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

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.

Magnon interactions in the frustrated pyrochlore ferromagnet Yb2Ti2O7

“…Extrinsic effects, such as structural disorder, can take many distinct forms. Several different types of structural disorder have been discussed for Yb 2 Ti 2 O 7 : these include pointdefects, such as "stuffed" Yb 3+ spins [26,33,83], oxygen vacancies [44,83], as well as internal strains and atomic displacements, such as those that might arise due to line-defects like dislocations [46] or through intrinsic frustration of the structural network [84]. For example, recent experiments have explored the effects of oxygen annealing and stuffed Yb 3+ spins [83], finding that the presence or absence oxygen vacancies has strong effects on the magnetic correlations.…”

“…For example, recent experiments have explored the effects of oxygen annealing and stuffed Yb 3+ spins [83], finding that the presence or absence oxygen vacancies has strong effects on the magnetic correlations. It has also been reported [46] that some single crystals are liable to contain defects such as "superdislocations" or anti-phase boundaries that can induce further lattice distortions.…”

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

“…As described previously, strain-fields in pyrochlores can dramatically affect their properties, thus, analysis of strain-fields around structural defects formed in pyrochlores formed during synthesis are critical to understand ( Shafieizadeh et al, 2018 ). Geometric phase analysis (GPA) ( Hÿtch et al, 1998 ) and peak analysis ( Bayle et al, 1994 ; Anthony and Granick, 2009 ) are two commonly used digital processing approaches that facilitate extraction of lattice parameter at high resolution, based in reciprocal- and real-space, respectively ( Zhu et al, 2013 ).…”

“…Heterointerface engineering of fluorites, perovskites and pyrochlores has been shown to control local oxygen diffusivity ( Schweiger et al, 2017 ), to affect local radiation response ( Spurgeon et al, 2020 ) and to produce novel topological phases ( Gallagher et al, 2016 ; Li et al, 2021 ). Other types of extended crystalline defects, such as dislocations ( Sheth et al, 2016 ; Shafieizadeh et al, 2018 ) and grain boundaries ( Bowman et al, 2015 ; Bowman et al, 2017 ; Perriot et al, 2017 ; Gupta et al, 2020 ; Syed et al, 2020 ; Tong et al, 2020 ) have been shown to behave as fast oxygen transport pathways in some cases. Even small, localized, atomic displacements such as local changes in bonding environment (e.g., coordination), polyhedral distortions (e.g., bond length or angle), polyhedral tilting and the adoption of short-range polyhedral configurations or ‘structural motifs’ ( Shamblin et al, 2016 ; Sun et al, 2016 ) can affect functional properties.…”

Multiphase magnetism in Yb 2 Ti 2 O 7

“…Its structure and properties were first reported over 50 y ago (7), but the precise nature of its ground state has remained a mystery. While some samples are found to develop ferromagnetism below 270 mK (8)(9)(10)(11), the specific nature of the low-temperature state is contested (11)(12)(13) and the magnetism is susceptible to disorder at the 1% level (14)(15)(16)(17)(18)(19). What is more, the zero-field neutron spectrum was reported to form a diffuse continuum rather than the spin wave excitations expected to accompany ferromagnetism (11,20,21).…”