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)(4)O-12, where 0 x 1. A metal-insulator transition (MIT) Occurs for Ru concentrations x similar to 0.75. At the same time, the Cu2+ antiferromagnetic state is destroyed and it's 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. (C) 2004 Elsevier Ltd. All rights reserved.