Complex interrupted tetrahedral frameworks in the nitridosilicates M7Si6N15 (M = La, Ce, Pr).

A new structure type of nitridosilicates with an interrupted framework has been identified for M(7)Si(6)N(15) with M = La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnace at temperatures between 1550-1625 degrees C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce(7)Si(6)N(15) has been determined by using single-crystal X-ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P1, no. 2; a = 13.009(3), b = 25.483(5), c = 25.508(10) A; alpha = 117.35(3), beta = 99.59(3), gamma = 99.63(3) degrees; V = 7114(2) A(3); Z = 18; R1 = 0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R3, no. 148) have also been observed (a = 43.420(6), c = 6.506(2) A; V = 10 623(3) A(3); Z = 27; R1 = 0.0309). Pr(7)Si(6)N(15) (3) and La(7)Si(6)N(15) (4) are isostructural with 1 as evidenced by twinned single-crystal data for 3 (P1, no. 2; a = 12.966(3), b = 25.449(10), c = 25.459(10) A; alpha = 117.28(3), beta = 99.70(4), gamma = 99.60(4) degrees; V = 7068(4) A(3); Z = 18; R1 = 0.0526) and powder diffraction data for 4 (P1, no. 2; a = 13.109(9), b = 25.606(18), c = 25.609(18) A; V = 7223(12) A(3); Z = 18; R(P) = 0.0194; R(F)(2) = 0.0936). The crystal structure of M(7)Si(6)N(15) (M = La, Ce, Pr) is built up exclusively of corner-sharing tetrahedrons that appear as Q(2)-, Q(3)-, and Q(4)-type tetrahedrons forming different ring sizes within a less condensed three-dimensional network. Among the characteristic structural motifs are saw-blade-shaped 12-rings and finite chains consisting of four corner-sharing SiN(4) tetrahedrons. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce(7)Si(6)N(15) show paramagnetic behavior with an experimental magnetic moment of 2.29 mu(B) per Ce, thereby corroborating the existence of Ce(3+).