Magnetic ordering temperature in lead and barium iron niobates studied by Moessbauer spectrometry

The multiferroic ceramics Ba(Fe_0.5Nb_0.5)O₃ (BFN) and Pb(Fe_0.5Nb_0.5)O₃ (PFN) to fall into the family of single-phase perovskites exhibiting high value of dielectric permittivity. These materials provide growing application potential and are used in electronics or sensors. The aim of our study is to estimate the values of the temperatures of magnetic (electric) ordering for PFN and BFN from the temperature dependences of the hyperfine parameters, measured by Moessbauer Spectrometry. Tablets of ceramics of BFN, PFN were prepared by three-step solid-state reaction. The first step was enriching Fe₂O₃ to 20 % of ⁵⁷Fe in contrast to the 2.17% content of Fe in the natural mixture of iron isotopes. The second step was preparation of FeNbO₄ precursor. The last step was final synthesis together with Pb(NO₃)₂ or Ba(NO₃)₂ to produce pure PFN and BFN. The samples were characterized by XRD to check their phase composition, crystal structure and lattice constant(s). Their chemical composition and homogeneity were confirmed by SEM and XRF. The BFN sample evinced pure perovskite structure, in PFN less than 3% of a different phase with pyrochlore-like structure was identified. The Moessbauer spectra of powdered samples were acquired in transmission geometry in the temperature range 4.2-300 K in a bath cryostat (Janis) and in the range 300 - 500 K in a Moessbauer furnace (Wissel). Temperature dependences of hyperfine parameters were evaluated from spectra fitted by the contemporary versions of Confit and MossWin softwares. The temperatures of the magnetic ordering transition was around 30 K for BFN sample and 150 K for PFN sample in good agreement with literature. Temperature dependences of Isomer Shift correspond to the second order Doppler shift. Quadrupole Splitting at high temperature is nearly constant for both samples. (authors)