Elastic properties of substances in the megabar pressure range: Inversion of shear rigidity

The behavior of elastic moduli of substances is analyzed in the megabar pressure range. A new effect-inversion of the shear moduli and mechanical properties upon compression is predicted for various classes of substances. The melting-curve data for different materials confirm the predicted phenomenon. The materials traditionally considered the softest, such as rare gas solids and molecular substances, may become the hardest in the megabar range. This should be taken into account in developing the experimental high-pressure technique.