The first layered analogue of Sr2FeMoO6; the structure and electronic properties of Sr4FeMoO8

Abstract

The n = 1 Ruddlesden–Popper phase Sr₄FeMoO₈ has been synthesised by high temperature ceramic methods under reducing conditions. Rietveld refinements of the structure against X-ray and neutron powder diffraction data shows that this phase adopts the space group I4/mmm (a = 3.92962(5), c = 12.6707(2) Å) and contains a crystallographically disordered arrangement of Fe and Mo on the single octahedral site in the structure. Mössbauer spectroscopy data show that iron is in the trivalent state and that the transition metals are fully disordered at a local scale. SQUID magnetometry measurements and low temperature neutron diffraction experiments have been used to examine the magnetic properties of this material. The magnetic susceptibility shows Curie–Weiss paramagnetism above a magnetic transition at 16(1) K. Below this temperature the sample shows magnetic hysteresis, but the neutron diffraction data show no evidence of the additional Bragg intensity associated with the formation of a magnetically ordered phase. The magnetic transition at 16 K is therefore assigned to the formation of a spin-glass phase. The absence of magnetic order in this material is due to the chemical disorder in the Fe/Mo occupancy of the octahedral site and thus the randomisation of the magnetic exchange interactions.