Chemical pressure control of exchange interaction in Mo pyrochlore

Structural parameters are systematically investigated for Mo pyrochlore R₂Mo₂O₇ (R=Dy, Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius r_R of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with r_R from 129.7° for Dy₂Mo₂O₇ (SG) to 131.5° for Nd₂Mo₂O₇ (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.