Metal-insulator transition in lead yttrium ruthenate

Lead yttrium ruthenate pyrochlore solid solutions (Pb2-xYxRu2O6.5+z) were investigated for a metal insulator transition (MIT) and the Seebeck effect. Introducing increasing moles x of Y instead of Pb into metallic Pauli paramagnetic Pb2Ru2O6.5 causes an MIT at x approximate to 0.2 and formation of semiconductor/insulator materials until the antiferromagnetic Mott insulator Y2Ru2O7 is reached. The Mott-Hubbard mechanism of electron localization was used to explain the MIT. A critical content of yttrium is needed to open the Mott-Hubbard gap and to fill the lower Hubbard band (LHB) with localized t(2g)(4) electrons. The Seebeck effect showed a maximum between 400 and 500 K, for at least all x <= 1.5. The effect can be explained in terms of S varying proportionally with the effective mass of the carriers and inversely with their concentration.