Experiment and theory of Pb(In_1/2Nb_1/2)O₃: Antiferroelectric, ferroelectric, or relaxor state depending on perovskite B-site randomness

We review the recent progress in our understanding of the effect of B-site randomness on Pb(In_1/2Nb_1/2)O₃ (PIN) from the experimental and theoretical viewpoints. PIN is one of the ideal systems for investigating the effect of perovskite B-site randomness on relaxor formation, because it can be in the antiferroelectric (AFE), ferroelectric (FE), or relaxor state depending on B-site randomness. Lattice dynamics measured in ordered and disordered PINs indicates the existence of FE instability regardless of the B-site randomness of PIN. AFE is stabilized when the B-site is spatially ordered, overwhelming FE instability. The hidden FE state starts to appear as B-site randomness becomes stronger and suppresses AFE instability. Ultimately, the randomness competes with the development of FE regions and blocks long-range FE ordering, which yields polar nanoregions (PNRs) resulting in relaxor behavior. In order to investigate the interesting behavior of PIN, we constructed a minimal model that includes dipolar interaction and easy-axis anisotropy which depends on B-site randomness. The results from Monte Carlo simulations show that the model well reproduces the phase diagram and dielectric constant of PIN qualitatively.