MS-12-P-2176 Transmission electron microscopy study of Yb₂Ti₂O₇

In the past two decades there has been a great interest in the magnetic behaviour of pyrochlore oxides with general formula A₂B₂O₇, in which A is a rare-earth ion and B is a transition metal. Such materials can exhibit geometric frustration with the A and B cations ordered into separate interpenetrating lattices of corner-sharing tetrahedra [1]. Magnetic A cations can also be “stuffed” onto the nonmagnetic B sites further modifying the magnetic behaviour of these materials. For instance, Yb₂Ti₂O₇ samples (especially single crystals) have broad features in specific heat capacity that vary in sharpness and temperature depending on the sample, indicating that the magnetic ground state may be qualitatively different in samples with different degrees of stuffing [2]. In the present work, the structure of several Yb₂Ti₂O₇ samples (polycrystalline and single crystal) has been studied by aberration-corrected annular dark field Scanning Transmission Microscopy (ADF-STEM). The polycrystalline powders were prepared by the standard solid state synthesis method in which the constituent powders were mixed together and heated at temperatures between 1150 and 1400 ºC for 6 days. Powder X-ray diffraction was used to check the phase purity of the synthesised powders. Large single crystal samples were grown by the optical floating zone technique [3]. Atomic resolution STEM images demonstrate lattice deformation in the structure of the pyrochlores. Moreover, intensity fitting on the atomic resolution images of the network of corner sharing Yb and Ti tetrahedra, together with Electron Energy Loss Spectroscopy (EELS) studies are used to look for the presence of Yb³⁺ cations in Ti⁴⁺ sites. We will examine the claims of Ross et al. [2] that the variation of the magnetic ground state of the pyrochlores is as a result of random exchange bond and local lattice deformation introduced by substituting (stuffing) Yb³⁺ on the Ti⁴⁺ sublattice.

[1] G C Lau et al, Journal of Solid State Chemistry 179 (2006) p. 3126.
[2] K A Ross et al, Physical Review B 86 (2012) p. 174424.
[3] G Balakrishnan et al, J. Phys. Condens. Matter 10 (1998) p. L723.