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

Abstract

Fe3P5SiO19 has been prepared by solid state reaction of Fe(PO3)3, FePO4, and SiO2 at 1000°C. The structure has been determined from a single crystal through direct methods and difference Fourier synthesis and refined to R=0.052. The unit cell is hexagonal, space group P63, with a=14.4804(8) Å, c=7.4256(2) Å, and Z=4. The three-dimensional framework is built up from [Fe2O9] units of two faces sharing octahedra and Si2O7 disilicates linked by PO4 tetrahedra. Fe3P5SiO19 is isotypic with V3P5SiO19. Fe3P5SiO19 is antiferromagnetic below TN=35 K. The magnetic structure has been determined by means of powder neutron diffraction methods: the magnetic moments are antiferromagnetically coupled inside the [Fe2O9] units, in agreement with the Goodenough rules. These units are linked to each other through several Fe–O–P–O–Fe super-superexchange pathways and form antiferromagnetic [001] rows. The moment direction lies in the (001) plane (μFe=4.56(5) μB at 2 K). There is a competition between the intra- and interunits interactions which all are antiferromagnetic and cannot be simultaneously satisfied without frustration. Mössbauer spectra are fitted with two doublets and two sextuplets in the paramagnetic and antiferromagnetic states, respectively. Their rather high isomer shifts are explained by the inductive effect. The magnetic interactions are discussed.