Figure 3.2 suggests that oxidative non stoichiometry of NiO at high temperature is achieved via formation of nickel vacancies and holes. The following data have been measured [5, 6] at 1100 °C in O₂: coefficient of (tracer) random diffusion for nickel atoms D_Ni = 10⁻¹⁵ m²/s, total electrical conductivity σ = 65.5 S/m, site fraction 0.0001 of nickel vacancies. Assuming that Ni diffuses with a vacancy mechanism via nickel sites (never at O site), and given a = 4.20 Å for the NaCl-type cell, calculate for each of the two majority defects: (a) Concentration per m³, (b) Diffusivity, (c) Ionic electrical conductivity and ionic transference coefficient in order to determine whether NiO is an ionic or electronic conductor, (d) Mobility.

Figure 3.2

Transcribed Image Text:

O nonstoichiometry log defect concentration 0.02 0.00 -0.02 0 N A 6 co -10 [VNI"][Vo"] -1/6 VO [vo"] = [VI] integer structure -14 2[VN] + [e] = [h*] + 2[vo"] excess of -20 + integer valence [e'] = [h'] log Poz -10 1/6 "1 V Ni -1/6 e [e'] = [h] integer valence - excess o -20 + integer structure [VO]= [VN"] -10 log Poz h 1/6 "F VNI 0