The pentazolates, the last all-nitrogen members of the azole series, have been notoriously elusive for the last hundred years despite enormous efforts to make these compounds in either gas or condensed phases. Here we report a successful synthesis of a solid state compound consisting of isolated pentazolate anions N − 5 , which is achieved by compressing and laser heating cesium azide (CsN 3 ) mixed with N 2 cryogenic liquid in a diamond anvil cell. The experiment was guided by theory, which predicted the transformation of the mixture at high pressures to a new compound, cesium pentazolate salt (CsN 5 ). Electron transfer from Cs atoms to N 5 rings enables both aromaticity in the pentazolates as well as ionic bonding in the CsN 5 crystal. This work provides a critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high nitrogen content species.

Sodium pentazolates NaN 5 and Na 2 N 5 , new high energy density materials, are discovered during first principles crystal structure search for the compounds of varying amounts of elemental sodium and nitrogen. The pentazole anion (N − 5 ) is stabilized in the condensed phase by sodium Na + cations at pressures exceeding 20 GPa, and becomes metastable upon release of pressure. The sodium azide (NaN 3 ) precursor is predicted to undergo a chemical transformation above 50 GPa into sodium pentazolates NaN 5 and Na 2 N 5 . The calculated Raman spectrum of NaN 5 is in agreement with the experimental Raman spectrum of a previously unidentified substance appearing upon compression and heating of NaN 3 .

Two new crystalline compounds, pentazole (NH) and ammonium pentazolate (NH)(N), both featuring cyclo-N are discovered using a first-principles evolutionary search of the nitrogen-rich portion of the hydro-nitrogen binary phase diagram (NH, x ≥ y) at high pressures. Both crystals consist of the pentazolate N anion and ammonium NH or hydrogen H cations. These two crystals are predicted to be thermodynamically stable at pressures above 30 GPa for (NH)(N) and 50 GPa for pentazole NH. The chemical transformation of ammonium azide (NH)(N) mixed with dinitrogen (N) to ammonium pentazolate (NH)(N) 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.

Polynitrogen compounds have attracted great interest due to their potential applications as high energy density materials. Most recently, a rich variety of alkali polynitrogens (RN; R = Li, Na, and Cs) have been predicted to be stable at high pressures and one of them, CsN 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 N chains, N rings, and N rings stable at high pressures are discovered. In addition, an unusual nitrogen-rich metallic crystal with stoichiometry KN 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 KN compound are investigated, and the pressure-dependent IR spectrum is obtained to assist in experimental discovery of this new high-nitrogen content material.

Determining the unreacted equation of state of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is challenging because it exhibits low crystal symmetry and low X-ray scattering strength. Here, we present the first high-pressure single-crystal X-ray diffraction (SXD) study of this material. Our SXD results reveal a previously unknown transition to a monoclinic phase above 4 GPa. No abrupt change of the volume occurs but the compressibility changes. Concomitant first principles evolutionary crystal structure prediction USPEX calculations confirm this transition and show that it involves a pressure-induced in-plane shift of the layers of TATB molecules with respect to the ambient-pressure phase.

Efforts to synthesize, characterize, and recover novel polynitrogen energetic materials have driven attempts to subject high nitrogen content precursor materials (in particular, metal and nonmetal azides) to elevated pressures. Here we present a combined theoretical and experimental study of the high-pressure behavior of ammonium azide (NH4N3). Using density functional theory, we have considered the relative thermodynamic stability of the material with respect to two other crystal phases, namely, trans-tetrazene (TTZ), and also a novel hydronitrogen solid (HNS) of the form (NH)4, that was recently predicted to become relatively stable under high pressure. Experimentally, we have measured the Raman spectra of NH4N3 up to 71 GPa at room temperature. Our calculations demonstrate that the HNS becomes stable only at pressures much higher (89.4 GPa) than previously predicted (36 GPa). Our Raman spectra are consistent with previous reports up to lower pressures and at higher pressures, while some additional subtle behavior is observed (e.g., mode splitting), there is again no evidence of a phase transition to either TTZ or the HNS.

Tin sulfides, SnxSy, 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 SnxSy 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 P3¯m1 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 Pm3¯m space group symmetry stable up to 100 GPa, which becomes a superconductor with a maximum Tc = 9.74 K at 40 GPa. Another new metallic compound, Sn3S4 with I4¯3d space group symmetry, is predicted to be stable at pressures above 15 GPa, which also becomes a superconductor with relatively high Tc = 21.9 K at 30 GPa.

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, RbN, RbN, and RbN, are discovered. Rubidium pentazolate (RbN) becomes thermodynamically stable at pressures above 30 GPa. The charge transfer from Rb to N atoms enables aromaticity in cyclo-N while increasing the ionic bonding in the crystal. Rubidium pentazolate can be synthesized by compressing rubidium azide (RbN) and nitrogen (N) precursors above 9.42 GPa, and its experimental discovery is aided by calculating the Raman spectrum and identifying the features attributed to N modes. The two other interesting compounds, RbN containing infinitely long single-bonded nitrogen chains and RbN consisting of single-bonded N hexazine rings, become thermodynamically stable at pressures exceeding 60 GPa. In addition to the compounds with high nitrogen content, RbN, a new compound with 1:1 RbN stoichiometry containing bent N azides is found to exist at high pressures.