Phase transitions, dielectric properties and valence of magnetic ions in PbFe_0.5−x Cr _x Nb_0.5O₃ multiferroic ceramics

Ceramic PbFe_0.5−x Cr _x Nb_0.5O₃ (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 PbFe_0.5−x Cr _x Nb_0.5O₃ is very similar to that in the Pb(Fe_0.5Nb_0.5)_1−x M _x O₃ (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 PbFe_0.5−x Cr _x Nb_0.5O₃ (x = 0.05, 0.10) samples contain Cr³⁺, Cr⁴⁺, Cr⁵⁺, and Cr⁶⁺ ions. It is assumed that the presence of Cr^{5 +} and Cr⁶⁺ ions is due to the formation of a small amount of non-perovskite phases, e.g., Pb _n ²⁺ Cr⁶⁺O_3+n and Fe³⁺Cr⁵⁺O₄. This assumption is supported by the presence of weak reflections of the Pb₂ ²⁺Cr⁶⁺O₅ phase in the XRD pattern of the composition with x = 0.15.