pubs.acs.org/IC Published on Web 07/07/2010 r 2010 American Chemical Society 6904 Inorg. Chem. 2010, 49, 6904–6908 DOI: 10.1021/ic100376u SrNp(PO 4 ) 2 : an Original Ordered Modification of Cheralite Karin Popa, † Gilles Wallez,* ,‡ Philippe E. Raison, § Damien Bregiroux, ‡ Christos Apostolidis, § Patric Lindqvist-Reis, ^ and Rudy J. M. Konings § † Department of Chemistry, Al. I. Cuza University, 11 Carol I Boulevard, 700506 Iasi, Romania, ‡ Laboratoire de Chimie de la Mati  ere Condens ee de Paris, UPMC Universit e Paris 06, CNRS-UMR 7574, ENSCP-ParisTech, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France, § European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany, and ^ Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Received February 24, 2010 The new compound SrNp(PO 4 ) 2 (orthorhombic Cmca) has been synthesized by a solid-state reaction and its crystal structure solved ab initio and refined by Rietveld analysis. Though chemically and structurally related to the cheralite CaTh(PO 4 ) 2 , SrNp(PO 4 ) 2 shows alternate layers of SrO 10 and NpO 8 polyhedra instead of a disordered array of 9-fold polyhedra. Raman and IR spectroscopic measurements also account for ordered cations. This novel structural type allows one to explain the boundaries of the cheralite domain in terms of the cation size and can be proposed as an archetype for a series of other actinide-bearing phosphate compounds. Introduction Under normal conditions, M II M 0 IV (PO 4 ) 2 double phosphates of medium- to large-size cations crystallize in two main families: layered yavapaiite-related structures made of M II O 10 (M II = Ca, Sr, Ba) and M 0 IV O 6 polyhedra (M 0 IV = Ge, Sn, Ti, Zr, Hf), 1-5 and cheralites, from the archetype CaTh(PO 4 ) 2 . 6,7 A third form, observed only for PbSn(PO 4 ) 2 , results from the stereochemically active lone pair of Pb II . 8 Formerly known as brabantite, cheralite is an analogue of monazite CePO 4 (also P2 1 /n), made of a phosphate tetrahe- dron and a unique 9-fold polyhedron for disordered cations Ca II and Th IV . 9-11 The early work of Rose has evidenced several other M II Th(PO 4 ) 2 cheralites with cations slightly smaller or larger than Ca II (Cd II , Sr II , and Pb II ) but not with Ba II , probably being too bulky. 6 Because of their remarkable durability both thermally (up to 1200 °C) 12,13 and chemically 14-19 and the possibility of fabri- cating and sintering monazite-cheralite solid solutions, 20-24 these mineral derivates have been the subject of intensive studies during the past years and figure as interesting candi- dates for the long-term immobilization of minor actinide radwastes. *To whom correspondence should be addressed. 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