6533b86ffe1ef96bd12ce643

RESEARCH PRODUCT

High-pressure Raman study of theN2stretching vibration in argon-nitrogen mixtures at room temperature

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We present room-temperature Raman investigations of the nitrogen molecule stretching vibration in ${\mathrm{Ar}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(${\mathrm{N}}_{2}$${)}_{\mathit{x}}$ from x=1 to x=0.59 in the pressure range 1\ensuremath{\le}p\ensuremath{\le}31 GPa. Ar substitution leads to an inhomogeneous broadening of the vibrational signal. The \ensuremath{\beta}-\ensuremath{\delta} phase transition of pure ${\mathrm{N}}_{2}$ is shifted to higher pressures for x=0.75. The substituted Ar atoms preferentially occupy the 2a sites of the cubic elementary cell of the orientationally disordered \ensuremath{\delta} phase. This interpretation of the experimental data is supported by our model calculations. For x=0.75 a superstructure forms in which all the a sites are occupied by Ar atoms. No direct experimental evidence is found for a disorder-order \ensuremath{\delta}-\ensuremath{\epsilon} phase transition observable in pure ${\mathrm{N}}_{2}$. However, calculated orientational potential energy barriers propose freezing of the orientational dynamics at reduced pressures. \textcopyright{} 1996 The American Physical Society.