Electrical conductivity of solid solutions between metal-like lead- and insulating yttrium ruthenate

A metal-insulator transition (MIT) is observed when doping the strongly correlated electron system lead ruthenate pyrochlore with yttrium. Lead ruthenate (Pb₂Ru₂O_6.5) has metal-like conductivity and is metallic Pauli paramagnetic, whereas yttrium ruthenate (Y₂Ru₂O₇) is an antiferromagnetic mott insulator. In Pb_2-xY_xRu₂O_6.5+z solid solutions, x is increased from 0 to 2. The MIT occurs at x ≈ 0.2, which is mathematically analyzed in a new way and explained by the Mott–Hubbard mechanism of electron localization. Obviously, a critical content of yttrium is needed to open the Mott–Hubbard energy gap and to fill the lower Hubbard band (LHB) with localized electrons.