Institut für Geowissenschaften, Abt. Kristallographie, Johann Wolfgang Goethe-Universität Frankfurt, Senckenberganlage 30, D-60054 Frankfurt a. M., Germany, MineralogischPetrologisches Institut, Poppelsdorfer Schloß, Universität Bonn, D-53115 Bonn, Germany, Department Chemie und Biochemie, Lehrstuhl für Anorganische Festkörperchemie, LudwigMaximilians-Universität München, Butenandtstraße 5-13 (D), D-81377 München, Germany, and Institut für Geowissenschaften, Christian-Albrechts Universität zu Kiel, Olshausenstraße 40, D-24098 Kiel, Germany

The structural compression mechanism of Ce 4 [Si 4 O 3 +  x N 7 −  x ]Cl 1 −  x O x , x ≃ 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6 GPa using the diamond–anvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6 GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. The main structural mechanisms, leading to a volume reduction of about 5% at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2, Ce3 split position, and a bending of some of the inter-polyhedral Si—N—Si angles in the arrangement of the corner-sharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure.

Institut für Geowissenschaften, Abt. Kristallographie, Johann Wolfgang Goethe-Universität Frankfurt, Senckenberganlage 30, D-60325 Frankfurt a.M., Germany, Institut für Geowissenschaften, Mineralogie, Kristallographie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, D-24098 Kiel, Germany, Department Chemie und Biochemie, Lehrstuhl für Anorganische Festkörperchemie, LudwigMaximilians-Universität München, Butenandtstraße 5-13 (D), D-81377 München, Germany, and European Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble CEDEX, France

The oxonitridoaluminosilicate chloride Pr-10[Si10-xAlxO9+xN17-x]Cl was obtained by the reaction of praseodymium metal, the respective chloride, AlN and Al(OH)(3) with "Si(NH)(2)" in a radiofrequency furnace at temperatures around 1900 degrees C. The crystal structure was determined by single-crystal X-ray diffraction (Pbam, no. 55, Z=2, a=10.5973(8) angstrom, b=11.1687(6) angstrom, c=11.6179(7) angstrom, R1=0.0337). The sialon crystallizes isotypically to the oxonitridosilicate halides Ce-10[Si10O9N17]Br, Nd-10[Si10O9N17]Br and Nd-10[Si10O9N17]Cl, which represent a new layered structure type. The structure refinement was performed utilizing an O/N-distribution model according to Paulings rules, i.e. nitrogen was positioned on all bridging sites and mixed O/N-occupation was assumed on the terminal sites resulting in charge neutrality of the compounds. The Si and Al atoms were refined equally distributed on their three crystallographic sites, due to their poor distinguishability by X-ray analysis. The tetrahedra layers of the structure consist of condensed [(Si,Al)N-2(O,N)(2)] and [(Si,Al)N-3(O,N)] tetrahedra of Q(2) and Q(3) type. The chemical composition of the compound was derived from electron probe micro analyses (EPMA).

The nitridoaluminosilicate oxide BaSM5[Si9Al3N20]O was obtained by the reaction of barium and samarium metal, BaCO3, AIN and '' Si(NH)(2)'' in a radiofrequency furnace at temperatures around 1850 degrees C. The crystal structure was determined by single-crystal X-ray diffraction at 200(2) K (P321, no. 150, Z = 1, a = 9.510(8) angstrom, c = 6.1207(5) angstrom, R1 = 0.0247). It represents a new structure type with channels filled with chains of oxygen anions with corner sharing trigonal-bipyramidal coordination spheres. The anionic (Si,Al)/N 3D-network is made up solely of '' star-shaped '' [N-[4]((Si,Al)N-3)(4)] units consisting of four (Si,Al)N-4 tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N-distribution model according to Paulings rules, i.e., nitrogen was positioned on all bridging sites and oxygen was positioned on the isolated position, resulting in charge neutrality of the compound. Thus a structure model representing a nitridoaluminosilicate oxide has been established. The Si and Al atoms have been distributed equally on their crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X-ray methods. The chemical composition of the compound was derived from electron probe micro analyses (EPMA). (c) 2006 Elsevier SAS. All rights reserved.

