Pb5Fe3TiO11Cl: A rare example of Ti(IV) in a square pyramidal oxygen coordination

Abstract A new oxychloride Pb 5 Fe 3 TiO 11 Cl has been synthesized using the solid state method. Its crystal and magnetic structure was investigated in the 1.5–550 K temperature range using electron diffraction, high angle annular dark field scanning transmission electron microscopy, atomic resolution energy dispersive X-ray spectroscopy, neutron and X-ray powder diffraction. At room temperature Pb 5 Fe 3 TiO 11 Cl crystallizes in the P 4/ mmm space group with the unit cell parameters a =3.91803(3) A and c =19.3345(2) A. Pb 5 Fe 3 TiO 11 Cl is a new n =4 member of the oxychloride perovskite-based homologous series A n +1 B n O 3 n −1 Cl. The structure is built of truncated Pb 3 Fe 3 TiO 11 quadruple perovskite blocks separated by CsCl-type Pb 2 Cl slabs. The perovskite blocks consist of two layers of (Fe,Ti)O 6 octahedra sandwiched between two layers of (Fe,Ti)O 5 square pyramids. The Ti 4+ cations are preferentially located in the octahedral layers, however, the presence of a noticeable amount of Ti 4+ in a five-fold coordination environment has been undoubtedly proven using neutron powder diffraction and atomic resolution compositional mapping. Pb 5 Fe 3 TiO 11 Cl is antiferromagnetically ordered below 450(10) K. The ordered Fe magnetic moments at 1.5 K are 4.06(4) μ B and 3.86(5) μ B on the octahedral and square-pyramidal sites, respectively.