[S-CG60] Petrology, Mineralogy and Resource Geology

[SCG60-01] Single crystal structure analyses by XRD and local structural analyses by XAFS on natural Ce-perovskite and Nb-perovskite

*Ginga Kitahara¹, Akira Yoshiasa¹, Makoto Tokuda¹, Tsubasa Tobase¹, Hidetomo Hongu¹, Hiroshi Arima², Kazumasa Sugiyama² (1.Kumamoto University, 2.Tohoku University)

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 (Ca_0.816 Na_0.077 Ce_0.045 La_0.018 Nd_0.018 Pr_0.005 Ag_0.004 Sm_0.003 Dy_0.002 Tb_0.002 Eu_0.001 Gd_0.001 Pm_0.001 Ho_0.001 Lu_0.001 Sr_0.001) _0.996 (Ti_0.942 Fe²⁺_0.007 Fe³⁺_0.013 Nb_0.017 Zn_0.012 Ge_0.007 W_0.003) _1.002O₃, space group Pbnm, a = 5.41620(3), b = 5.48350(3), c = 7.70340(5). Nb-perovskite (Ca_0.937 Ce_0.021 Na_0.020 La_0.015 Sr_0.003) _0.996 (Ti_0.730 Nb_0.122 Fe³⁺_0.108 Al_0.020 Zr_{0 .009} V_0.008) _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.