ELSEVIER Inorganica Chimica Acta 244 (1996) 247-252 Note Monazite- and zircon-type structures of seven mixed (Ln/Ln) P04 compounds D.F. Mullica a.., E.L. Sappenfield ", L.A. Boatner b aDepartment of Chemistry, Baylor University, Waco, TX 76798, USA b Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056, USA Received 27 March 1995; revised 17 August 1995 Abstract The ( 1:1 ) compound ortho-phosphates (Ce/Tb), (Nd/Tb) and (Sm/Tb) crystallize in the monoclinic system conforming to space group P21/n (No. 14), and the lanthanide metal atoms are nine-coordinated to oxygen atoms forming a polyhedron that is best described as a pentagonal interpenetrating tetrahedron. The (1:1) compound ortho-phosphates (Nd/Er), (Sm/Er), (Sm/Yb) and (Sm/Lu) crystallize in the tetragonal system (space group 141/amd (No. 141 ) ), and the metal atoms are eight-coordinatedforming two orthogonal interpenetrating tetrahedra,a bis-bisphen0idal geometry. The phosphategroup in each compoundinvestigatedis a distortedtetrahedron.Single-crystalstructural analyseshave been carded out for the seven indicated mixed (Ln/Ln) PO4 ortho-phosphates and coordinationgeometrieshave been determined by means of automated three-dimensional single-crystal diffractometry and full-matrix least-squares refinements. Important bond distances and angles, experimental summaries and ORTEP-II drawings related to the geometries are presented. Keywords: Crystal structures; Lanthanide complexes; Mixed complexes; Phosphate complexes 1. Introduction Due to the relatively unique combination of their physical and chemical properties, the lanthanide ortho-phosphates (LnPO4) and mixed solid-solution lanthanide ortho-phos- phates (Ln/LnPO4) have previously been investigated as ceramic waste forms for the incorporation and long-term dis- posal of commercial, defense and actinide high-level radio- active wastes [1-7]. This combination of lanthanide ortho-phosphate physical and chemical properties encom- passes: (i) an extremely low solubility in aqueous media over a wide range of pH values; (ii) a high tolerance in the solid state for the inc~orporation of actinide ions as well as other constituents normally found in radioactive wastes; (iii) an established ability to retain both thorium and uranium over geologic time periods; (iv) the ability to recover readily from displacive radiation damage [ 8,9]; (v) a negative tempera- ture coefficient of solubility; and (vi) a high degree of ther- mal stability. Additionally, plutonium, americium and curium are all reported to form ortho-phosphates with the monoclinic (monazite) structure characteristic of lanthanide ortho-phos- phates in the first half of the rare-earth transition series [ 10,11 ], and this fact has recently led to discussions regard- * Corresponding author. 0020-1693/96/$15.00 © !996 Elsevier Science S.A. All fights reserved SSDIO020-1693 ( 95 ) 04783-9 ing the use of lanthanide ortho-phosphates as a medium for storing fissile material resulting from the dismantling of nuclear weapons. Most recently, the potential of the mixed- lanthanide ortho-phosphates for applications as laser hosts, thermophosphor materials for remote temperature sensing, and as scintillators for gamma-ray detection [ 12-14] have led to renewed interest in the structural, optical and electronic properties of these materials, and accordingly, the present investigations were motivated by new applications of this type. 2. Experimental Single crystals of Ce/Tb (I), Nd/Tb (II), Sm/Tb (III), Nd/Er (IV), Sm/Er (V), Sm/Yb (VI) and Sm/Lu (VII) ortho-phosphates were synthesized using equimolar ratios of the metal constituents (purity > 99.9%). A high-temperature flux technique described previously [ 15,16] was employed in the crystal-growth process. Conoscopic studies of the mixed (Ln/Ln)PO4 compounds provided evidence that the systems (both monazite- and zircon-types) were biaxial in nature, i.e., optically birefringent. Single crystals of each spe- cies, selected on the basis of optical homogeneity, were mounted on glass fibers and transferred to goniometer heads