Phase behavior of the N-2-Ar system at high pressures: A Raman spectroscopy study - art. no. 024103

We have studied the spectral behavior of the Raman Q branch of nitrogen in the binary system nitrogen-argon for nitrogen mole fractions of 0.04, 0.10, 0.25, 0.50, and 0.75 at room temperature. These results have been used to construct the phase diagram. The orientational degrees of freedom of N-2 pl ay an essential role in the shape of the phase diagram. The melting pressur e is almost linear dependent on composition although the melting pressures of the pure components are quite different. The amount of nitrogen dissolve d in the argon lattice is significantly smaller than the amount of argon in beta- and also in delta -N-2. The higher compression of the nitrogen molec ule in the argon lattice is reflected in a higher frequency compared to bet a -nitrogen. The beta* -delta* transition in N-2-Ar is shifted to higher pr essures. The solubility in the delta phase increases rapidly from 4.7 to 6 GPa, but is restricted to about 30% since it is difficult for the argon ato ms to occupy the c sites. As a result there is a large solubility gap compa red to hard sphere systems with the same diameter ratio (nearly 1). No indi cation for compound formation was found. In contrast to the pure component, the linewidth in the solid phases strongly increases as a function of pres sure.