Valence-electron transfer and a metal-insulator transition in a strongly correlated perovskite oxide

We present transport and thermal data for the quadruple-perovskites MCu3(Ti1−xRux)4O12, where 0<x<1. A metal-insulator transition (MIT) occurs for Ru concentrations x∼0.75. At the same time, the Cu2+ antiferromagnetic state is destroyed and itʹs magnetic entropy suppressed by Ru on a 1:1 basis. This implies that each Ru transfers an electron to a Cu ion and thus the MIT correlates with filling the Cu 3d shell. The Cu spin entropy in this strongly correlated electron material provides a unique probe among MIT systems.