Valence states of Cr and the insulator-to-metal transition in Cr-doped SrTiO3

Resistivity measurements and x-ray absorption near-edge structure spectroscopy were performed on single crystals of 0.2 mol % Cr-doped SrTiO3, which exhibit an electrical-field-induced insulator-to-conductor transition with a resistive memory effect. The electrically conducting state is shown to be metallic. The insulator-to-metal transition is accompanied by an increase of the Cr valence underneath the anode from Cr3+ to approximately Cr4+. This indicates that the insulator-to-metal transition is associated with internal doping due to a change of the Cr valence, thereby providing free carriers to the Ti 3d band. © 2005 The American Physical Society.