MEASUREMENT OF (DP/DT)V AND RELATED PROPERTIES IN SOLIDIFIED GASES .2. SOLID H2

We report measurements and their analysis of pressure changes with temperature and ortho-H2 concentration in solid H2 in both the hcp and cubic phases. The temperature range extended from 0.4 to 4.2°K, and the concentration c of ortho-H2 was between 0.005 and 0.94. The measurements were carried out by means of a sensitive capacitance strain gauge capable of resolving pressure changes of 2×10-5 bar. The categories of experiments performed were (1) determination of the pressure P in the hcp phase as a function of ortho concentration at several temperatures; (2) determination of the pressure difference P (hcp)-P (cubic) as a function of ortho concentration, and study of the hysteresis in both pressure and temperature of the hcp-to-cubic transition; and (3) measurement of ( P T)V at constant ortho concentration in the hcp phase at several different ortho concentrations. The results were analyzed in terms of a lattice contribution and an electric quadrupole-quadrupole (EQQ) interaction, neglecting any effects from other interactions and from crystalline fields. The EQQ interaction parameter determined experimentally was Γ=6e2Q225R5, where eQ is the quadrupole moment of the orthomolecule in the state J=1, and R is the nearest-neighbor distance. The theoretical value for a rigid lattice is ΓkB=1.00°K. The results from (1) and (2), extrapolated to pure ortho-H2, were analyzed using the theory of Miyagi and Nakamura and gave ΓkB=0.82±0.04°K (value extrapolated to P=0). This value was confirmed from the temperature of the maximum of (P T)V at low ortho concentrations. The discrepancy between the experimental and the theoretical values of Γ is briefly discussed. From (P T)V data with almost pure para-H2 and from comparison with specific-heat data due to Ahlers, a lattice Grüneisen constant γL=2.06±0.1 was found. The Grüneisen constant of the EQQ interaction was found to be γEQQ=1.62±0.1, in agreement with the theoretically expected value. Evidence was found for redistribution of orthomolecules at low ortho concentration as a function of time. The theoretical expectations for a thermodynamic-equilibrium distribution of molecules in the lattice and that for a random high-temperature distribution are compared with experimental results. © 1969 The American Physical Society.