Structural properties of the geometrically frustrated pyrochlore Tb2Ti2O7

TL;DR

This study demonstrates that Tb2Ti2O7 has a highly ordered, disorder-free pyrochlore structure, supporting its role as a geometrically frustrated magnet that does not order magnetically down to very low temperatures.

Contribution

The paper provides detailed structural analysis confirming the ideal pyrochlore structure of Tb2Ti2O7, resolving previous doubts about disorder in this system.

Findings

Tb2Ti2O7 has a well-ordered pyrochlore structure

No evidence of site interchange or oxygen deficiency within 2%

The structure remains disorder-free down to low temperatures

Abstract

Although materials that exhibit nearest-neighbor-only antiferromagnetic interactions and geometrical frustration theoretically should not magnetically order in the absence of disorder, few such systems have been observed experimentally. One such system appears to be the pyrochlore Tb2Ti2O7. However, previous structural studies indicated that Tb2Ti2O7 is an imperfect pyrochlore. To clarify the situation, we performed neutron powder diffraction (NPD) and x-ray absorption fine structure (XAFS) measurements on samples that were prepared identically to those that show no magnetic order. The NPD measurements show that the long-range structure of Tb2Ti2O7 is well ordered with no structural transitions between 4.5 and 600 K. In articular, mean-squared displacements (u^2's) for each site follow a Debye model with no offsets. No evidence for Tb/Ti site interchange was observed within an upper limit of 2%. Likewise, no excess or deficiency in the oxygen stoichiometry was observed, within an upper limit of 2% of the nominal pyrochlore value. Tb L3- and Ti K-edge XAFS measurements from 20-300 K similarly indicate a well-ordered local structure. Other aspects of the structure are considered. We conclude that Tb2Ti2O7 has, within experimental error, an ideal, disorder-free pyrochlore lattice, thereby allowing the system to remain in a dynamic, frustrated spin state to the lowest observed temperatures.