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Effects of pressure-temperature protocols on the properties of crystals and ageing effects – an analogy with glasses

ABSTRACT Properties of a crystal are used to determine the point defects concentration, n, the self diffusion coefficient, D and variation of n and D with tempertaure, T. Also, spontaneous change in the properties of a crystal’s non-equilibrium state is used to determine the decrease in n with time. Both n and D decrease with increase in the pressure, P, until the pressurising-rate dependent, kinetic-freezing pressure for defects disorder, PD-F , is reached. At P > PD-F , a crystal is in a non-equilibrium state. We consider such configurationally-frozen states of a crystal produced by using three unusual P-T protocols: (i) pressurising a crystal to P > PD-F , cooling to a low T and depressurising, (ii) pressurising to P < PD-F , cooling through the defects freezing temperature, TD-F , and depressurising, and (iii) cooling a crystal at 1 bar to T < TD-F pressurising and maintaining at high P. The non-equilibrium state of the crystal would have defect concentration that kinetically froze at P = PD-F or T = TD-F , but its volume and phonon properties would not correspond to the kinetically-frozen state at P = PD-F or at T = TD-F . On aging, their properties would change differently than those of a non-equilibrium state of a crystal produced by quenching at a fixed P. We relate n, D and the electrical resistivity to thermal conductivity, κ, by the Wiedemann-Franz equation, and discuss how κ would change on aging of a crystal. The above-given effects alter the properties of metallic and non-metallic, metastable materials during their commercial use.