  V2O3 is the prototype system for the Mott transition, one of the most
fundamental phenomena of electronic correlation. Temperature, doping or
pressure induce a metal to insulator transition (MIT) between a paramagnetic
metal (PM) and a paramagnetic insulator (PI). This or related MITs have a high
technological potential, among others for intelligent windows and field effect
transistors. However the spatial scale on which such transitions develop is not
known in spite of their importance for research and applications. Here we
unveil for the first time the MIT in Cr-doped V2O3 with submicron lateral
resolution: with decreasing temperature, microscopic domains become metallic
and coexist with an insulating background. This explains why the associated PM
phase is actually a poor metal. The phase separation can be associated with a
thermodynamic instability near the transition. This instability is reduced by
pressure which drives a genuine Mott transition to an eventually homogeneous
metallic state.
