Crystal Structure and Magnetic Properties of a New Iron(III) Silicophosphate Fe3P5SiO19 Studied by Neutron Diffraction and M?ssbauer Techniques

Substituted phases with the composition La5Mo4-xTxO16-delta (T=Co, Fe, Mn, and Mg and x~0.7) were prepared by fusedsalt electrolysis and/or conventional solid-state methods. The crystal structure of the parent compound, La5Mo4O16, contains perovskite-like corner-sharing MoO6 octahedral units in the ab plane separated by Mo2O10 bioctahedral units along the c direction. Detailed single-crystal X-ray diffraction studies on the Co-substituted phase, La5Mo3.31Co0.69O16-delta, indicated that the unit cell is triclinic (space group C-1) with Co exclusively replacing Mo atoms in the perovskite layers. X-ray absorption measurements revealed that the transition metal ions are divalent, consistent with the crystal structure analysis. The anomalous magnetic transition observed at 180 K in the parent compound shifts to lower temperatures upon substitution with transition metal ions. No long-range magnetic order was evident in the Mg2+-substituted compositions. The electrical resistivity of all the substituted phases was at least 3 orders of magnitude higher than that of the parent compound. Variations in the magnetic and electrical properties have been ascribed to the disruption of exchange correlations caused by substitutional disorder at the Mo sites.