Keywords:perovskite, structure refinement, XAFS, local structure, radioactive element
As a perovskite type compounds ABO3, minerals with various compositions are occurred from the Earth’s crust to the lower mantle. There are few detailed structural studies of natural perovskite-type minerals which easily contains rare earth elements and various kinds of heavy metals. In this study, we conducted a comparative study on Ce-perovskite from Russia, Kola Peninsula and Nb-perovskite from USA, Arkansas. We report on structural properties of solid solution, local structural characteristics of contained elements and irregularities in structure due to radioactive elements. The chemical compositions were determined using a scanning electron microscope SEM JSM-7001F, JEOL and an energy dispersive X-ray spectrometry EDS INCA SYSTEM, Oxford. The local structures and oxidation states were analyzed by XAFS analysis using BL-9C and NW10A beam line at High Energy Accelerator Research Organization (KEK). The single crystal structure analyses were performed using Rigaku XtaLAB Super Nova. As a result of chemical analysis and structural analysis, both minerals were tetragonal perovskite: Ce-perovskite (Ca0.816 Na0.077 Ce0.045 La0.018 Nd0.018 Pr0.005 Ag0.004 Sm0.003 Dy0.002 Tb0.002 Eu0.001 Gd0.001 Pm0.001 Ho0.001 Lu0.001 Sr0.001) 0.996 (Ti0.942 Fe2+0.007 Fe3+0.013 Nb0.017 Zn0.012 Ge0.007 W0.003) 1.002O3, space group Pbnm, a = 5.41620(3), b = 5.48350(3), c = 7.70340(5). Nb-perovskite (Ca0.937 Ce0.021 Na0.020 La0.015 Sr0.003) 0.996 (Ti0.730 Nb0.122 Fe3+0.108 Al0.020 Zr0 .009 V0.008) 0.997O3, space group Pbnm, a = 5.40260(3), b = 5.46750(2), c = 7.67360(3). A slight inhomogeneity of the composition and a domain structure in the crystal were observed in the compo- images. It was revealed that the oxidation state of Fe in Ce-perovskite and Nb-perovskite is largely different state by the comparison of the XANES spectra. The total valence of each A and B site was almost divalent and tetravalent in both perovskites, The charges are balanced in sites by the substitution of Na+ and REE3+ at the A site and the substitution of Fe and Nb5+ at the B site. These solid solutions take large values of Debye-Waller factors. In particular, Ce-perovskite has a particularly large value due to the influence of contained radioactive elements. It can be interpreted that irregularization corresponding to local metamictization occurs due to radioactive decay of contained actinoid elements.