  The cubic symmetry of pyrochlore iridium oxides R2Ir2O7 (R = Nd, Eu, and Pr)
has been investigated by high resolution X-ray diffraction experiments down to
4 K,in order to clarify the relationship between the metal-insulator transition
(MIT) and the small structural phase transition suggested by Raman scattering
experiments in these compounds. We have found that a small negative thermal
expansion of the order of 10^-3 angstroms appears only in Nd2Ir2O7 below the
MIT, T_MIT = 34 K, resulting in the magnetovolume effect of the long-range
order of Ir moments. However, any breaking of the cubic symmetry of three
iridates has not been observed as appearance of superlattice reflections nor
splittings of cubic reflections below T_MIT. These results imply that lowering
of the cubic symmetry plays a minor role for the change in the electronic state
of these compounds, while a formation of the special arrangement of Ir moments
plays a major role for the MIT.
