Local disorder and structure relation induced by magnetic exchange interactions in A_2(Mo_(1-y)Mn_y)_2O_7 pyrochlores

We present an extended X-ray absorption fine structure study at the Mo K-edge of A_2(Mo_(1-y)Mn_y)_2O_7 (A = Gd, Ho; y = 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A_2Mo_2O_7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples A_2(Mo_(1-y)Mn_y)_2O_7 (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO octahedron and of the Mo - Ho and Mo - Mo second shell. On the other hand, two Gd_2(Mo_(1-y)Mn_y)_2O_7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones