The minimum conductivity and electron localisation in the metallic phase of transition metal compounds in the vicinity of a metal-insulator transition

A value of the minimum metallic conductivity of order 10 3 Omega -1 cm -1 is observed for many transition metal compounds which show a metal-insulator phase transition. When the conductivity at the transition is above this value, the resistance shows the normal metallic behaviour increasing with increasing temperature. When it is below this value, the resistance is activated, as for a semiconductor. In a single valence compound (e.g. V 2 O 3 , VO 2 ) a change of conductivity from one form to the other is, due to structural disorder. Compounds with mixed valence (V 4 O 7 , V 3 O 5 , etc.) always have an activated conductivity. An hypothesis is offered that this behaviour is connected with electron localisation, due both to the structural disorder and to electron correlation.