The hafnium-hydrogen system at room temperature consists of three phases. The lattice parameters of the hafnium metal used in this investigation and of the three phases are as follows: hafnium, ao=3.200A, Co = 5.061A, and c = 1.58; deformed cubic phase, ao= 4.702±0.012A, Co.= 4.678±0.012A, and c = 0.995; facecentered cubic phase, ao=4.708±0.OO2A; and face-centered tetragonal phase, ao=4.882±0.OO2A, co=4.384 ±0.OO2A, and c=0.898. The deformed cubic phase extends up to HfH1.63 and converts into the face-centered cubic phase between HfHu3 and HfH\. 70. The face-centered cubic phase transforms into the tetragonal phase between HfH1.8o and HfHI.87. The tetragonal phase extends up to HfH1.98.

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Oral Session 7A: Fast Z-Pinches -111, Dense Plasma Focus; High Energy Density Using the Los Alamos high explosive pulsed power (HEPP) system, isentropic equation of state (EOS) data may be obtained for a wide range ofmatenals. Current pulses with risetimes of-SO0 ns and curHydrodynamics and Equation of State -rent densities exceeding 400 MNm, create continuous magnetic load-ing of samples at megabar pressures. We will summarize the technique and the problems that had to be overcome to perform the HEPP-ICE experiments at these pressures. We will then present our EOS results obtained with the conventional Lagrangian analysis and the Hayes' "Backwars' integration method', and compare the data with the published principal isentrope of OFHC copper.