“Lost in Translation”: what do the negative values of effective Gr$\ddot{\rm u}$neisen coefficients mean in shock-wave experiments? (Extended comment on “Shock compression of porous metals and silicates” by A.B. Medvedev and R.F. Trunin, Phys. Usp. 55 773 (2012))

The possible reasons for the negative effective Gr$\ddot{\rm u}$neisen coefficient in silicates under shock compression are considered. It is shown that the negative effective values of the Gr$\ddot{\rm u}$neisen and thermal expansion coefficients are, as a rule, a consequence of diffuse structural changes with a change in the short-range order structure in the solid or liquid state. We discuss the thermodynamic and kinetic aspects of these transformations, as well as the possibility of chemical decomposition of compounds at high pressures and temperatures.