Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3

TL;DR

This study uses neutron and x-ray diffraction to investigate the effects of a [111] electric field on the structure of relaxor ferroelectric PMN, revealing a cubic bulk phase with a surface rhombohedral transition and enhanced order below 210 K.

Contribution

It provides detailed insights into the structural behavior of PMN under electric fields, showing a stable cubic bulk phase and a surface transition, challenging previous notions of a bulk rhombohedral phase.

Findings

Bulk unit cell remains cubic up to 8 kV/cm

Surface undergoes rhombohedral transition below 210 K for E > 4 kV/cm

Diffuse scattering decreases and becomes asymmetric below 210 K

Abstract

Neutron and x-ray diffraction techniques have been used to study the competing long and short-range polar order in the relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E $>$ 1.7 kV/cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90$^{\circ}$-$\alpha$ =0.03$^{\circ}$)for fields up to 8 kV/cm. Furthermore, we observe a structural transition confined to the near surface volume or `skin' of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T$_{c}$=210 K for E $>$ 4 kV/cm, for which 90$^{\circ}$-$\alpha$=0.08 $\pm$ 0.03$^{\circ}$ below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T$_{c}$=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T$_{c}$, however below T$_{c}$ the diffuse scattering is reduced in intensity and adopts an asymmetric lineshape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin maybe necessary for a relaxor ground state.