Electrical, magnetic and thermal properties of ternary A<inf>2</inf>B<inf>2</inf>O pyrochlores-a family of magnetic semiconductors

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 = Nd, Sm and Gd the behaviour is metal-like, in most other cases it is like that of a degenerate semiconductor with a thermally activated mobility probably arising from hopping of small polarons. This is evidenced by the typical T-174 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 = Nd, Sm and Gd. The ordering could arise from Mo4+ ions. In the case of Y2MO2O7 as well as for R = 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 behaviour. For R = Tb, Dy and Ho deviations from Curie-Weiss behaviour are seen below 20K while for R = Er, Tm and Yb simple paramagnetic behaviour persists down to 4.2K. Magnetic and heat capacity results also point to the existence of Schottky anomalies arising from crystal field effects at the Mo4+ and R3+ sites.