S.-W. Han

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 ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7.$ However, previous structural studies indicated that ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ 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 ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is well ordered with no structural transitions between 4.5 and 600 K. In particular, mean-squared displacements $u^2\mathrm{’}\mathrm{s}$ 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 $L_{\mathrm{III}}$- and Ti K-edge XAFS measurements from 20 K to 300 K similarly indicate a well-ordered local structure. Other aspects of the structure are considered. We conclude that ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ 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.