Possible Negative Value of the Grüneisen Coefficient of Hydrogen in the Area of Pressures from 40 to 75 GPa and Temperatures from 3500 to 7500 K.

Experimental data on single and double shock compression of initially liquid and gaseous (compressed by initial pressure) hydrogen isotopes (protium and deuterium) at pressures of ≈10-180 GPa and temperatures of ≈3000-20 000 K are considered. The mean values of the measured variables (pressure, density, internal energy, and temperature) show that hydrogen at a pressure of ≈41 GPa in the temperature interval of ≈3500-5700 K and at a pressure of ≈74 GPa in the temperature interval of ≈5000-7500 K is characterized by a negative value of the Grüneisen coefficient. Such an anomaly may play a key role in some processes, including those proceeding in the Jupiter gas envelope, which mainly consists of protium (≈90%) and helium (≈10%). In the range of pressures (depths) of its manifestation, convection in the protium envelope is forbidden with an increase in temperature in the envelope with increasing pressure. Possibly, a comparatively low fraction of helium does not suppress the anomaly, and it serves as a barrier for large-scale convection in the Jupiter envelope. Additional refining experiments are required to confirm this anomaly. [ABSTRACT FROM AUTHOR]