The compressibilities of the nitridosilicate SrYb[Si(4)N(7)] and the oxonitridoaluminosilicates MYb[Si(4-x)Al(x)O(x)N(7-x)] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond-anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch-Murnaghan equation-of-state to the p-V data results in V(0) = 302.91 (6) A(3), B(0) = 176 (2) GPa and B' = 4.4 (2) for SrYb[Si(4)N(7)]; V(0) = 310.4 (1) A(3), B(0) = 161 (2) GPa and B' = 4.6 (2) for SrYb[Si(4-x)Al(x)O(x)N(7-x)]; and V(0) = 317.3 (5) A(3), B(0) = 168 (2) GPa and B' = 4.7 (2) for BaYb[Si(4-x)Al(x)O(x)N(7-x)]. While the linear compressibilities of the a and c axes of BaYb[Si(4-x)Al(x)O(x)N(7-x)] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si(4)N(7)] and SrYb[Si(4-x)Al(x)O(x)N(7-x)], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role.

The isotypic oxonitridosilicate chlorides Ln4[Si4O3+xN7−x]Cl1−xOx with Ln=Ce, Pr, Nd and x≈0.2 were obtained by the reaction of the respective rare-earth metals, their oxides and chlorides with “Si(NH)2” in a radiofrequency furnace at temperatures around 1800°C, using CsCl as a flux. The crystal structures were determined by single-crystal X-ray diffraction (P213, no. 198, Z=4, Ce: a=10.4461(12)pm, R1=0.0524; Pr: a=10.3720(12)pm, R1=0.0415; Nd: a=10.3618(12)pm, R1=0.0257) and found to be isotypic with Ce4[Si4O4N6]O. In order to characterize the incorporation of chlorine into the structure, the crystallographic site occupation factors of O, N and Cl were determined by neutron powder diffraction. Furthermore, these results were substantiated by the chemical analyses for Pr4[Si4O3+xN7−x]Cl1−xOx and electron microprobe analyses for all of the synthesized oxonitridosilicate chlorides. Temperature-dependent magnetic susceptibility measurements of the cerium and the praseodymium compound indicate Curie–Weiss behavior with experimentally determined magnetic moments of 2.15(5)μB/Ce and 3.50(5)μB/Pr, respectively. No magnetic ordering could be detected down to 2K. The 4f1 of cerium has been confirmed by XAS measurements.

The isotypic oxonitridosilicate halides Ce10[Si10O9N17]Br, Nd10[Si10O9N17]Br and Nd10[Si10O9N17]Cl were obtained by the reaction of the respective lanthanide metals, their oxides and halides with "Si(NH)2" in a radiofrequency furnace at temperatures around 1800 °C, using CsBr, resp. CsCl, as a flux. The crystal structures were determined by single-crystal X-ray diffraction (Pbam, no. 55, Z=2; Ce/Br: a=10.6117(9) Å, b=11.2319(10) Å, c=11.688(8) Å, R1=0.0356; Nd/Br: a=10.523(2) Å, b=11.101(2) Å, c=11.546(2) Å, R1=0.0239; Nd/Cl: a=10.534(2) Å, b=11.109(2) Å, c=11.543(2) Å, R1=0.0253) and represent a new layered structure type. The structure refinements were performed utilizing an O/N-distribution model according to Paulings rules, i.e. nitrogen was positioned on all bridging sites and mixed O/N-occupation was assumed on the terminal sites resulting in charge neutrality of the compounds. The layers consist of condensed [SiN2(O/N)2] and [SiN3(O/N)] tetrahedra of Q2 and Q3 type. The chemical composition of the compounds was derived from chemical analyses for Nd10[Si10O9N17]Br and electron probe micro analyses (EPMA) for all three compounds. The results of IR spectroscopic investigations are reported.

Single phase K5H(CN2)(3) was synthesized by reaction of KHCN2 with metallic potassium in liquid ammonia or by reaction of KNH2 with melamine C3N3(NH2)(3) at 320 degrees C, respectively. The crystal structure was determined from X-ray powder and single crystal data: K5H(CN)(3), space group Im3m, a = 795.68(7) pm, Z = 2, R1 = 0.025, wR2 = 0.0438. In the solid K5H(CN2)(3) contains K+ and CN22-, the anions exhibit D-infinity h symmetry. According to H-1 and C-13 Solid State MAS-NMR investigations, temperature dependent impedance spectroscopy, and FTIR spectroscopy the protons are only loosely bound to the CN22- ions. The proton conductivity shows a sharp increase above 70 degrees C.

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