Orthonitridoborate ions [BN3]6- in oxonitridosilicate cages: synthesis, crystal structure, and magnetic properties of Ba4Pr7[Si12N23O][BN3], Ba4Nd7[Si12N23O][BN3], and Ba4Sm7[Si12N23O][BN3

The isotypic title compounds Ba4Pr7[Si12N23O][BN3], Ba4Nd7[Si12N23O][BN3], and Ba4Sm7[Si12N23O][BN3] were prepared by reaction of Pr, Nd, or Sm, with barium, BaCO3, Si(NH)2, and poly(boron amide imide) in nitrogen atmosphere in tungsten crucibles using a radiofrequency furnace at temperatures up to 1650 C. They were obtained as main products (approximately 70%) embedded in a very hard glass matrix in the form of intense dark green (Pr), orange-brown (Sm), or dark red (Nd) large single crystals, respectively. The stoichiometric composition of Ba4Sm7[Si12N23O][BN3] was verified by a quantitative elemental analysis. According to the single-crystal X-ray structure determinations (Ba4Ln7[Si12N23][BN3], Z= , P6 with Ln = Pr: a = 1225.7(1), c = 544.83(9) pm, R1 = 0.013, wR2 = 0.030; Ln = Nd: a = 1222.6(1), c = 544.6(1) pm, R1 = 0.017, wR2 = .039; Ln = Sm: a = 1215.97(5), c = 542.80(5) pm, R1 = 0.047, wR2 = 0.099) all three compounds are built up by a framework structure [Si12N23O]23- of corner-sharing SiX4 tetrahedrons (X = O, N). The oxygen atoms are randomly distributed over the X positions. The trigonal-planar orthonitridoborate ions [BN3]6- and also the Ln(3)3+ are situated in hexagonal cages of the framework (bond lengths Si-(N/O) 169-179 pm for Ln=Pr). The remaining Ba2+ and Ln3- ions are positioned in channels of the large-pored network. The trigonal-planar [BN3]6- ions have a B-N distance of 147.1(6) pm (for Ln = Pr). Temperature-dependent susceptibility measurements for Ba4Nd7[Si12N23O][BN3] revealed Curie-Weiss behavior above 60 K with an experimental magnetic moment of muexp = 3.36(5) microB/Nd. The deviation from Curie-Weiss behavior below 60 K may be attributed to crystal field splitting of the J = 9/2 ground state of the Nd3+ ions. No magnetic ordering is evident down to 4.2 K.