  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.
