Clathrate hydrates in the system H2O-Ar at pressures and temperatures up to 30 kbar and 140 degrees C

Studies of the binary mixture H2O-Ar-by means of a quasi-isochoric scanning method-have revealed the range of stability of clathrate hydrates in the h igh-pressure and high-temperature region. The results obtained show an exte nsion of the decomposition curve above the melting curve of pure argon and up to the melting curve of pure ice VII. At low pressures the argon decompo sition temperature first increases and then decreases with pressure, showin g a local maximum and minimum temperature. At higher pressures the slope of the decomposition curve remains positive but undergoes three more breaks. Two new quadruple points have been determined; at 105 degrees C, 20 kbar (Q (4)) and at 136 degrees C, 29 kbar (Q(5)). In the lower region (below 10 kb ar) the decomposition curve shows two breaks; at 31.5 degrees C, 6.2 kbar ( Q(2)) and at 37.5 degrees C, 9.6 kbar (Q(3)), indicating that three differe nt clathrate structures are formed in this region. The positions of the thr ee-phase lines separating the two clathrate structures at lower pressure co uld be obtained via Raman spectroscopy since the pressure and temperature d ependence of the vibrational frequencies of the coupled O-H oscillations is different for each clathrate structure. (C) 1999 American Institute of Phy sics. [S0021-9606(99)51945-4].