Accelerating ferroic ageing dynamics upon cooling

Once a structural glass is formed, its relaxation time will increase exponentially with decreasing temperature. Thus, the glass has little chance of transforming into a crystal upon further cooling to zero Kelvin. However, a spontaneous transition upon cooling from amorphous to long-range ordered ferroic states has been observed experimentally in ferroelastic, ferroelectric and ferromagnetic materials. The origin for this obvious discrepancy is discussed here conceptually. We present a combined theoretical and numerical study of this phenomenon and show that the diffusive and displacive atomic processes that take place in structural glass and amorphous ferroics, respectively, lead to markedly different temperature-dependent relaxation behaviors, one being ‘colder is slower’ and the other being ‘colder is faster’. Once a structural glass (e.g., window glass) is formed, it will not turn into a crystal upon further cooling to zero Kelvin. This is, however, not true for ferroic glasses — disordered states of magnetization, polarization or strain. Junyan Zhang from Xi'an Jiao Tong University in China and collaborators developed a theoretical framework to shed light on how the relaxation processes in these two types of glasses are fundamentally different, one being dominated by atomic diffusion and activation enthalpy, and the other being dominated by collective displacements and activation volume/entropy; one being ‘colder is slower’ as expected, and the other being ‘colder is faster’. Numerical simulations unveil the change from ‘colder is faster’ to ‘colder is slower’ with an increasing defect concentration in a generic ferroelastic system. Microstructure evolutions upon cooling in three ferroelastic systems having different defect concentrations, c=0.02, 0.14 and 0.40, respectively. Green color in the figures represents the parent phase; blue and red colors represent the two different martensitic variants. As the defect concentration increases, the behavior of the ferroelastic system shifts from ‘the cooler the faster’ (c=0.02 and 0.14) to ‘the cooler the slower’ (c=0.40).