Local disorder and structure relation induced by magnetic exchange interactions in A 2(Mo1−yMny)2O7 pyrochlores

Highlights: • In Gd₂(Mo_1−yMn_y)₂O_7, the spin-glass phase component is strengthened. • A MoO₆ octahedral distortion is observed for all the studied compounds. • A frustration effect due to Mn doping on the Mo site is observed. • The general pyrochlore predisposition towards structural disorder is verified. -- Abstract: We present an extended X-ray absorption fine structure study at the Mo K-edge of A₂(Mo_1−yMn_y)₂O₇ (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 A₂Mo₂O₇ 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 Ho₂(Mo_1−yMn_y)₂O₇ (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)₆ octahedron and of the Mo‒Ho and Mo‒Mo second shell. On the other hand, two Gd₂(Mo_1−yMn_y)₂O₇ 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.