Synthesis, structural, impedance, optical, Mossbauer characterizations and magnetic studies of Pb-containing germanate compounds

The orthorhombic antiferromagnetic germanate minerals Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9 have been prepared via solid state reaction procedure. An extensive study of structural, dielectric, optical and magnetic properties of both the germanates have been investigated by employing different measurements such as X-ray diffraction, FTIR, SEM, impedance, UV-Vis-NIR & Mossbauer spectroscopic and magnetometary. Rietveld refinement of room temperature XRD patterns revealed that both the compounds crystallize in the orthorhombic space group Pbcn. FTIR spectrum exhibits the presence of M-O & Ge-O bonds which confirms strong chemical bonding. Morphological analysis through SEM showed good crystallinity of both the samples. The variation of dielectric constant, loss tangent and impedance with frequency have been studied and it has been established that both the germanates are semiconducting at 300K (RT) which may be attributed to the stacking of ions at grain boundaries. The optical band gaps estimated by UV-Vis-NIR spectroscopic measurements further confirms the semiconducting nature of the prepared germanates. Fe-57 Mossbauer spectroscopic measurement on synthesized Pb2Fe2Ge2O9 sample was carried out at RT. The obtained values of IS and QS showed that iron is in Fe3+ high spin state (t(2g)(3)e(g)(2), s = 5/2) and occupy two different sites, Fe1 and Fe2. The field dependent magnetization measurements show that both the compounds display dominant paramagnetic nature at 300K. The temperature dependent magnetization data reveal very sharp first order type AFM to PM transition at TN similar to 50K and TN similar to 8K for Pb2Fe2Ge2O9 and Pb2Mn2Ge2O9 , respectively, and weak FM nature below Neel temperature. The field dependent properties of both the compounds have been analyzed by drawing the M versus H plots at low and room temperatures. The values of calculated effective magnetic moment 6.07 mu B and 3.95 mu B for Fe3+ and Mn3+ ions are in the high spin states (t(2g)(3)e(g)(2), s = 5/2 and )t(2g)(3)e(g)(1), s = 2.