Brad Steele

Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure

Two new crystalline compounds, pentazole (N5H) and ammonium pentazolate (NH4)(N5), both featuring cyclo-N5- are discovered using a first-principles evolutionary search of the nitrogen-rich portion of the hydro-nitrogen binary phase diagram (NxHy, x ≥ y) at high pressures. Both crystals consist of the pentazolate N5- anion and ammonium NH4+ or hydrogen H+ cations. These two crystals are predicted to be thermodynamically stable at pressures above 30 GPa for (NH4)(N5) and 50 GPa for pentazole N5H. The chemical transformation of ammonium azide (NH4)(N3) mixed with dinitrogen (N2) to ammonium pentazolate (NH4)(N5) is predicted to become energetically favorable above 12.5 GPa. To assist in identification of newly synthesized compounds in future experiments, the Raman spectra of both crystals are calculated and mode assignments are made as a function of pressure up to 75 GPa.

Novel Potassium Polynitrides at High Pressures

Polynitrogen compounds have attracted great interest due to their potential applications as high energy density materials. Most recently, a rich variety of alkali polynitrogens (RxNy; R = Li, Na, and Cs) have been predicted to be stable at high pressures and one of them, CsN5 has been recently synthesized. In this work, various potassium polynitrides are investigated using first-principles crystal structure search methods. Several novel molecular crystals consisting of N4 chains, N5 rings, and N6 rings stable at high pressures are discovered. In addition, an unusual nitrogen-rich metallic crystal with stoichiometry K2N16 consisting of a planar two-dimensional extended network of nitrogen atoms arranged in fused 18 atom rings is found to be stable above 70 GPa. An appreciable electron transfer from K to N atoms is responsible for the appearance of unexpected chemical bonding in these crystals. The thermodynamic stability and high pressure phase diagram is constructed. The electronic and vibrational properties of the layered polynitrogen K2N16 compound are investigated, and the pressure-dependent IR spectrum is obtained to assist in experimental discovery of this new high-nitrogen content material.

Ternary Inorganic Compounds Containing Carbon, Nitrogen, and Oxygen at High Pressures

Ternary CxNyOz compounds are actively researched as novel high energy density and ultrahard materials. Although some synthesis work has been performed at ambient conditions, very little is known about the high pressure chemistry of of CxNyOz compounds. In this work, first-principles variable-composition evolutionary structure prediction calculations are performed with the goal of discovering novel mixed CxNyOz materials at ambient and high pressure conditions. By systematically searching ternary variable composition crystalline materials, the full ternary phase diagram is constructed in the range of pressures from 0 to 100 GPa. The search finds the C2N2O crystal containing an extended covalent network of C, N, and O atoms, having space group symmetry Cmc21, and stable above just 10 GPa. Several other novel metastable (CO)x-(N)y crystalline compounds discovered during the search, including two polymorphs of C2NO2 and two polymorphs of C3N2O3 crystals, are found to be energetically favorable compared to polymeric carbon monoxide (CO) and nitrogen. Predicted new compounds are characterized by their Raman spectra and equations of state.

Cesium pentazolate: A new nitrogen-rich energetic material

We report theoretical and experimental evidence for a new class of high-nitrogen content energetic material compounds consisting of molecular pentazoles, which are stabilized in the crystal phase upon introduction of elemental cesium. First-principles structural predictions show that the material with composition CsN5 is thermodynamically stable above 15 GPa. Indexing of the measured X-ray diffraction spectra indicate the synthesis of this material at 60 GPa as well its stability upon decompression down to 24 GPa.

Structural and spectroscopic studies of nitrogen-carbon monoxide mixtures: Photochemical response and observation of a novel phase

Mixtures of nitrogen and carbon monoxide in two molar compositions (90-10 and 95-5 N2—CO) have been studied with Raman spectroscopy, X-ray diffraction, and first-principles density functional theory. Near 16 GPa, there is a discontinuous change in the X-ray diffraction patterns indicating a transition to phase I, which is distinct from any known phase of nitrogen. With the help of theory, the X-ray diffraction pattern was indexed to a triclinic unit cell. The evolutionary crystal structure search also identified several metastable stoichiometries of C—O—N phases, which produce distinct signatures in the experimental Raman spectra, thus explaining anomalous Raman behavior. Decompression studies showed that phase I did not persist below the melt line of nitrogen and, as such, it can be concluded that all observations are reversible.

