Journal of Solid State Chemistry 178 (2005) 800–810 Solid solubilities of (La Nd,) 2 (Zr,Ti) 2 O 7 phases deduced by neutron diffraction Elizabeth J. Harvey a , Karl R. Whittle a , Gregory R. Lumpkin a , Ronald I. Smith b , Simon A.T. Redfern a,Ã a Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK b ISIS Facility, Rutherford Appleton Laboratory, Oxon OX11 OQX, UK Received 6 October 2004; received in revised form 3 December 2004; accepted 17 December 2004 Abstract Time-of-flight powder neutron diffraction has been performed on oxides with composition (La 1x Nd x ) 2 Zr 2 O 7 and Nd 2 (Zr 1x Ti x ) 2 O 7 , where x ¼ 0; 0.2, 0.4,y1.0, in order to determine the solid solution behaviour across each series. Between La 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 , a cubic pyrochlore phase is observed (Fd ¯ 3m; Z ¼ 8). A linear decrease in the lattice parameter from 10.8047 to 10.6758A ˚ indicates complete miscibility of the two end-members. For the same series, the 48f oxygen x-parameter increasesfrom0.3313to0.3348,suggestingincreaseddistortionofthe6coordinate B sitesandreduceddistortionofthe8coordinate A sites. There is limited solubility of Nd 2 Ti 2 O 7 in Nd 2 Zr 2 O 7 . Exsolution of a monoclinic phase (P2 1 ; Z ¼ 8) rich in Nd 2 Ti 2 O 7 is observed at approximately x ¼ 0:56: The compositional range over which a solid solution exists is more extensive than that which has been previously reported. The solubility of Nd 2 Zr 2 O 7 in Nd 2 Ti 2 O 7 is very low. r 2004 Elsevier Inc. All rights reserved. Keywords: Pyrochlore; Neutron diffraction; Solid solution 1. Introduction Crystalline materials with the pyrochlore structure have been suggested as candidates for the immobilisa- tion of actinide species found in high-level nuclear waste (HLW) streams from the nuclear power industry and weapons decommissioning programmes. The introduc- tion of pyrochlore to Synroc [1] phase assemblages tailors them for direct waste loading of actinide-rich fuel elements [2,3]. The resilience of crystalline ceramics, such as these, to intrinsic radiation damage and degradation by repository groundwater is likely to far exceed that of the borosilicate glasses currently em- ployed for immobilisation [4]. The pyrochlore structure A 2 B 2 O 6 X, (space group Fd ¯ 3m; Z ¼ 8) is an ordered superstructure of fluorite with a cubic lattice parameter of about 10A ˚ , roughly twice that of fluorite (see Fig. 1). The A-cations (typically rare earths) occupy 16d Wyckoff positions ( 1 2 ; 1 2 ; 1 2 ),whilst B-cations(Zr,Ti)resideon16c sites (0,0,0). Relative to fluorite, one-eighth of the anions have been removed in an ordered manner, producing an unoccu- pied site (8a; ( 1 8 ; 1 8 ; 1 8 )). Six of the seven anions per formula unit of pyrochlore reside in 48f positions (x, 1 8 ; 1 8 ), with x  3 8 which relax towards the unoccupied 8a sites. The remaining anion occupies 8b positions ( 3 8 ; 3 8 ; 3 8 ). The magnitude of the 48f oxygen relaxation depends on the types of cations, 8b anions and site occupancies and isindicatedbythe48fx-positional parameter—the only variable atomic coordinate within the pyrochlore structure. This parameter, which can theoretically take values from 0.3125 to 0.375 [5], affects the local coordination geometry around A and B cations. The geometry of the 8 coordinate A sites changes from scalenohedral with 6+2 local coordination, to that of a ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$-see front matter r 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2004.12.030 Ã Corresponding author. Fax +441223333450. E-mail address: satr@cam.ac.uk (S.A.T. Redfern).