The measurement of DC conductivity of thin films deposited from vanadium pentoxide gels between 200 and 600 K in air, oxygen and a vacuum of 5 multiplied by 10** minus **7 torr has revealed that reversible changes in conductivity are determined by hydration/dehydration phenomena. The removal of weakly bonded water results in a conductivity decrease from about 2 S/m to about 0. 3 S/m at room temperature. Heat treatment between 430 and 550 K in oxygen leads to the maximally dehydrated phase in which sigma approximately equals 9 multiplied by 10** minus **3 S/m at 300 K. The essentially unchanged activation energies in all of the phases suggest that hydration affects the charge carrier concentration only.
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