Novel rubidium poly-nitrogen materials at high pressure

First-principles crystal structure search is performed to predict novel rubidium poly-nitrogen materials at high pressure by varying the stoichiometry, i.e., relative quantities of the constituent rubidium and nitrogen atoms. Three compounds of high nitrogen content, RbN5, RbN2, and Rb4N6, are discovered. Rubidium pentazolate (RbN5) becomes thermodynamically stable at pressures above 30 GPa. The charge transfer from Rb to N atoms enables aromaticity in cyclo-N5- while increasing the ionic bonding in the crystal. Rubidium pentazolate can be synthesized by compressing rubidium azide (RbN3) and nitrogen (N2) precursors above 9.42 GPa, and its experimental discovery is aided by calculating the Raman spectrum and identifying the features attributed to N5- modes. The two other interesting compounds, RbN2 containing infinitely long single-bonded nitrogen chains and Rb4N6 consisting of single-bonded N6 hexazine rings, become thermodynamically stable at pressures exceeding 60 GPa. In addition to the compounds with high nitrogen content, Rb3N3, a new compound with 1:1 RbN stoichiometry containing bent N3 azides is found to exist at high pressures.

New phase of ammonium nitrate: A monoclinic distortion of AN-IV

A new phase of ammonium nitrate (AN) is found using first principles evolutionary crystal structure search. It is this polymorph that is associated with the phase transition to previously unidentified phase, which was detected in experiment at 17 GPa upon appearance of the two extra peaks in Raman spectrum. The new phase has a monoclinic unit cell in the P21/m space group symmetry (AN-P21/m) and is similar to the known phase IV of AN (AN-IV) except the ammonium molecules are oriented differently relative to the nitrate molecules. The calculated free energy of AN-P21/m is found to be lower than AN-IV at pressures above 10.83 GPa. The equation of state of both AN-P21/m and AN-IV phases (volume vs hydrostatic pressure at room temperature) has been obtained within the quasi-harmonic approximation. The calculated Raman spectrum of both AN-P21/m and AN-IV as a function of pressure is in a good agreement with experiment. The energetic competitiveness of AN-IV and AN-P21/m at ambient conditions suggests a possibility of the phase transition in a small pressure-temperature range near ambient pressure and temperature.

Density Functional Theory Investigation of Sodium Azide at High Pressure

High pressure experiments utilizing Raman spectroscopy indicate that the a phase of sodium azide undergoes a polymeric phase transition at high pressure. In this work, the structural and vibrational properties, including the first order Raman and infrared spectra, of the a phase of sodium azide are calculated using first-principles density functional theory up to 92 GPa. The equation of state of ? NaN3 is obtained within the quasi-harmonic approximation at various temperatures. Each Raman-active mode blue shifts under compression whereas the doubly degenerate IR-active azide bending mode red-shifts under compression. However, at 70 GPa, the intensity of the Bu IR-active bending mode decreases substantially, and a new distorted azide bending lattice mode appears in the IR spectrum. In contrast to the bending mode, this new mode blue-shifts under compression. No new modes appear in the Raman spectra at high pressure, indicating that the changes in the Raman spectrum seen in experiment at high pressure are signs of new high nitrogen content structures, but not due to sodium azide.

Novel phases and superconductivity of tin sulfide compounds

Tin sulfides, Sn x S y , are an important class of materials that are actively investigated as novel photovoltaic and water splitting materials. A first-principles evolutionary crystal structure search is performed with the goal of constructing the complete phase diagram of Sn x S y and discovering new phases as well as new compounds of varying stoichiometry at ambient conditions and pressures up to 100 GPa. The ambient phase of SnS2 with P 3 ¯ m 1 symmetry remains stable up to 28 GPa. Another ambient phase, SnS, experiences a series of phase transformations including α-SnS to β-SnS at 9 GPa, followed by β-SnS to γ-SnS at 40 GPa. γ-SnS is a new high-pressure metallic phase with P m 3 ¯ m space group symmetry stable up to 100 GPa, which becomes a superconductor with a maximum T c = 9.74 K at 40 GPa. Another new metallic compound, Sn3S4 with I 4 ¯ 3 d space group symmetry, is predicted to be stable at pressures above 15 GPa, which also becomes a superconductor with relatively high T c = 21.9 K at 30 GPa.

First-principles investigation of iron pentacarbonyl molecular solid phases at high pressure

The polymeric phases of carbon monoxide (p-CO), an extended non-molecular solid, represent a new class of low-Z energetic materials. The presence of transition metal ions is believed to stabilize polymeric carbon monoxide (p-CO) at ambient conditions. Since p-CO forms at high pressures, it becomes important to investigate the high-pressure behavior of one of the potential precursors, iron pentacarbonyl Fe(CO)5. In this work, a first-principles evolutionary structure search method is used to determine the crystal phases of Fe(CO)5 at high pressure. The calculations predict the crystal structure of Phase I in agreement with experiment. Moreover, the previously unidentified crystal structure of Phase II is found. The calculated pressure-dependent Raman spectra are used to demonstrate that the changes in Raman spectra as a function of pressure observed in recent experiment can be explained without invoking a phase transition to a new phase III.