Superdislocations and point defects in pyrochlore Yb2Ti2O7 single crystals and implication on magnetic ground states

Zahra Shafieizadeh,Haidong Zhou,Qing Huang,Seyed M Koohpayeh,Yan Xin

doi:10.1038/s41598-018-35283-w

Abstract

This study reports atomic-scale characterization of structural defects in Yb2Ti2O7, a pyrochlore oxide whose subtle magnetic interactions is prone to small perturbations. Due to discrepancies in the reported magnetic ground states, it has become a pressing issue to determine the nature of defects in this system. In the present study, we use atomic resolution scanning transmission electron microscopy techniques to identify the type of defects in the ytterbium titanate single crystals grown by the conventional optical floating zone (FZ) method. In addition to the known point defects of substitution Yb on Ti B-sites, extended defects such as dissociated superdislocations and anti-phase boundaries were discovered for the first time in this material. Such defects were prevalently observed in the FZ grown single crystals (of a darker color), in contrast to the stoichiometric white polycrystalline powders or high quality colorless single crystals grown by the traveling solvent floating zone technique. The lattice strains from these extended defects result in distortions of Yb-tetrahedron. A change of Ti valance was not detected at the defects. Our findings provide new insights into understanding the nature of defects that are of great importance for the physical property studies of geometrically frustrated compounds. Furthermore, this work sheds light on the complicated core structure of superdislocations that have large Burgers vectors in oxides with complex unit cells.

Highlights

Pyrochlore oxides A2B2O7, RE2Ti2O7 (RE = Yb, Ho, Tb and Dy) has been of great interests due to their geometrically frustrated magnetic interactions, displaying different magnetic ground states arising from the interplay between geometry frustration and spin orbital interaction, such as spin ice, spin liquid, long-range, or short-range ferromagnetic ordering1–5
Three sample types were used for this study, the stoichiometrically synthesized powder and the FZ single crystal, together with single crystals grown by the FZ and the traveling solvent floating zone (TSFZ) techniques9
For the closely stoichiometric powders and the TSFZ single crystals, a sharp peak was obtained for the specific heat data6,9

Summary

Introduction

The single crystal form of the Yb2Ti2O7 compound can be readily grown by the conventional FZ method, the quality of the crystals varies considerably, e.g. showing broader specific heat features at 150 to 200 mK and different magnetic ground states This is in contrast with a sharp peak observed at 250 mK in the stoichiometric polycrystalline samples synthesized by a solid-state reaction or the high-quality colorless single crystals. For the first time, extended defects such as dissociated superdislocations (with Burgers vector size larger than 0.6 nm) and anti-phase boundaries (APBs) are observed, in addition to the known point defects of substitution Yb atoms on Ti B-sites The existence of these extended defects induces considerable strain and such strain causes the distortion of Yb-tetrahedron. Size of Burgers vector reported in the past is usually in the range from 0.159 nm for α-Al2O3 to 0.421 nm for MgO, while it is larger than 0.6 nm in the pyrochlore material

Methods

Results

Conclusion