Probing the full strain tensor at ferroelectric domain walls by scanning X-ray diffraction microscopy

The recent discovery of electrically conductive ferroelectric domains walls (DWs) has sparked much research exploring their potential as re-configurable conducting pathways, or as means to access distinct resistive states through deterministic modification of their population [1]. A large proportion of studies has focussed on LiNbO3 (LN), where the DW conductivity is believed to result from inclined DWs with partial head-to-head components, and seems to be further enhanced by an unexpected local lattice bending close to the free crystal surface [2].

Typical LN growth methods give rise to Li vacancies and Nb antisites that interact with the DWs, influencing their properties [3]. In addition, DWs in LN appear to display meanders and kinks resulting in local wall charging [4]. Overall, the peculiarities of DWs in LN are considered representative of the general issues in local DW structure and defect-wall interactions in all ferroelectrics [1].

Here, we employ synchrotron-based scanning X-ray diffraction microscopy (SXDM) to probe the local lattice structure of LN across a pair of adjacent DWs [5,6]. We map a series of domains in a single-crystal of periodically-poled defective LN with 65nm resolution. At each position of the map the three-dimensional reciprocal space volume occupied by a Bragg peak is measured. Selecting three non-collinear Bragg reflections allows us to unambiguously determine the full 3D strain and rotation tensors for the mapped region, averaged along the z crystal direction. This allows us to quantify the complex strain state at DWs in a prototypical ferroelectric, providing important input for the rapidly evolving field of domain-wall nanoelectronics.

[1] Nataf et al., Nature Reviews Physics, 2(11), 634–648 (2020)

[2] Lu et al., Advanced Materials, 31(48) (2019)

[3] Gopalan et al., Annual Review of Materials Research, 37(1), 449–489 (2007)

[4] Gonnissen et al., Advanced Functional Materials, 26(42), 7599–7604 (2016)

[5] Girard et al., Journal of Applied Physics, 129(9) (2021)

[6] Hadjimichael et al., Physical Review Letters, 120(3), 037602 (2018)