Calculation of structural properties and vibrational frequencies of α- and γ-N2 crystals

Abstract

The effects of the zero-point energy, residual forces and torques on the molecules, and molar density, on the calculated lattice frequencies and structural properties of solid α- and γ-N2, are investigated. An intramolecular potential and a parametrized 12-6 atom–atom intermolecular potential are used to calculate lattice modes, intramolecular modes, sublimation energy, equilibrium unit cell parameters, Grüneizen coefficients, and P–V data. The second virial coefficient is also reasonably reproduced. The α-to-γ phase transition is not revealed by the employed intermolecular potential.

High-resolution infrared study of the translational lattice modes in α-N2 single crystals

“…In fact the only available experimental ␥ ͑Ref. 6͒ is relative to the T u (1) phonon and is in good agreement with the calculated values 37,38 which are very similar for both translational modes. The anharmonic shift ⌬(T) is calculated including cubic and quartic contributions in the following form:…”

Perturbation theory for the thermodynamic properties of liquid nitrogen using model potentials

“…To put these results in perspective it is useful to first recall previous uses of these potentials. It is well-known that the two-centre Lennard-Jones potential reproduces the broad features of liquid and low pressure solid properties [1,[41][42][43][44]. The CP parameters [1] we used for this potential were obtained by fitting liquid state properties through computer simulation techniques (see w 2).…”

Towards an intermolecular potential for nitrogen

“…In our determination of the zero-point energy, we have used the approach of Zunger and Huler [34], which was found to give reliable results and was also adopted by Raich and Gillis. Using this approach, the zero-point energy for the 17 external optical and 3 external acoustic modes is calculated to a good approximation using experimental vibrational frequencies, wl, at the centre of the Brillouin zone (~= 0).…”

“…To take proper account of the volume dependence of the zero point energy for these modes, the corresponding Grtineisen parameters, Vi = -(~ In w i/~ In V), are incorporated. Values of w i and Yi were obtained respectively from the experimental data for ~= 0 frequencies measured at 15 K from neutron scattering [35] and experimental [36,37] and theoretical Griineisen parameters [34] for the external optical modes. For the internal optical modes, the zero point energy contribution was determined from the difference in vibrational energy between the measured modes in the solid and the molecular gas.…”