First-principles Studies Of Solid Hydrazine Under High Pressure

Hydrazine is a binary inorganic chemical compound with the formula N2H4.Hydrazine has been widely used in various rocket fuels, fuel cells and organicchemical industry. According to the Italian catalyst manufacturer Acta, a hydrazinefuel cell exhibits a maximum power generation performance of700mW/cm2, whichexceeds by40percent the500mW/cm2for a hydrogen fuel cell. Hydrazine’selectromotive force is1.56V, which is27percent higher than the1.23V forhydrogen. Therefore, hydrazine could be a new, green replacement for hydrogen inthe field of fuel cells.There is some doubt as to the crystal structure of solid hydrazine underatmospheric pressure. In1951, R. L. Collin and W. N. Lipscomb indicated that thesymmetry group is-P21/m and there are two molecules in a unit cell. In1961, W.R. Busing et al. reported that the symmetry is-P21and contains two N2H4per unitcell. In1967, F. G. Baglin et al. analyzed the optical modes of the crystal lattice in thefar infrared and pointed out that the crystal symmetry has the space group. Thenin1969, M. Guay and R. Savoie analyzed the raman spectra of solid hydrazine andpointed out that the symmetry group is one of the C2hspace groups and there are fourN2H4in a unit cell. In1974, J. R. Durig et al. analyzed the raman spectra of solidhydrazine and hydrazine-d4and found themselves in agreement with M. Guay and R.Savoie’s proposal, however, they pointed out that the number of molecules perprimitive cell may be larger than four.For many materials, structural phase transitions may occur under high pressure.These high-pressure structures usually have some very different properties with thosestructures under atmospheric pressure. Thus high-pressure methods have been widelyused in searching novel materials and studying phase diagrams. First Principle studieshave proved to be successful in predicting and simulating the structures and propertiesof materials under high pressure, and many results obtained by calculations have beenconsistent with experiments. So in this paper, we use first-principles based on DFT to have a series of investigations of solid hydrazine under high pressure. Under pressuresranging from0to300GPa, we use USPEX to predict the crystal structures of solidhydrazine and find four high-pressure phases with the crystal symmetry of C2(below3.5GPa), P21(3.5-99.0GPa), Cc (99.0-235GPa), and C2/c (above235GPa),respectively. The band gaps of these phases are always over3.4eV under pressuresbelow300GPa, which indicates that solid hydrazine is keeping insulation withoutmetallization. The C2â†'P21and P21â†'Cc phase transitions in hydrazine are thefirst-order reconstructive type with the volume discontinuity of2.1%and2.4%respectively, while the Ccâ†'C2/c transition can be the second-order displacive typewithout volume discontinuity. Calculations on ELF and interatomic distances indicatethat the hydrogen bond symmetrization occurs at235GPa which is also the Ccâ†'C2/cphase transition pressure.