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The A2B2O7 pyrochlore compounds exhibit a wide variety of interesting electrical transport, magnetic and thermal properties due to the diverse possibilities of substitution at the A (rare earth, R) and B (transition metal) sites. While for R equals Nd, Sm and Gd the behavior is metal-like, in most other cases it is like that of a degenerate semiconductor with a thermally activated mobility probably arising from the hopping of small polarons. This is evidenced by the typical T-1/4 variation of the hopping conductivity at low temperatures and a difference between activation energies derived from conductivity and thermopower measurements. Magnetic ordering of V4+ ions is clearly seen in R2V2O7 pyrochlores whereas with R2Mo2O7 compounds this is true only for R equals Nd, Sm and Gd. The ordering could arise from Mo4+ ions. In the case of Y2Mo2O7 as well as for R equals Sm and Gd maxima in susceptibility are accompanied by a difference between zero field and field cooled magnetization in low applied fields. This is characteristic of spin glass behavior. For R equals Tb, Dy and Ho deviations from Curie-Weiss behavior are seen below 20 K, while for R equals Er, Tm and Yb simple paramagnetic behavior persists down to 4.2 K. Magnetic and heat capacity results also point to the existence of Schottky anomalies arising from crystal field effects at the Mo4+ and R3+ sites.
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