A novel carbon nitride compound, structurally related to the proposed graphitic phase of C(3)N(4), has been synthesized in a bulk well-crystallized form. The new material, with stoichiometry C(6)N(9)H(4)Cl, was prepared through a solid-state reaction of 2,4,6-triamino-1,3,5-triazine with 2,4,6-trichloro-1,3,5-triazine at 1.0-1.5 GPa and 500-550 degrees C and also through a self-reaction of 2-amino-4,6-dichloro-1,3,5-triazine at similar conditions. X-ray and electron diffraction measurements on the yellowish compound indicate a hexagonal space group (P6(3)/m) with cell parameters of a = 8.4379(10) A and c = 6.4296(2) A. This new compound possesses a two-dimensional C(6)N(9)H(3) framework that is structurally related to the hypothetical P6m2 graphitic phase of C(3)N(4), but with an ordered arrangement of C(3)N(3) voids. The large voids in the graphene sheets are occupied by chloride ions with an equivalent number of nitrogen atoms on the framework protonated for charge balance. The composition of the sample was determined by bulk chemical analysis and confirmed by electron energy loss (EELS) spectroscopy. The chemical and structural model is consistent with bulk density measurements and with the infrared and (13)C NMR spectra. This work represents the first bulk synthesis of a well-characterized and highly crystalline material containing a continuous network of alternating carbon and nitrogen atoms.

g-Ge 3 N 4 is formed from a-or b-Ge 3 N 4 at pressures greater than 12 GPa and temperatures above 1000 8C. It has the spinel structure, symmetry Fd3 Å m, and lattice parameter a 8.2125(1) . Germanium has both tetrahedral [GeÀN 1.879(2) ] and octahedral [Ge À N 1.996(1) ] coordination to nitrogen. The difference between the octahedral and tetrahedral bond lengths in this nitride is close to that expected from systematics, which were largely derived from oxides.

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Abstract. New high-pressure orthorhombic (GdFeOatype) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) ~, c=7.690 (8) A_ and V= 221.99 (15) ~ 3. Volume compression data were collected between 7 and 20 GPa, The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a= 5.022 (6) ~, b=5.169 (5) A,, c=7.239 (9)/k and V= 187.94 (36)~3. Based on published data on the quench phases, the FeTiOa perovskite breaks down to a rocksalt § mixture of "FeO" and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.

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