Continuous metal-insulator transition in the pyrochlore Cd2Os2O7 - art. no. 195104

Cd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-ins ulator transition (MIT) near 226 K. We have characterized the MIT in Cd2Os2 O7 using x-ray diffraction, resistivity at ambient and high pressure, speci fic heat, magnetization, thermopower, Hall coefficient, and thermal conduct ivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-c ell volume of less than 0.05%. The resistivity shows little temperature dep endence above 226 K, but increases by 3 orders of magnitude as the sample i s cooled to 4 K. The specific heat anomaly resembles a mean-held transition and shows no hysteresis or latent heat. Cd2Os2O7 orders magnetically at th e MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coeffi cients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd 2Os2O7. These calculations indicate that Cd2Os2O7 is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data i n terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic ord er. A Slater transition-unlike a Mott transition-is predicted to be continu ous, with a semiconducting energy gap opening much like a BCS gap as the ma terial is cooled below T-MIT.