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 d iffractometer 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 a nd 181 variable parameters. The structure of the isotypic compound CeS i2P6 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 distingui shed by comparing their occupancy parameters obtained from both struct ure refinements. The assignments agree with those deduced by structure -chemical arguments. These atoms form a three-dimensionally infinite f ramework polyanion, which accommodates four different kinds of rare ea rth atoms: three with nine and one with ten phosphorus neighbors. The silicon atoms are all in tetrahedral phosphorus coordination. There ar e phosphorus atoms which have only two rare earth and two silicon neig hbors, but most phosphorus atoms have-in addition to the rare earth an d 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-), wh ere 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.