Phase transitions, dielectric properties and valence of magnetic ions in PbFe0.5−xCrxNb0.5O3 multiferroic ceramics

Ceramic PbFe0.5−xCrxNb0.5O3 (x = 0.025, 0.05, 0.10, 0.15) perovskite solid solutions are synthesized using a solid-state reaction method. The samples of solid solutions are studied using the methods of X-ray diffraction, dielectric, Mössbauer and X-ray photoelectron spectroscopy, electron microscopy, and X-ray microanalysis. XRD studies have shown that the content of the parasitic pyrochlore phase increases dramatically with x, approaching 50% for x = 0.15. The averaged lattice parameter and the temperature of ferroelectric phase transition decrease monotonically with x. Concentration dependence of the magnetic phase transition temperature for PbFe0.5−xCrxNb0.5O3 is very similar to that in the Pb(Fe0.5Nb0.5)1−xMxO3 (M = Ti, Zr, Sn) solid solutions implying the lack of magnetic exchange between Fe3+ and Cr3+ ions. Bulk and surface elemental compositions are found to differ from each other and from nominal composition. XPS studies revealed that the PbFe0.5−xCrxNb0.5O3 (x = 0.05, 0.10) samples contain Cr3+, Cr4+, Cr5+, and Cr6+ ions. It is assumed that the presence of Cr5 + and Cr6+ ions is due to the formation of a small amount of non-perovskite phases, e.g., Pbn2+Cr6+O3+n and Fe3+Cr5+O4. This assumption is supported by the presence of weak reflections of the Pb22+Cr6+O5 phase in the XRD pattern of the composition with x = 0.15.