Direct observation of polar tweed in LaAlO3.

Ekhard K H Salje,Jens Kreisel,Marin Alexe,Mads C Weber,Guillaume F Nataf,Sergey Kustov,Jason Schiemer

doi:10.1038/srep27193

Abstract

Polar tweed was discovered in mechanically stressed LaAlO3. Local patches of strained material (diameter ca. 5 μm) form interwoven patterns seen in birefringence images, Piezo-Force Microscopy (PFM) and Resonant Piezoelectric Spectroscopy (RPS). PFM and RPS observations prove unequivocally that electrical polarity exists inside the tweed patterns of LaAlO3. The local piezoelectric effect varies greatly within the tweed patterns and reaches magnitudes similar to quartz. The patterns were mapped by the shift of the Eg soft-mode frequency by Raman spectroscopy.

Highlights

It is likely, that our observation can be generalised to other compounds and leads credence to the initial hypothesis that most tweed structures involving anion and cation lattices are polar
Weak electric fields applied at frequencies between 100 kHz and 10 MHz excite strong piezoelectric vibrations in LaAlO3 with a tweed structure but not in uniform samples
Considering the diffraction based point group symmetry 3 m of LaAlO3 we find that this piezoelectric point group symmetry is polar

Summary

Introduction

That our observation can be generalised to other compounds and leads credence to the initial hypothesis that most (or perhaps all) tweed structures involving anion and cation lattices are polar. A 2-inch-disc of LaAlO3 (MTI Corporation, USA) was analysed by microprobe analysis. The sample was slightly more defect-rich than typical LaAlO3 specimen: the purity was 99.98% rather than 99.99% as commonly seen in other samples (e.g. Crystal GmbH, Berlin, Germany). The chemical composition is listed, the main impurities are Cl and Si. The disc was cut into narrow stripes of 1 cm width. The as-grown sample was optically free of tweed at room temperature and contained a small number of needle twins. The cutting induced stress fields in the sample. Additional needle domains were induced at the edge of the sample and the entire sample assumed an almost uniform, coarse-grained tweed microstructure. A tweed microstructure is seemed throughout the sample. The inset shows a map of strain order parameter of a tweed structure obtained by Monte-Carlo simulation (reproduced from ref. 41)

Methods

Results

Discussion

Conclusion