Calculation of high-pressure phase transitions in solid $N_{2}$ and the pressure dependence of intramolecular mode frequencies

A calculation that minimizes the energy of solid

N_{2}

with respect to a rhombohedral distortion of the

Pm 3 n

structure shows that a low-temperature phase transition occurs into the

R 3 c

calcite structure at

P ≃ 19.2

kbar with a volume change of 0.125

{cm}^{3}

/mole. This transition agrees with recent Raman scattering measurements. Another transition from

R 3 c

into

R \bar{3} m

is predicted at

P ≃ 67.5

kbar, with a volume change of 0.1

{cm}^{3}

/mole. The pressure dependence of the intramolecular mode frequencies for the

R 3 c

structure are in reasonably good agreement with the two main branches observed experimentally.

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Calculation of high-pressure phase transitions in solidN2and the pressure dependence of intramolecular mode frequencies