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Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates |
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| Authors: | Philippe E Raison Regis Jardin Daniel Bouëxière Rudy J M Konings Thorsten Geisler Claudiu C Pavel Jean Rebizant Karin Popa |
| Institution: | 1. European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box?2340, 76125, Karlsruhe, Germany 2. Institut für Mineralogie, Westf?lische Wilhelms-Universi?t, Corrensstr. 24, 48149, Münster, Germany 3. Department of Chemistry, Al.I. Cuza University, 11—Carol I Blvd., 700506, Ia?i, Romania |
| Abstract: | The crystallographic structures of the synthetic cheralite, CaTh(PO4)2, and its homolog CaNp(PO4)2 have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO4 (Ln = La to Gd). The space group is P21/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO4)2 are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R wp = 4.87%, R p = 3.69% and R B = 3.99%. For CaNp(PO4)2, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R wp = 6.74%, R p = 5.23%, and R B = 6.05%. The results indicate significant distortions of bond length and angles of the PO4 tetrahedra in CaTh(PO4)2 and to a lesser extent in CaNp(PO4)2. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO4)2. A comparison with the isostructural compounds LnPO4 (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca2+ and Th4+ introduces a distortion of the PO4 tetrahedra. |
| Keywords: | Cheralite Tetravalent actinide phosphates Rietveld analysis Thorium Neptunium |
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