A neutron diffraction study of structure and thermal motion in several monovalent metal azides

The First Alkaline-Earth Azidoaurate(III), Ba[Au(N3 )4 ]2 ⋅ 4 H2 O

Transparent, dark orange Ba[Au(N₃ )₄ ]₂  ⋅ 4 H₂ O was synthesized by reaction of Ba(N₃ )₂ and AuCl₃ or HAuCl₄ in aqueous solution. The novel barium tetraazidoaurate(III) tetrahydrate crystallizes in the monoclinic space group Cc (no. 9) with a=1813.68(17) pm, b=1737.95(11) pm, c=682.04(8) pm and β=108.849(4)°. The predominant structural features of Ba[Au(N₃ )₄ ]₂  ⋅ 4 H₂ O are two crystallographically independent discrete anions [Au(N₃ )₄ ]^- with gold in a tetragonal planar coordination by nitrogen. Vibrational spectra show good agreement with those of other azidoaurates(III). Upon drying, this salt was shown to be a highly explosive material.

A unified model of impact sensitivity of metal azides

A comprehensive theoretical study of 18 metal azides is reported.

The determination of ionic transport properties at high pressures in a diamond anvil cell

A two-electrode configuration was adopted in an in situ impedance measurement system to determine the ionic conductivity at high pressures in a diamond anvil cell. In the experimental measurements, Mo thin-films were specifically coated on tops of the diamond anvils to serve as a pair of capacitance-like electrodes for impedance spectrum measurements. In the spectrum analysis, a Warburg impedance element was introduced into the equivalent circuit to reveal the ionic transport property among other physical properties of a material at high pressures. Using this method, we were able to determine the ionic transport character including the ionic conductivity and the diffusion coefficient of a sodium azide solid to 40 GPa.

Elastic, thermoelastic and piezoelastic properties of crystals of the KSCN family (KN3, KHF2, KSCN, NH4HF2, NH4SCN)

Large single crystals of KN3, KHF2, NH4HF2, KSCN and NH4SCN, which are structurally related to those of the CsCl type, were grown from aqueous solutions by controlled evaporation. Elastic, thermoelastic and piezoelastic constants were determined from ultrasonic resonance frequencies of thick plane-parallel plates. In addition, the tensors of thermal expansion were measured.

Elastic properties and thermal expansion reflect the close relationship to the CsCl-type structure. The anisotropy is mainly caused by the preferential orientation of the long axes of the anionic dumb-bells within one distinct plane. KSCN exhibits weak elastic anomalies approaching the phase transition at 413 K. These effects, which are much smaller than those found in the high-temperature phases of the alkali cyanides, indicate, together with the almost missing hysteresis and the small transition enthalpy, a transition of mainly second order. The variation of the elastic behaviour of KHF2 and NH4SCN passing through the transition at 471 K and 367 K, respectively, express first-order processes.

Ab initio study of electronic structure and optical properties of heavy-metal azides: TlN3, AgN3, and CuN3

Detailed ab initio studies on the electronic structure and optical properties of the heavy-metal azides have been performed using density functional theory within the generalized gradient approximation. An analysis of band structure, density of states, charge transfer, and bond order shows that the heavy-metal azides are ionic compounds but have covalent character. The valence bands of AgN3 and CuN3 are strongly dominated by Ag- and Cu-d, respectively, but that of TlN3 arises from the contributions of Tl-s and terminal N-p and not from Tl-d. The real and imaginary parts of the dielectric function, adsorption coefficient, and electron energy-loss spectra are calculated and compared with available experimental data.

Comparative First-Principles Study of Structural and Optical Properties of Alkali Metal Azides

A comparative first-principles study of the structural and optical properties of the alkali metal azides has been performed with density functional theory within the generalized gradient approximation. The crystal structures of the alkali azides compare well with experimental data. Their ionic character is manifested by the closeness of their internitrogen distances to the calculated N-N bond length for the free azide ion. An analysis of electronic structure, charge transfer, and bond order shows that the alkali azides are all wide-gap insulators and ionic compounds. The energy band and density of states for lithium azide and alpha-sodium azide are very similar, while these for potassium azide, alpha-rubidium azide, and alpha-cesium azide are alike, but some modifications are observed with the increment of alkali metals' electropositivity. These changes are closely related to the differences of the crystal structures. The general shapes of the real and imaginary parts of the dielectric function, adsorption coefficient, and electron energy-loss spectra are quite similar. The peaks originate from the electron transitions from the alkali metal s and p states to the conduction band. Our calculated optical properties for the alkali azides are found to be in good agreement with available experimental data. The absorption spectra of the alkali azides show a number of absorption peaks, which are believed to be associated with different exciton states, in the fundamental absorption region. In general, the electron energy-loss spectra have two plasma frequencies.

Polymerization of nitrogen in sodium azide

The high-pressure behavior of nitrogen in NaN(3) was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N(3-) are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.

Synthesis, structure determination, and quantum-chemical characterization of an alternate HgNCN polymorph

A novel, possibly metastable form of HgNCN, designated HgNCN(II), is accessible under soft-chemical reaction conditions, and its existence has been established from combined X-ray/neutron Rietveld refinements (P2(1)/a, a = 6.8521(4) A, b = 6.9797(4) A, c = 5.5516(4) A, beta = 113.212(4) degrees ), vibrational spectroscopy investigations, and plane-wave DFT calculations utilizing pseudopotentials. In contrast to the known mercury carbodiimide HgNCN(I) with two practically identical N-C double bonds of 1.22 A, the true cyanamide HgNCN(II) is characterized by a short ("triple") N-C bond (1.12 A) and a long ("single") C-N bond (1.35 A); two Hg atoms coordinate the asymmetrically shaped NCN(2)(-) unit only on the side of the C-N "single" bond. Mercury carbodiimide HgNCN(I) and mercury cyanamide HgNCN(II) are thermochemically separated from each other by such a high energy barrier that thermal decomposition prevents structural interconversion. The structure-chemical relationship of the two HgNCN phases is discussed in terms of bond-stretch and linkage isomerism.