On the case of the negative activation volumes of defects in solids

When conductivity, self-diffusion or electrical relaxation measurements are carried out at various pressures, the results are described in terms of a defect activation volume. Here, we discuss the rare case of negative activation volumes e.g., the one found by Fontanella et al. [Phys. Rev. Lett. 51 (1983) 1992] in the electrical relaxation measurements under pressure in β-PbF2 doped with lanthanum and cerium. This might be of importance for the explanation of electric signals detected before earthquakes, We suggest that this might be due to a small Gruneisen constant γi, namely smaller than 13.