Experimental compressions for normal hydrogen and normal deuterium to 25 kbar at 4.2 K

The piston-displacement technique has been used to determine the pressure-volume relations for normal hydrogen (n-H₂) and normal deuterium (n-D₂) at pressures to 25 kbar at 4.2 K. The accuracy of the relative compressions VV₀ ranges from ±{}10^-3 at low pressures to ±{}3×{}10^-3 at 25 kbar. The data, especially for n-H₂, agree well with earlier 20-kbar results, and the extrapolated $P=0$ bulk moduli, 1.70 ±{} 0.06 kbar for n-H₂ and 3.15 ±{} 0.06 kbar for n-D₂, are consistent with recent ultrasonic data. The shapes of the pressure-volume relations resemble more closely those for the helium isotopes than those for the heavier-rare-gas solids, and suggest that the two-body repulsive interaction for hydrogen molecules (and helium atoms) varies more slowly with intermolecular spacing than that for the heavier-rare-gas atoms. These experiments also give maximum values for the pressure-dependent shear yield stress of solid hydrogen.