  We have recently reported a new mechanism of colossal magnetoresistance in
electron doped Mn oxypnictides NdMnAsO1-xFx. Magnetoresistances of up to -95
percent at 3 K have been observed. Here we show that upon replacing Nd for Pr,
the CMR is surprisingly no longer present. Instead a sizeable negative
magnetoresistance is observed for PrMnAsO0.95F0.05 below 35 K (MR7T (12 K) =
-13.4 percent for PrMnAsO0.9F0.05. A detailed neutron and synchrotron X-ray
diffraction study of PrMnAsO0.95F0.05 has been performed, which shows that a
structural transition, Ts, occurs at 35 K from tetragonal P4/nmm to
orthorhombic Pmmn symmetry. The structural transition is driven by the Pr 4f
electrons degrees of freedom.
