The Polar Nano Regions $\rightleftharpoons$ Relaxor Transition in   $Pb_{1-X}(Sc_{1/2}Nb_{1/2})O_{3-X}$; $X$ = bulk concentration of nearest   neighbor [Pb-O] divacancies
  In previous work, molecular dynamics simulations based on a first-principles-derived effective Hamiltonian for $Pb_{1-X}(Sc_{1/2}Nb_{1/2})O_{3-X}$~ (PSN), with nearest-neighbor Pb-O divacancy pairs, was used to calculate $X_{\rm [Pb-O]}$~vs.~T, phase diagrams for PSN with: ideal rock-salt type chemical order; nanoscale chemical short-range order; and random chemical disorder. Here, we show that the phase diagrams should include additional regions in which a glassy relaxor-phase (or state) is predicted. With respect to phase diagram topology, these results strongly support the analogy between relaxors and magnetic spin-glass-systems. 
B. P. Burton, Eric Cockayne, D. B. Gopman, Gunay Dogan, and Sarah Hood
physics.comp-ph cond-mat.mtrl-sci