MONAZITE-TYPE AND ZIRCON-TYPE STRUCTURES OF 7 MIXED(LN LN)PO4 COMPOUNDS/

The (1:1) compound ortho-phosphates (Ce/Tb), (Nd/Tb) and (Sm/Tb) cryst allize in the monoclinic system conforming to space group P2(1)/n (No. 14), and the lanthanide metal atoms are nine-coordinated to oxygen at oms forming a polyhedron that is best described as a pentagonal interp enetrating tetrahedron. The (1:1) compound ortho-phosphates (Nd/Er), ( Sm/Er), (Sm/Yb) and (Sm/Lu) crystallize in the tetragonal system (spac e group I4(1)/amd (No. 141)), and the metal atoms are eight-coordinate d forming two orthogonal interpenetrating tetrahedra, a bis-bisphenoid al geometry. The phosphate group in each compound investigated is a di storted tetrahedron. Single-crystal structural analyses have been carr ied out for the seven indicated mixed (Ln/Ln) PO4 ortho-phosphates and coordination geometries have been determined by means of automated th ree-dimensional single-crystal diffractometry and full-matrix least-sq uares refinements. Important bond distances and angles, experimental s ummaries and ORTEP-II drawings related to the geometries are presented .