Structural Modifications of Cold and Dense Cesium, Calcium, Barium, and Selenium

Following the development of high brilliance synchrotron x-ray sources, high density crystalline structures of elemental solids have been vastly studied at room temperature and elevated pressures. In the last decades, experimental and computational results have unveiled a vast diversity of crystalline structures adopted by many dense elements. Both complex modulated and exotic structures have been observed [1] and predicted [2]. In this communication, we report results of systematic searches for structural modifications taking place at very low temperature (T>10 K) and high pressure (P<50 GPa) in selected elementals solids. Results for cesium, calcium, barium, and selenium are presented. An extension of the known P-T phase diagram to lower temperature for cesium and selenium indicates that both elements do not adopt crystalline structures different that those already known and documented. We show that calcium at low temperature and high pressure, however, exhibits unusual and large dynamical fluctuations leading to a tetragonal distortion of the simple cubic structure known to exist at room temperature and about 30 GPa. The large amplitude fluxional behaviour leads to the appearance of a new phase, nested at T<30K between 40 and 45 GPa. Finally, barium when compressed at low temperature, transforms into a crystalline structure unobserved at high pressure and room temperature. It is found that, below 140K and in the pressure range of 13 to 35 GPa, barium does not adopt the phase IV structure, i.e., the modulated incommensurate cell, but undergoes a transition from phase II (P63/mmc) to an orthorhombic (Pmna) cell. This new structure corresponds to phase VI. On the basis of an x-ray diffraction study along quasi-isobaric and isothermal paths, we conclude that Ba-VI is most likely metastable. Our results suggest the need to scrutiny other dense elements at very low temperature. Under those conditions, unusual structural modifications are ought to be observed.

Nitrogen-Rich Carbon Nitrides as Novel High Energy Density Materials

Nitrogen-rich carbon nitride materials hold the promise of constituting novel high density energetic materials if recoverable as metastable polymeric networks of single-bonded atoms at ambient conditions. Upon transition to a lowest-energy configuration, this high pressure synthesized nitrogen-heavy material would release a large amount of energy. In this work, two nitrogen-rich molecular precursors, namely, 5'-bis(1H-tetrazolyl)amine (BTA) and cyanuric triazide (CTA), were studied in their condensed states at elevated pressures and room temperature. Powder x-ray diffraction using synchrotron radiation and micro-Raman spectroscopy were carried out to pressures as high as 12.9 and 59.6 GPa, for BTA and CTA, respectively. In our study, dense BTA is shown to conserve its room condition crystalline structure, an orthorhombic unit cell (Pbca), up to the highest pressure. In the case of CTA, results of Raman spectroscopy and x-ray diffraction indicate structural changes between 29.6 and 33.4 GPa. From numerical simulations of dense CTA [1], a phase transition into either tritetrazole (hexagonal lattice, P-6) or the sought-after polymeric CTA (monoclinic lattice, P21) is expected to take place at a pressure close to 30 GPa. Preliminary results of x-ray diffraction data indicate a transition from a hexagonal to a monoclinic unit cell with parameters similar to those predicted. Moreover, theoretically calculated polymeric nitrogen Raman peaks [2] are well matched to those observed for the high-density phase of CTA [1]. Studies of BTA and CTA under extreme conditions provide a deeper understanding of the behaviour of dense nitrogen-rich materials and guidance for further developments of high energy density compounds.

Simple Binary Molecular Mixtures at High Density: the Methane-Nitrogen System

Mixing different molecular species at high density may yield weakly bound compounds or van der Waals solids. These novel solids are stabilized by the application of high pressure and differ in physical properties from solids formed by pure molecular species at comparable thermodynamic conditions. In this contribution, we address the importance in studying the miscibility in dense molecular mixtures and present results of the formation of binary methane-nitrogen compounds at low temperature and high pressure. Methane and nitrogen, with similar potentials and molecular sizes, are expected to be partly miscible in the condensed state. Indeed, binary van der Waals solid phases of methane and nitrogen do occur with the application of pressure. Using single crystal and powder X-ray diffraction with synchrotron radiation, and vibrational spectroscopy, the pressure-concentration phase diagram for this system has been explored from 1 to 16 GPa at room temperature. The existence of novel van der Waals solid phases for samples with concentrations above 10% (methane per volume) is demonstrated. For example, at 7.6 GPa and at room temperature, whereas pure nitrogen and methane exist in a cubic (Pm3n) and in a rhombohedral structure (R-3/m), respectively, our study indicates that a methane-nitrogen sample with ~ 40% methane by volume exhibits, under the same conditions, a novel phase with a tetragonal symmetry with lattice parameters a ~ 11.9 Å and c ~ 6.2 Å. Other novel structures in methane-nitrogen samples with different concentrations under varying pressure conditions have also been observed and will be discussed.