Deprotonating Melamine to Gain Highly Interconnected Materials: Melaminate Salts of Potassium and Rubidium.

A new class of materials, melaminate salts of potassium and rubidium, has been obtained by deprotonating molecular melamine in liquid ammonia. Potassium melaminate KC3N6H5·NH3 and rubidium melaminate RbC3N6H5·1/2NH3 were characterized by single-crystal XRD, showing that the melaminate anion is slightly distorted compared to the neutral molecule due to the ionic imine group, but it still forms extensive hydrogen bonding networks. The melaminate anion also displays an increased coordination ability of µ4 and µ6+1 (the maximum for melamine is µ3). Thermal gravimetry coupled with mass spectrometry evidence a multistep decomposition with liberation of ammonia first and then cyanamide and larger fragments. A plausible decomposition mechanism is proposed. The infrared spectrum allows to identify the fingerprint of the melaminate vibrations such as to partially characterize the also synthesized amorphous sodium melaminate NaC3N6H5·nNH3 and the proposed tripotassium melaminate K3C3N6H3.

Relative stability of diamond and graphite as seen through bonds and hybridizations.

The relative stability of the two most important forms of elemental carbon, diamond and graphite, is readdressed from a newly developed perspective as derived from historically well-known roots. Unlike other theoretical studies mostly relying on numerical methods, we consider an analytical model to gain fundamental insight into the reasons for the quasi-degeneracy of diamond and graphite despite their extremely different covalent bonding patterns. We derive the allotropes' relative energies and provide a qualitative picture predicting a quasi-degenerate electronic ground state for graphite (graphene) and diamond at zero temperature. Our approach also gives numerical estimates of the energy difference and interatomic separations in good agreement with experimental data and recent results of hybrid DFT modeling, although obtained with a much smaller numerical but highly transparent effort. An attempt to extend this treatment to the lowest energy allotropes of silicon proves to be successful as well.

Ammonothermal Synthesis, Crystal Structure, and Properties of the Ytterbium(II) and Ytterbium(III) Amides and the First Two Rare-Earth-Metal Guanidinates, YbC(NH)3 and Yb(CN3H4)3.

We report the oxidation-controlled synthesis of the ytterbium amides Yb(NH2)2 and Yb(NH2)3 and the first rare-earth-metal guanidinates YbC(NH)3 and Yb(CN3H4)3 from liquid ammonia. For Yb(NH2)2, we present experimental atomic displacement parameters from powder X-ray diffraction (PXRD) and density functional theory (DFT)-derived hydrogen positions for the first time. For Yb(NH2)3, the indexing proposal based on PXRD arrives at R3̅, a = 6.2477(2) Å, c = 17.132(1) Å, V = 579.15(4) Å(3), and Z = 6. The oxidation-controlled synthesis was also applied to make the first rare-earth guanidinates, namely, the doubly deprotonated YbC(NH)3 and the singly deprotonated Yb(CN3H4)3. YbC(NH)3 is isostructural with SrC(NH)3, as derived from PXRD (P63/m, a = 5.2596(2) Å, c = 6.6704(2) Å, V = 159.81(1) Å(3), and Z = 2). Yb(CN3H4)3 crystallizes in a structure derived from the [ReO3] type, as studied by powder neutron diffraction (Pn3̅, a = 13.5307(3) Å, V = 2477.22(8) Å(3), and Z = 8 at 10 K). Electrostatic and hydrogen-bonding interactions cooperate to stabilize the structure with wide and empty channels. The IR spectra of the guanidinates are compared with DFT-calculated phonon spectra to identify the vibrational modes. SQUID magnetometry shows that Yb(CN3H4)3 is a paramagnet with isolated Yb(3+) (4f(13)) ions. A CONDON 2.0 fit was used to extract all relevant parameters.

Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing a Hole-Extraction Gradient for Photoelectrochemical Water Oxidation.

A nanowire photoanode SrTaO2N, a semiconductor suitable for overall water-splitting with a band gap of 2.3 eV, was coated with functional overlayers to yield a core-shell structure while maintaining its one-dimensional morphology. The nanowires were grown hydrothermally on tantalum, and the perovskite-related oxynitride structure was obtained by nitridation. Three functional overlayers have been deposited on the nanowires to enhance the efficiency of photoelectrochemical (PEC) water oxidation. The deposition of TiO x protects the oxynitride from photocorrosion and suppresses charge-carrier recombination at the surface. Ni(OH) x acts a hole-storage layer and decreases the dark-current contribution. This leads to a significantly improved extraction of photogenerated holes to the electrode-electrolyte surface. The photocurrents can be increased by the deposition of a cobalt phosphate (CoPi) layer as a cocatalyst. The heterojunction nanowire photoanode generates a current density of 0.27 mA cm-2 at 1.23 V vs the reversible hydrogen electrode (RHE) under simulated sunlight (AM 1.5G). Simultaneously, the dark-current contribution, a common problem for oxynitride photoanodes grown on metallic substrates, is almost completely minimized. This is the first report of a quaternary oxynitride nanowire photoanode in core-shell geometry containing functional overlayers for synergetic hole extraction and an electrocatalyst.

Tin(ii) oxide carbodiimide and its relationship to SnO.

The compound Sn2O(CN2) was obtained as a brick-red crystalline powder from a solid-state reaction of equimolar amounts of SnCl2, SnO and Li2(CN2). Thermal analysis of the reaction indicates the formation of two intermediate compounds until Sn2O(CN2) is formed at 400 °C. The crystal structure of Sn2O(CN2) was determined in the space group Pccn (a = 13.3949(7) Å, b = 5.2954(3) Å, c = 5.5369(2) Å, Z = 4) from single-crystal X-ray diffraction data. The structure contains a Sn2+ ion with a 5s2 lone pair, situated in a fourfold, mixed O-/N-coordination environment. The crystal structure of Sn2O(CN2) is analyzed and discussed in relation to that of SnO, namely by electronic-structure calculations and a COHP bonding analysis of Sn2O(CN2).