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Charge migration in Pb(Zr,Ti)O3 ceramics and its relation to ageing, hardening and softening

The dielectric response of hard (Fe-doped) and soft (Nb-doped) rhombohedral Pb(Zr0.58Ti0.42)1-xMexO3 (PZT; Me= Fe or Nb) ceramics was studied at subswitching conditions over a wide range of temperatures (50 C to 450 C) and frequencies (10 mHz to 10 kHz). The results show qualitative differences in behavior of the acceptor and donor doped samples. Hard materials exhibit a steep increase of the complex permittivity with decreasing frequency. The onset of the dispersion is thermally activated with activation energy of about 0.6-0.8 eV and is attributed here to oxygen vacancy hopping. Activation energy for ac conductivity observed in soft materials is estimated to about 1.7 eV, corresponding to the half of the energy gap of Pb(Zr,Ti)O3 and is thus consistent with electronic conduction. The relevance of ionic hopping conductivity in hard materials to ferroelectric aging / deaging and hardening is analyzed. Strong ionic conductivity in hard and its absence in soft samples agree well with the dipolar mechanism of ageing in hard materials and the absence of significant ageing in soft materials.