Intrinsic structure of relaxor ferroelectrics from first principles

Abstract

We hybridize the swap Monte Carlo and geometric relaxation methods to determine the intrinsic compositional structure (CS) of the lead magnesium niobate (PMN) relaxor. We verify the stability of a Nb-rich sublattice in PMN, as prescribed by the prevailing random-site model. However, ions in the complementary sublattice are not randomly mixed. Most Nb ions collapse into a single percolating cluster with a mesh-like structure. This specific geometry serves to prevent large space charges, and this behavior differs from typical phase separation in metallic alloys. Subsequent molecular dynamics simulations predict a pair distribution function that is consistent with neutron scattering experiments. Analysis of dipolar structures in the Nb cluster sheds light on the unique dielectric properties of PMN.

Figures (5)

• Figure 1: (a) Sketch of the FIRE-Swap algorithm. (b) Energy stability of different models of CS.
• Figure 2: (a) Order parameter $O(\mathbf{s})$ as a function of accepted FIRE-Swap steps. The insets show the CS of arbitrary cross sections of the $6 \times 6 \times 6$ lattice. sublattice $\beta^{\mathrm{II}}$ are represented by gray tiles. (b) The number of Nb sites in all Nb clusters (gray) and in the largest cluster (blue). The insets show the geometry of the largest cluster in the $12 \times 12 \times 12$ lattice. (c) Upper panel: The predicted (purple) and experimental eremenko2019local (black) pair distribution functions associated with neutron scattering of PMN powder samples at $T=300$K. Lower panel: The predicted room-temperature radial distribution function.
• Figure 3: (a) Histograms of PDF $p(d_i^x, d_i^y)$ associated with different types of B sites and different temperatures. The PDF $p(d_i^x, d_i^z)$ has similar features and are omitted here. (b) ($d_i^x, d_i^y$) as arrows on an arbitrary perovskite layer (in the XY plane) of the simulation box. Blue shades mark the Nb sites (blue spheres) in Nb clusters. Red sphere represents a Mg site. (c) The correlation functions $C_{ij}$ (averaged over eligible pairs) for nearest-neighbor Nb-Nb pairs and Nb-Mg pairs .
• Figure 4: Error distribution of the DP model. Blue trainset. orange test set
• Figure 5: Left panel: A sketch of energy changes, $\Delta E_{\text{FIRE}}$ and $\Delta E_{\text{Total}}$, in one iteration of FIRE-Swap. Right panel: the probability density distribution of $\Delta E_{\text{FIRE}}$ and $\Delta E_{\text{Total}}$ in a FIRE-Swap simulation with around 10000 iterations.