The rare earth silicon phosphides LnSi(2)P(6) (Ln=La,Ce,Pr, and Nd)

The title compounds were prepared in well-crystallized form from a tin flux and their crystal structure was determined from single-crystal diffractometer data of LaSi2P6: Cmc2(1), a = 1012.9(3) pm, b = 2817.5(7) pm, c = 1037.4(5) pm, Z = 16, R = 0.034 for 3303 structure factors and 181 variable parameters. The structure of the isotypic compound CeSi2P6 was also refined from single-crystal X-ray data: a = 1011.8(4) pm, b = 2803.1(8) pm, c = 1031.1(4) pm, R = 0.035 for 2132 F values and 181 variables. The silicon and the phosphorus atoms could be distinguished by comparing their occupancy parameters obtained from both structure refinements. The assignments agree with those deduced by structure-chemical arguments. These atoms form a three-dimensionally infinite framework polyanion, which accommodates four different kinds of rare earth atoms: three with nine and one with ten phosphorus neighbors. The silicon atoms are all in tetrahedral phosphorus coordination. There are phosphorus atoms which have only two rare earth and two silicon neighbors, but most phosphorus atoms have-in addition to the rare earth and silicon atoms-phosphorus neighbors, thus forming P-3, P-4, P-5, and P-6 units. Using oxidation numbers, tile compounds can be rationalized with the formulas Ln(3+)(Si2P6)(3-) and Ln(3+)(Si4+)(2)(P-6)(11-), where the octet rule is obeyed for the silicon and phosphorus atoms and two electrons are counted for each Si-P and P-P interaction. (C) 1996 Academic Press, Inc.