As3O10 and distilled water. Single-crystal structure determination (MoKa X-radiation, CCD area detector, 293 K, Rl(F) = 0.027-0.056) revealed that KFe3(AsO4) 2(HAsO4)2 represents a novel sheet structure-type, containing shared edges between ASO4 tetrahedra and FeO6 octahedra, whereas K(H2O)M3+(H 1.5AsO4)2(H2AsO4) is a novel chain structure-type consisting of decorated krohnkite-type (octahedral-tetrahedral) chains with water molecules and K atoms in-between. Structural relationships to related compounds are discussed in detail for both types. Supplementary SEM and Raman-spectroscopic data are presented. The two new K-Fe3+-arsenate compounds may occur in strongly acidic environments in nature. © 2007 E. Schweizerbart'sche Verlagsbuchhandlung.