Synthesis, properties and phase transitions of pyrochlore- and fluorite-like Ln2RMO7 (Ln = Sm, Ho; R = Lu, Sc; M = Nb, Ta)

We have studied the new compounds with fluorite-like (Ho2RNbO7 (R = Lu, Sc)) and pyrochlore-like (Sm2ScTaO7) structure as potential oxide ion conductors. The phase formation process (from 1200 to 1600 °C) and physical properties (electrical, thermo mechanical, and magnetic) for these compounds were investigated. Among the niobate materials the highest bulk conductivity is offered by the fluorite-like Ho2ScNbO7 synthesized at 1600 °C: 3.8 × 10−5 S/cm at 750 °C, whereas in Sm system the highest bulk conductivity, 7.3 × 10−6 S/cm at 750 °C, is offered by the pyrochlore Sm2ScTaO7 synthesized at 1400 °C. In Sm2ScTaO7 pyrochlore we have observed the first-order phase transformation at ∼650–700 °C is related to rearrangement process in the oxygen sublattice of the pyrochlore structure containing B-site cations in different valence state and actually is absent in the defect fluorites. o holmium niobates show Curie–Weiss paramagnetic behavior, with the prevalence of antiferromagnetic coupling. The magnetic susceptibility of Sm2ScTaO7 is a weak function of temperature, corresponding to Van Vleck paramagnetism.