Synthesis and Characterization of New Strontium Iron(II) Phosphates, SrFe2(PO4)2 and Sr9Fe1.5(PO4)7

The structural and electronic consequences of the metal–metal sigma bonding in the hexagonal perovskite sulfides were examined by calculating the electronic band structures of Sr9/8TiS3 and Sr8/7TiS3, by synthesizing Sr8/7[Ti6/7Fe1/7]S3 and characterizing its crystal structure, and by measuring the magnetic susceptibilities of Sr9/8TiS3, Sr8/7TiS3, and Sr8/7[Ti6/7Fe1/7] S3. The (TiS3)infinity chains of Sr9/8TiS3 and Sr8/7TiS3 contain oligomer units (Oh)n (n=4, 5) made up of face-sharing TiS6 octahedra (Oh). Given the (TiS3)infinity chains aligned along the z direction, the only occupied d-block levels of each (TiS3) infinity chain are the most sigma bonding level of each (Oh)n oligomer, which is constructed from its nz2 orbitals. This "norbital two-electron" metal–metal sigma bonding controls the d-orbital density distribution and the Ti–Ti distances of the (Oh) n oligomers and is ultimately responsible for the semiconducting property of Sr9/8TiS3, Sr8/7TiS3, and Sr8/7[Ti6/7Fe1/7]S3. The Sr8/7[Ti6/7 Fe1/7]S3 structure is described by the trigonal symmetry, R3m (00?????)0s superspace group with the following parameters: as=11.4935(11) ?, cs=2.9986(7) ?, Q=0.57161(15) c*, and Vs=343.05(15) ?3