Correlating electromechanical and dielectric properties with nanometre-scale order is the defining challenge for the development of piezoelectric oxides. Current lead (Pb)-based relaxor ferroelectrics can serve as model systems with which to unravel these correlations, but the nature of the local order and its relation to material properties remains controversial. Here we employ recent advances in diffuse scattering instrumentation to investigate crystals that span the phase diagram of PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-xPT) and identify four forms of local order. From the compositional dependence, we resolve the coupling of each form to the dielectric and electromechanical properties observed. We show that relaxor behaviour does not correlate simply with ferroic diffuse scattering; instead, it results from a competition between local antiferroelectric correlations, seeded by chemical short-range order, and local ferroic order. The ferroic diffuse scattering is strongest where piezoelectricity is maximal and displays previously unrecognized modulations caused by anion displacements. Our observations provide new guidelines for evaluating displacive models and hence the piezoelectric properties of environmentally friendly next-generation materials.

将机电和介电性能与纳米级量级相关联是压电氧化物发展的决定性挑战。当前基于铅（Pb）的弛豫铁电体可以作为模型系统来解开这些相关性，但是局部有序的性质及其与材料特性的关系仍然存在争议。在这里，我们利用扩散散射仪器的最新进展来研究跨越PbMg _1/3 Nb _2/3 O ₃ - x PbTiO ₃（PMN- xPT），并确定本地订单的四种形式。从组成的依赖性，我们解决了每种形式的耦合到观察到的介电和机电性能。我们表明，弛豫器的行为并不简单地与铁性弥散散射相关。相反，它是由化学短程有序的局部反铁电相关性与局部铁性有序之间的竞争导致的。在压电性最大的地方，铁性扩散散射最强，并且显示出以前由阴离子位移引起的无法识别的调制。我们的观察结果为评估替代模型以及评估环境友好的下一代材料的压电性能提供了新的指导原则。