Grüneisen parameter of cesium and rubidium at high pressure and the nature of the isostructural electronic transition

The Grüneisen parameter ($\gamma$) for cesium has been measured in the solid to 43 kbar at 22°C and in the liquid to 20 kbar at 200°C. This parameter decreases in value with increasing pressure and demonstrates a dramatic decrease prior to the isostructural electronic transition at 42 kbar. After the transition, $\gamma$ increases. $\gamma$ for liquid Cs to 20 kbar is essentially the same as that for the solid. These results support the suggestion of Glötzel and McMahan that an anomaly in the lattice vibrations is responsible for this transition. For Rb, $\gamma$ approaches an asymptotic value of 0.5 at 35 kbar. The melting temperatures of these substances have been reinvestigated to 30 kbar.