  In this research the electrochemical reduction of 3YSZ was investigated in
various atmospheres with different oxygen partial pressures under an electric
field of 25 V/cm at an environmental temperature of 800 {\deg}C. At a certain
oxygen partial pressure insufficient incorporation of oxygen in the sample led
to electrochemical reduction of YSZ which shows two clearly distinguishable
states. First, greying of the material without a significant change in
properties was detected which then transitioned into a second stage where a
fundamental phase transition in the material happened within seconds from
tetragonal 3YSZ towards FCC rocksalt ZrO or ZrON, dependent on the atmosphere.
This phase transition is accompanied by blackening of the material, sudden
increase in electrical conductivity, current concentration, and an obvious
change in Raman spectrum.
