First-principles-based simulations of relaxor ferroelectrics

The phenomenology of Pb(B,B')O-3 perovskite-based relaxor ferroelectrics (RFE) is reviewed, with emphasis on the relationship between chemical short-range order and the formation of polar nanoregions in the temperature range between the "freezing" temperature, T-f, and the Burns temperature, T-B. Results are presented for first-principles-based effective Hamiltonian simulations of Pb(Sc1/2Nb1/2)O-3 (PSN), and simulations that were done with empirically modified variants of the PSN Hamiltonian. Arbitrarily increasing the magnitudes of local electric fields, caused by an increase in chemical disorder, broadens the dielectric peak, and reduces the ferroelectric (FE) transition temperature; and sufficiently strong local fields suppress the transition. Similar, but more dramatically glassy results are obtained by using the PSN dielectric model with a distribution of local fields that is appropriate for Pb(Mg1/3Nb2/3)O-3 (PMN). The results of these simulations, and reviewed experimental data, strongly support the view that within the range T-f < T < T-B, polar nanoregions are essentially the same as chemically ordered regions. In PSN a ferroelectric phase transition occurs, but in PMN, a combination of experimental and computational results indicate that pinning from local fields is strong enough to suppress the transition and glassy freezing is observed.